[1] Volume and salt transports are estimated from three dimensional variational analysis (3DVAR) reanalysis in the Baltic Sea. With the assimilation of temperature and salinity profiles, we find that 3DVAR assimilation causes larger volume and salt transports into the Baltic Sea than directly estimated in the Skagerrak for the 2003 major inflow. In addition, the daily transport obtained as the derivatives of total salt and volume reveals pronounced difference from what is directly estimated. The reason is found to be related to the unbalanced part in the reanalysis obtained from weak constraint 3DVAR. The reanalysis of hydrography is further used to estimate the transport in the Danish strait. For the 2003 major inflow, the total net volume/salt transports across the Darss Sill and Drogden Sill are comparable to observations and other model studies. However, the effect of the 3DVAR assimilation amounts to a volume decrease of 7 km³ and a salt increase of 0.23 Gt in the Arkona region. The reason is related to the barotropic flow through the Danish strait that was supposed to be enhanced in the reanalysis. It suggests that the solution of 3DVAR is statistically optimal but not physically balanced. In coastal regions, reanalysis data must be used with special care when transport is estimated. Meanwhile, misfit of the transports acquired in different ways helps to identify problems in the application of 3DVAR and the model quality.

[1] We have developed, validated, and applied a synthetic method to monitor the off-equatorial eastward currents in the central tropical Atlantic. This method combines high-density expendable bathythermograph (XBT) temperature data along the AX08 transect with altimetric sea level anomalies (SLAs) to estimate dynamic height fields from which the mean properties of the North Equatorial Countercurrent (NECC), the North Equatorial Undercurrent (NEUC) and the South Equatorial Undercurrent (SEUC), and their variability can be estimated on seasonal to interannual timescales. On seasonal to interannual timescales, the synthetic method is well suited for reconstructions of the NECC variability, reproduces the variability of the NEUC with considerable skill, and less efficiently describes variations of the SEUC, which is located in a region of low SLA variability. A positive correlation is found between interannual variations of the NECC transport and two indices based on an interhemispheric sea surface temperature (SST) gradient and southeasterly wind stress in the central tropical Atlantic. The NEUC is correlated on interannual timescales with SSTs and meridional wind stress in the Gulf of Guinea and zonal equatorial wind stress. This study shows that both altimetry and XBT data can be effectively combined for near-real-time inference of the dynamic and thermodynamic properties of the tropical Atlantic current system.

[1] The objective of this paper is to evaluate the baroclinic and barotropic components of sea surface height (SSH) anomalies in the Southern Ocean and investigates the causes for the weak correlation between hydrographically derived and satellite measured SSH anomalies. To this end, data obtained by six Pressure Inverted Echo Sounders (PIES) deployed south of Africa were used to derive baroclinic and barotropic SSH anomalies. Our results show that the barotropic component accounts for 30%–60% of the variability between the jets of the Antarctic Circumpolar Current (ACC). In contrast, the jets associated with the major ACC fronts are predominantly baroclinic. The deep baroclinic component is important in the ACC and accounts for the major part of the baroclinic variability. The comparison with along-track satellite altimetry (Jason 1/2) shows correlations coefficients of 0.24–0.92, with low values in regions of high-barotropic variability. The comparison with gridded Aviso satellite altimetry generally shows higher correlation coefficients (0.33–0.92) and lower root mean square (RMS) errors compared to the along-track product. In conclusion the barotropic SSH anomaly plays a major role in this region and has to be accounted for when assimilating SSH into ocean models. Due to their high-baroclinic component, the correct representation of the time and space varying ACC fronts seems to be crucial for the right SSH anomaly partitioning used for assimilation purposes. Gridded products, namely Aviso, seems to be more suitable compared to along-track products (Jason 1/2) in representing the variability of SSH anomalies.

[1] The sensitivity of passive microwave observations to the sea surface temperature (SST) is carefully analyzed, with the objective of designing an optimized satellite instrument, MICROwave Wind And Temperature (MICROWAT), dedicated to an “all-weather” estimation of the SST at high spatial resolution (15 km). Our study stresses the importance of low-frequency observations around 6 GHz for accurate SST retrieval. Compared to the 11 GHz channel, the 6 GHz channel provides more sensitivity to the low SSTs and offers lower instrument noise, thanks to possibly broader channel bandwidths. However, it requires much larger antenna size for a given spatial resolution. Two instrument concepts have been suggested, one using a classic real aperture antenna and the other using synthetic interferometric antennas. This first analysis shows that 2-D interferometric systems would be very complex and would not satisfy the user requirements in terms of SST accuracy. A 1-D interferometric system could be proposed, but its development requires additional investigation. A dedicated conical scanner onboard a microsatellite with a 6 m antenna and channels at 6.9 and 18.7 GHz (both with V and H polarizations) can provide an SST accuracy of 0.3 K with a 15 km spatial resolution, with today's technology.

[1] Spur and groove (SAG) formations are found on the fore reefs of many coral reefs worldwide. Although these formations are primarily present in wave-dominated environments, their effect on wave-driven hydrodynamics is not well understood. A two-dimensional, depth-averaged, phase-resolving nonlinear Boussinesq model (funwaveC) was used to model hydrodynamics on a simplified SAG system. The modeling results show that the SAG formations together with shoaling waves induce a nearshore Lagrangian circulation pattern of counter-rotating circulation cells. The mechanism driving the modeled flow is an alongshore imbalance between the pressure gradient (PG) and nonlinear wave (NLW) terms in the momentum balance. Variations in model parameters suggest the strongest factors affecting circulation include spur-normal waves, increased wave height, weak alongshore currents, increased spur height, and decreased bottom drag. The modeled circulation is consistent with a simple scaling analysis based on the dynamical balance of NLW, PG, and bottom stress terms. Model results indicate that the SAG formations efficiently drive circulation cells when the alongshore SAG wavelength allows for the effects of diffraction to create alongshore differences in wave height without changing the mean wave angle.

[1] Previous studies have shown that the properties of optically significant water constituents (phytoplankton, suspended matter, and colored dissolved organic matter (CDOM)) in Hudson Bay are different from other Arctic regions. A new bio-optical data set collected in summer 2010 shows that this region also presents seasonal variability of the light absorption coefficients by the different constituents, with a higher relative proportion of CDOM absorption in summer than in the fall as a result of decreased phytoplankton absorption in summer. The slope of the exponential function describing nonalgal particles and CDOM spectral absorption shows little variability between fall and summer. Seasonal variability of light absorption coefficients and water optical constituents is more pronounced near the coast, while less variability is observed in the central part of the bay. Very low summertime chlorophyll-specific absorption coefficients by phytoplankton, among the lowest reported in the literature, are attributed to the high proportion of large size phytoplankton (microphytoplankton) and important packaging effect. There is also a smaller contribution of accessory pigments to total pigments in the summer than in the fall, resulting in a lower blue-to-red phytoplankton absorption ratio. These results emphasize that it is necessary to take into account the seasonal variability of light absorption properties in bio-optical models for further remote sensing applications in Hudson Bay.

[1] The present-day availability of an 18 year record of merged Mediterranean Sea sea level anomaly (SLA) data enables a contemporary description of long-term mesoscale activity in the Balearic Sea. SLA data from satellite altimetry are used to study the variability of sea level and surface geostrophic circulation at different spatial and temporal scales within this complex and relatively understudied region in the western Mediterranean (WMED). We find that the mean Northern Current along the Iberian slope is strongest in autumn, although higher variability in winter leads to stronger peaks in kinetic energy. The Balearic Current, which flows along the northern slopes of the Balearic islands, also has its maximum expression in autumn. Across the two Balearic channels (Ibiza and Mallorca), key locations that partly regulate meridional exchange in the WMED, observed seasonal variability in geostrophic velocity anomalies conforms rather well to prior descriptions, suggesting cautious confidence in the use of the Mediterranean merged altimeter product in nearshore regions. Circulation through the channels is maximum in winter. The channel data support the hypothesis that the channel circulation may be hindered by the intermittent presence of the Western Intermediate Water mass, which sometimes forms in winter in the Gulf of Lions. This is the first time that an analysis of variability in the Balearic channels has been performed using altimetric data.

[1] Phycobiliproteins are a family of water-soluble pigment proteins that play an important role as accessory or antenna pigments and absorb in the green part of the light spectrum poorly used by chlorophyll a. The phycoerythrins (PEs) are one of four types of phycobiliproteins that are generally distinguished based on their absorption properties. As PEs are water soluble, they are generally not captured with conventional pigment analysis. Here we present a statistical model based on in situ measurements of three transatlantic cruises which allows us to derive relative PE concentration from standardized hyperspectral underwater radiance measurements (L_u). The model relies on Empirical Orthogonal Function (EOF) analysis of L_u spectra and, subsequently, a Generalized Linear Model with measured PE concentrations as the response variable and EOF loadings as predictor variables. The method is used to predict relative PE concentrations throughout the water column and to calculate integrated PE estimates based on those profiles.

[1] We developed a model for partitioning the spectral absorption coefficient of suspended marine particles, a_p(λ), into phytoplankton, a_ph(λ), and nonalgal, a_d(λ), components based on the stacked-constraints approach. The key aspect of our model is the use of a set of inequality constraints that account for large variability in the a_ph(λ) and a_d(λ) coefficients within the world's oceans. The bounds of inequality constraints were determined from the analysis of a comprehensive set of 505 field determinations of absorption coefficients in various oceanic environments. The feasible solutions of the model are found by simultaneously satisfying all inequality constraints. The optimal solutions represented by the median values of feasible solutions for a_ph(λ) and a_d(λ) generally agree well with field measurements and are superior in terms of error statistics compared with previous partitioning models. For example, on the basis of comparisons of optimal model solutions with field determinations of absorption coefficients, the systematic error calculated as the median ratio of model-derived to measured values for both a_ph(443) and a_d(443) is within ±1% for our model. The random error represented by the mean absolute percent difference for a_ph(443) and a_d(443) is <5% and <20%, respectively. This study suggests that our model has the potential for successful applications with input data of a_p(λ) which can be collected from various oceanographic platforms.

[1] Global near-surface currents are calculated from satellite-tracked drogued drifter velocities on a 0.5° × 0.5° latitude-longitude grid using a new methodology. Data used at each grid point lie within a centered bin of set area with a shape defined by the variance ellipse of current fluctuations within that bin. The time-mean current, its annual harmonic, semiannual harmonic, correlation with the Southern Oscillation Index (SOI), spatial gradients, and residuals are estimated along with formal error bars for each component. The time-mean field resolves the major surface current systems of the world. The magnitude of the variance reveals enhanced eddy kinetic energy in the western boundary current systems, in equatorial regions, and along the Antarctic Circumpolar Current, as well as three large “eddy deserts,” two in the Pacific and one in the Atlantic. The SOI component is largest in the western and central tropical Pacific, but can also be seen in the Indian Ocean. Seasonal variations reveal details such as the gyre-scale shifts in the convergence centers of the subtropical gyres, and the seasonal evolution of tropical currents and eddies in the western tropical Pacific Ocean. The results of this study are available as a monthly climatology.

[1] The ocean's western boundary current regions display the greatest rate of twentieth century warming and global climate models project that the accelerated rate of warming will continue with climate change. All existing global climate change projections come from simulations that do not fully resolve either these boundary currents or their eddies. Using an Ocean Eddy-resolving Model (OEM) that captures the dynamics of the East Australian Current (EAC) and its eddies we show the response of the Tasman Sea to climate change differs from what is projected with a coarse resolution Global Climate Model (GCM). With climate change, the OEM projects increased EAC transport with increased eddy activity and an approximately 1° southward latitudinal shift in the point where the EAC separates from the shelf and flows eastward. The OEM increased eddy activity in the Tasman Sea with climate change increases the nutrient supply to the upper ocean and causes an increase in the phytoplankton concentrations and primary productivity by 10% in the oligotrophic waters of the Tasman Sea. The increase in primary productivity is absent in the GCM climate change projection, which projects the region will have a decrease in primary productivity with climate change. Applying the OEM climate change projection for the Tasman Sea to other western boundary current regions suggests the projected intensification of all western boundary currents with climate change should increase eddy activity and provide an important nutrient supply mechanism to counter the increased stratification projected with global warming.

[1] Prescribing open boundary conditions for regional coastal ocean models encounters the challenge of imposing information on sea level, velocity and tracers that characterize the unrepresented far field ocean. Deriving such information from a larger domain model without communicating information from the “nested” model back to the exterior model is “downscaling”. We evaluate whether real-time models presently in operation for the Mid-Atlantic Bight (MAB) can deliver useful predictions of subtidal frequency currents and subsurface temperature and salinity for this downscaling purpose. The MAB is a broad continental shelf region where several models run in real time and there is a dense observational data set available for skill assessment. We examine seven real-time models that cover the MAB: three global models, and four regional models. A regional climatology is included as an eighth model. Skill metrics with respect to model bias, centered root mean square error and cross correlation are computed for temperature and salinity profile data from 16 autonomous underwater glider vehicle missions and four hydrographic voyages in 2010–2011. Two years of hourly HF-radar surface current observations that span the shelf are used to evaluate modeled mean surface currents and daily time scale variability in speed and direction. Skill metrics, with uncertainty estimates, are reported for inner and outer shelf subregions, and for stratified and unstratified seasons. A group of models is identified that offers useful skill for the purposes of providing open boundary data to inner shelf and estuary models for real-time applications.

[1] The glaciers draining into the Amundsen Sea Embayment are rapidly losing mass, making a significant contribution to current sea level rise. Studies of Pine Island Glacier (PIG) in this region indicate that the mass loss is associated with rapid melting of its floating ice shelf driven by warm Circumpolar Deep Water (CDW) that is able to penetrate all the way to its grounding line, and that recent intensification of the mass loss is associated with higher melt rates and stronger subice-shelf circulation. CDW is sourced from within the Antarctic Circumpolar Current (ACC) situated well north of the glacial ice fronts. To be able to access the Amundsen Sea glaciers, CDW must first cross the continental shelf break where the deep ocean meets the shallower waters of the continental shelf. Here, we present data that shows how CDW moves along the continental slope and across the shelf break into the Amundsen Sea. On-shelf flow of CDW is enhanced where a subsea trough bisects the shelf edge. A previously unreported undercurrent is observed flowing eastward along the shelf edge and when this current encounters the trough mouth it circulates southward into the trough and toward the glaciers. Upwelling associated with this trough circulation appears to allow Lower CDW onto the shelf that would otherwise be blocked by the topography. These observations concur with the results of a theoretical modeling study of circulation in a similar topographic setting and also with the results of a regional ocean/ice modeling study of the Amundsen Sea specifically.

[1] Sea-level trends and their forcing have been investigated in the Caribbean Sea using altimetry and tide gauge time series from 19 stations. The basin average sea-level rise is 1.7 ± 1.3 mm yr⁻¹ for the period 1993–2010. Significant spatial variability of the trends is found. The steric variability above 800 m combined with the global isostatic adjustment explains the observed trends for the altimetry period in most of the basin. Wind forcing changes causes the trends in the southern part of the basin, modulating the sea level through changes in the ocean circulation. The longest time series (102 years) of Cristobal shows a trend of 1.9 ± 0.1 mm yr⁻¹ insignificantly different from the global mean sea-level rise for the twentieth century. By contrast Cartagena, a world heritage site, has a large trend (5.3 ± 0.3 mm yr⁻¹) significantly affected by local vertical land movements. Stations dominated by the steric contribution have smaller trends (∼1.3 ± 0.2 mm yr⁻¹). Sea-level trends at tide gauges are not affected by atmospheric pressure changes or by the open ocean steric contribution at most stations. Decadal variability in the sea-level trends can partly be explained by steric and wind variability. The decadal variability in the trends is not spatially coherent. Interannual sea-level variability accounts for one third of the total sea-level variability and can be partly explained by the influence of El Niño-Southern Oscillation at different time and spatial scales. No correlation with the North Atlantic Oscillation is found.

[1] The source of the Canary Current has been inferred from an inverse box model applied to the hydrographic data of a survey carried out in 2009 in the northeast subtropical gyre (29–37°N, 9–24°W). The Portugal Current is observed between 13.5 and 14.8°W at 37°N carrying Sv southward. This current presumably merges with the eastward transport of the Azores Current System and partly contributes to the Mediterranean inflow and partly to the northward recirculation of the Azores Current through the Gulf of Cadiz. The Azores Current System is located in the meridional range 33.50–36.25°N at 24.50°W. This System transports eastward Sv in the thermocline layers and Sv at intermediate layers. The Azores Current intermediate water mass has the highest portion of Sub-Arctic intermediate water (SAIW) in the region, while the Azores Countercurrent intermediate waters mass is mainly Mediterranean water. The Canary Current extends from 22.25° to 18.50°W at 29°N, the westernmost position ever observed. This current transports southward Sv in the thermocline layers and Sv in the intermediate layers. This intermediate flow shows a relative maximum of oxygen and a relative minimum in nutrient concentration, indicating the presence of SAIW. The study concludes that, at least in fall 2009, the Canary Current extends to the intermediate waters ( approximately 1600 dbar) and that Azores Current feeds the Canary Current at surface and intermediate layers.

[1] Snow depth on sea ice (SD) is a key geophysical variable, knowledge of which is critical for calculating the energy and mass balance budgets. Moreover, accurate knowledge of the SD distribution is important to retrieve sea-ice thicknesses from altimetry data. So far, only space-based microwave radiometers (e.g., Advanced Microwave Scanning Radiometer for Earth Observing System; AMSR-E) provide operational SD on seasonal sea-ice retrievals. A thorough assessment of these retrievals is needed on a large scale and on a variety of sea-ice types. Our study presents such an assessment on Arctic sea ice using NASA's airborne Operation IceBridge (OIB) SDs, retrieved from radar measurements. Between 2009 and 2011, ∼610 12.5 km satellite grid cells were covered by seasonal sea ice where both satellite SD retrievals and OIB data were available. Using all the available data, the difference between the AMSR-E product and the averaged OIB snow-radar-derived SD is 0.00±0.07 m. Satellite-derived SD was accurate in the Beaufort Sea and the Canadian Archipelago but underestimated (∼0.07 m) in the Nares Strait. The RMSE between the two products ranges between 0.03 and 0.15 m. The RMSE is less than 0.06 m over a shallow snow cover (<0.20 m), in areas where satellite-retrieved ice concentrations are higher than 90%, surface smooth, and ice thicker than ∼0.5 m. Locally the AMSR-E algorithm can significantly underestimate SD. Several regions where the retrievals were less accurate (error >0.10 m) have been identified and related to the presence of either low ice concentration or significant fraction of multiyear ice within the grid cell that has not been flagged.

[1] Analysis of data sets based on satellite measurements during 1992–2011 reveals pronounced interannual-to-decadal variations of the west Pacific North Equatorial Countercurrent (NECC) system, involving the intensity (INT), position (Y_CM), and path length (L_CM) of the surface NECC jet, together with the associated recirculation gyres and mesoscale eddies. During the 1997–1998 and 2009–2010 El Niño events, the NECC jet showed increased INT before, and decreased INT, northerly position, and lengthened path after the mature phase. During the 1993–1995 and 2002–2005 central Pacific warming events, it also showed increased INT but no evident changes in Y_CM and L_CM. The varied responses caused the different natures of individual El Niño events. In 1998 and 2010, reflected upwelling Kelvin waves south of the NECC jet, together with the downwelling Rossby waves north of it induced by the following strong La Niña events, weakened the NECC jet through geostrophy and shifted it northward. This process was however absent during the 1993–1995 and 2002–2005 events. Intensified NECC jet during warm conditions gives rise to anomalously active mesoscale eddies, which also contribute to the unstable states of the NECC system in 1998 and 2010. Hindcast with a linear Rossby wave model reveals a slow change of the western Pacific NECC system during the past 50 years. Since 1990s, low-frequency variance of the NECC system has been dominated by quasi-decadal signals and more closely associated with wind forcing in the western Pacific Ocean, which corresponds to the slow changes of the El Niño/Southern Oscillation (ENSO)-related wind forcing pattern.

[1] A variational data assimilation system based on an incremental 4D-Var approach is proposed for use with a zero-dimensional model of the diurnal cycle of sea surface temperature (SST). Traditional 4D-Var, which seeks to find the initial state of a system, is not appropriate for diurnal SST which is a wind and heat flux driven system that has only a limited memory of its prior state. Instead the proposed assimilation system corrects both the initial SST and the heat and wind fluxes applied throughout the day. The assimilation system is tested using ensembles in a set of idealized twin experiments. In these tests controlling parameters are varied around reasonable “default” values with the quality of the analyses assessed against a known “truth”. Within our tests data assimilation is shown to improve diurnal SST under most circumstances. Analyzed heat fluxes are also sometimes improved, although the improvement is much less than that observed for diurnal SST. The system was not found to improve the wind stress. The only circumstances where diurnal SST was not found to be improved by the assimilation were where either observational errors were large (greater than 0.5 K in our tests), or biases in the observations were too big (less than −0.3 K or greater than 0.2 K). The non-Gaussian behavior of the wind stress was found to have an impact on the assimilation in low-wind conditions and under these conditions the best analyses were obtained by artificially inflating the observation error.

[1] The response of the Bay of Fundy and Gulf of Maine to large-scale tidal power plants and future sea-level rise is investigated using an established numerical tidal model. Free stream tidal turbines were simulated within the Bay of Fundy by implementing an additional bed friction term, K_t. The present-day maximum tidal power output was determined to be 7.1 GW, and required K_t = 0.03. Extraction at this level would lead to large changes in the tidal amplitudes across the Gulf of Maine. With future SLR implemented, the energy available for extraction increases with 0.5–1 GW per m SLR. SLR simulations without tidal power extraction revealed that the response of the semidiurnal tides to SLR is highly dependent on how changes in sea level are implemented in the model. When extensive flood defenses are assumed at the present-day coast line, the response to SLR is far larger than when land is allowed to (permanently) flood. For example, within the Bay of Fundy itself, the M₂ amplitude increases with nearly 0.12 m per m SLR without flooding, but it changes with only 0.03 m per m SLR with flooding. We suggest that this is due to the flooding of land cells changing the resonant properties of the basin.

[1] The evolution of the upper Southern Ocean hydrographic structure in response to the representative concentration pathways 4.5 (RCP 4.5) forcing scenario is analyzed using model data drawn from the coupled model intercomparison project phase 5 (CMIP5) archive. A robust freshening trend is evident, associated with an increase in stratification and decoupling of the upper ocean as the mixed layer gains buoyancy at a faster rate than the underlying ocean. The magnitudes of the individual terms of the salinity and heat budgets are evaluated. Convection-driven entrainment from the thermocline into the mixed layer is found to play a significant role in modulating the mixed layer salinity, whilst the heat budget of the mixed layer is dominated by a primary balance between atmospheric warming and the entrainment-modulated supply of oceanic heat from below the mixed layer. The relationship between oceanic heat storage below the mixed layer, ice thickness and atmospheric temperature is investigated, and a very disparate response noted amongst the models considered here. Based on this analysis, we hypothesize that the balance between the entrainment-modulated supply of oceanic heat from below the mixed layer and the heat supplied by the atmosphere may play an important role in determining the simulated sea ice variability.

[1] The effects of solar penetration on the annual cycle of extratropical North Pacific sea surface temperature (SST) are investigated based on coupled ocean-atmosphere model simulations. It is found that the solar penetration can significantly improve the seasonal cycle of the North Pacific SST in the model. In summer, the solar penetration can partly reduce the SST warm bias in the model by diluting the shortwave radiation to the seasonal thermocline, while in winter, it can reduce the model SST cold bias through the entrainment of the warm thermocline water into the mixed layer. The solar penetration forces significant changes not only in the ocean but also in the atmosphere. The changes in the atmospheric circulation are characterized by a baroclinic structure with ridge/trough in the lower/upper troposphere in summer and an equivalent barotropic trough in winter. As a result, the annual mean of the oceanic subtropical gyre is intensified. Our study echoes the potential importance of the ecosystem in modulating the coupled ocean-atmosphere interaction over the extratropical oceans.

[1] A high-frequency asymptotics approach within the Lagrangian framework shows that some exact equatorially trapped three-dimensional waves are linearly unstable when their steepness exceeds a specific threshold.

[1] The spatial distribution of turbulent dissipation rates and internal wavefield characteristics is analyzed across two contrasting regimes of the Antarctic Circumpolar Current (ACC), using microstructure and finestructure data collected as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Mid-depth turbulent dissipation rates are found to increase from in the Southeast Pacific to in the Scotia Sea, typically reaching within a kilometer of the seabed. Enhanced levels of turbulent mixing are associated with strong near-bottom flows, rough topography, and regions where the internal wavefield is found to have enhanced energy, a less-inertial frequency content and a dominance of upward propagating energy. These results strongly suggest that bottom-generated internal waves play a major role in determining the spatial distribution of turbulent dissipation in the ACC. The energy flux associated with the bottom internal wave generation process is calculated using wave radiation theory, and found to vary between 0.8 mW m⁻² in the Southeast Pacific and 14 mW m⁻² in the Scotia Sea. Typically, 10%–30% of this energy is found to dissipate within 1 km of the seabed. Comparison between turbulent dissipation rates inferred from finestructure parameterizations and microstructure-derived estimates suggests a significant departure from wave-wave interaction physics in the near-field of wave generation sites.

[1] A realistic primitive-equation model of the Southern Ocean at eddying spatial resolution is used to examine the effect of ocean-surface-velocity dependence of the wind stress on the strength of near-inertial oscillations. Accounting for the ocean-surface-velocity dependence of the wind stress leads to a large reduction of wind-induced near-inertial energy of approximately 40% and of wind power input into the near-inertial frequency band of approximately 20%. A large part of this reduction can be explained by the leading-order modification to the wind stress if the ocean-surface velocity is included. The strength of the reduction is shown to be modulated by the inverse of the ocean-surface-mixed-layer depth. We conclude that the effect of surface-velocity dependence of the wind stress should be taken into account when estimating the wind-power input into the near-inertial frequency band and when estimating near-inertial energy levels in the ocean due to wind forcing.

[1] Lateral diffusivity is computed from a tracer release experiment in the northeastern tropical Atlantic thermocline. The uncertainties of the estimates are inferred from a synthetic particle release using a high-resolution ocean circulation model. The main method employed to compute zonal and meridional components of lateral diffusivity is the growth of the second moment of a cloud of tracer. The application of an areal comparison method for estimating tracer-based diffusivity in the field experiments is also discussed. The best estimate of meridional eddy diffusivity in the Guinea Upwelling region at about 300 m depth is estimated to be m² s⁻¹. The zonal component of lateral diffusivity is estimated to be m² s⁻¹, while areal comparison method yields areal equivalent zonal diffusivity component of m² s⁻¹. In comparison to K_y, K_x is about twice larger, resulting from the tracer patch stretching by zonal jets. Employed conceptual jet model indicates that zonal jet velocities of about m s⁻¹ are required to explain the enhancement of the zonal eddy diffusivity component. Finally, different sampling strategies are tested on synthetic tracer release experiments. They indicate that the best sampling strategy is a sparse regular sampling grid covering most of the tracer patch.

[1] A 3-D hydrodynamic model (Regional Ocean Modeling System) is used to investigate the dynamics and structure of hyperpycnal river plumes over sloping continental shelves. The focus is on the plume's response to varying slopes and settling velocities (w_s). The idealized model is configured to represent small mountainous river systems during a flood event. A hyperpycnal sediment concentration of 60 g/L is specified at the river mouth such that the sediment–freshwater mixture is denser than the seawater, causing the plumes to traverse the shelves as undercurrents. A realistic range of shelf slope of 0.001–0.03 is chosen. The settling velocity is varied based on river's carrying capacity. The model-derived velocity profiles and the entrainment rate compare favorably against prior laboratory experiments. Both cross-shore and alongshore momentum balances are primarily between gravitational forcing and bottom friction. But, the Coriolis deflection is significant at the plume core in the alongshore momentum budget (i.e., Ekman balance). As the slope increases and settling velocity decreases, hyperpycnal plumes transition from depositional to autosuspending regime. An estimate of critical slope governed by a dimensionless parameter (q_b is buoyancy input) reasonably captures the regime transition. In the depositional regime, the plume's runout (cross-shore penetration) scales with advective distance: increasing slopes and discharge enhance the gravitational forcing and plume velocity, leading to an increase in runout. In contrast, increasing settling velocity shortens the vertical settling time, thereby reducing the plume's horizontal footprint. For the range of parameters considered, the runout of depositional plumes is confined within 15 km from the mouth, whereas the penetration of autosuspending plumes is essentially unlimited.

[1] On 19 September 2009, Typhoon Choi-wan passed ∼40 km to the southeast of the Kuroshio Extension Observatory (KEO) surface mooring, located at 32.3°N, 144.5°E. We use an atmosphere-wave-ocean coupled model that incorporated an oceanic carbon equilibrium model to investigate the typhoon-induced CO₂ outgassing observed by the KEO mooring. KEO data are used to provide atmospheric surface boundary conditions for partial pressure of CO₂ ( ) and to validate the numerical results. The model simulated the observed sea-level pressure variations reasonably well, although the simulated-typhoon translation was 3 h slower than the estimated best track. The simulation resulted in lower than observed sea-surface temperature (SST), sea-surface salinity (SSS), and partial pressure of surface ocean CO₂ ( ). Better agreement was found with the grid point south of the buoy that corresponded roughly to the buoy location in the simulated-typhoon reference frame. In situ observations show CO₂ outgassing during the Choi-wan's passage. Forty percent of observed outgassing was explained by decreasing (∼20 µatm), and thus, the remainder (∼30 µatm) must be explained by increasing . The model simulated only one third of the increase in observed surface variation (∼9.6 µatm), suggesting that not only SST but also high salinity and dissolved inorganic carbon caused by vertical turbulent mixing and horizontal advection are important in simulating surface variation. The simulations also reveal that surface roughness length affects surface wind asymmetry during the passage and variation in SSS and (∼1 µatm) after the passage.

[1] We investigate whether the Quasi-geostrophic (QG) modes and the Surface Quasi-geostrophic (SQG) solutions are consistent with the vertical structure of the subinertial variability off southeast Brazil. The first-order empirical orthogonal function (EOF) of current meter time series is reconstructed using different QG mode combinations; the first EOF is compared against SQG solutions. At two out of three moorings, the traditional flat-bottom barotropic (BT) and first baroclinic (BC1) mode combination fails to represent the observed sharp near-surface decay, although this combination contains up to 78% of the depth-integrated variance. A mesoscale broad-band combination of flat-bottom SQG solutions is consistent with the near-surface sharp decay, accounting for up to 85% of the first EOF variance. A higher-order QG mode combination is also consistent with the data. Similar results are obtained for a rough topography scenario, in which the velocity vanishes at the bottom. The projection of the SQG solutions onto the QG modes confirms that these two models are mutually dependent. Consequently, as far as the observed near-surface vertical structure is concerned, SQG solutions and four-QG mode combination are indistinguishable. Tentative explanations for such vertical structures are given in terms of necessary conditions for baroclinic instability. “Charney-like” instabilities, or, surface-intensified “Phillips-like” instabilities may explain the SQG-like solutions at two moorings; traditional “Phillips-like” instabilities may rationalize the BT/BC1 mode representation at the third mooring. These results point out to the presence of a richer subinertial near-surface dynamics in some regions, which should be considered for the interpretation and projection of remotely sensed surface fields to depth.

[1] Hurricane Bob moved up the U.S. east coast and crossed over southern New England and the Gulf of Maine [with peak marine winds up to 54 m/s (100 mph)] on 19–20 August 1991, causing significant damage along the coast and shelf. A 3-D fully wave-current-coupled finite-volume community ocean model system was developed and applied to simulate and examine the coastal ocean responses to Hurricane Bob. Results from process study-oriented experiments showed that the impact of wave-current interaction on surge elevation varied in space and time, more significant over the shelf than inside the inner bays. While sea level change along the coast was mainly driven by the water flux controlled by barotropic dynamics and the vertically integrated highest water transports were essentially the same for cases with and without water stratification, the hurricane-induced wave-current interaction could generate strong vertical current shear in the stratified areas, leading to a strong offshore transport near the bottom and vertical turbulent mixing over the continental shelf. Stratification could also result in a significant difference of water currents around islands where the water is not vertically well mixed.

[1] We assimilate satellite observations of surface chlorophyll into a three-dimensional biological ocean model in order to improve its state estimates using a particle filter referred to as sequential importance resampling (SIR). Particle Filters represent an alternative to other, more commonly used ensemble-based state estimation techniques like the ensemble Kalman filter (EnKF). Unlike the EnKF, Particle Filters do not require normality assumptions about the model error structure and are thus suitable for highly nonlinear applications. However, their application in oceanographic contexts is typically hampered by the high dimensionality of the model's state space. We apply SIR to a high-dimensional model with a small ensemble size (20) and modify the standard SIR procedure to avoid complications posed by the high dimensionality of the model state. Two extensions to the SIR include a simple smoother to deal with outliers in the observations, and state-augmentation which provides the SIR with parameter memory. Our goal is to test the feasibility of biological state estimation with SIR for realistic models. For this purpose we compare the SIR results to a model simulation with optimal parameters with respect to the same set of observations. By running replicates of our main experiments, we assess the robustness of our SIR implementation. We show that SIR is suitable for satellite data assimilation into biological models and that both extensions, the smoother and state-augmentation, are required for robust results and improved fit to the observations.

[1] The meridional heat flux required to balance the heat lost by ocean to atmosphere at high latitudes must be accomplished by some mechanism other than mean advection and the heat flux by eddies crossing the Antarctic Circumpolar Current (ACC) may be a candidate. In this study, the positions of the main ACC fronts are determined based on 23 expendable bathythermographs (XBT) transects collected from 1994 to 2010 and are compared with those detected through satellite altimetry. Then, cold core anomalies in XBT sections are identified and altimetry is used to follow the spatial-temporal evolution of these cold, low sea level anomalies. Mean values of main parameters, such as speed (0.35 km/h), lifetime (79 weeks), and diameter (105 km), are estimated. Moreover, estimations of rotational speed (0.9–76.8 cm/s), ocean surface layer heat content along temperature sections and eddy available heat anomaly (mean value −9.74 × 10⁹ Jm⁻²) give a wider description of the detected eddies. In our study area, the spawning of eddies is found to occur downstream of the Southeast Indian Ridge and in correspondence of the polar front (PF) with regard to the ACC frontal structure. The contribution of eddies to the global heat budget is not only linked to their ability to cross the ACC fronts but also to the capacity of keeping partially unaltered the properties of water inside them. Analysis of the relation between the translation and rotational speeds shows that a typical eddy may effectively be a significant part (0.8%) of the net meridional heat transport across the PF with a mean heat content/anomaly of −7.65 × 10¹⁹ J.

[1] Coastal polynyas are areas in an ice-covered ocean where the ice cover is exported, mostly by off-shore winds. The resulting reduction of sea ice enables an enhanced ocean-atmosphere heat transfer. Once the water temperatures are at the freezing point, further heat loss induces sea ice production. The heat exchange and ice production in coastal polynyas in the southwestern Weddell Sea is addressed using the Finite-Element Sea-ice Ocean Model, a primitive-equation, hydrostatic ocean circulation model coupled with a dynamic-thermodynamic sea-ice model, which allows to quantify the amount of heat associated with cooling of the water column. Three important polynya regions are identified: at Brunt Ice Shelf, at Ronne Ice Shelf and along the southern part of the Antarctic Peninsula. Multiyear winter means (May–September 1990–2009) give an upward heat flux to the atmosphere of 311 W/m² in the Brunt polynyas, 511 W/m² in Ronne Polynya and 364 W/m² in the Antarctic Peninsula polynyas, whereof 57 W/m², 49 W/m² and 48 W/m², respectively, are supplied as oceanic heat flux from deeper layers. The mean winter sea ice production is 7.2 cm/d in the Brunt polynyas corresponding to an ice volume of 1.3 ×10¹⁰ m³/winter, 13.2 cm/d at Ronne polynya (4.4 ×10¹⁰ m³/winter), and 9.2 cm/d in the Antarctic Peninsula polynyas (2.1 ×10¹⁰ m³/winter). The heat flux to the atmosphere inside polynyas is 7 to 9 times higher than the heat flux in the adjacent area; polynya ice production per unit area exceeds adjacent values by a factor of 9 to 14.

[1] The role of asymmetric tidal mixing (ATM) in subtidal estuarine dynamics is investigated using a series of generic numerical experiments that simulate narrow estuaries under different stratification and external forcing conditions. The focus is on quantifying the characteristics of ATM-induced flow and its contributions to stratification and salt transport. The flow induced by ATM has a two-layer vertical structure in periodically stratified estuaries, similar to that of the density-driven flow. It has a three-layer vertical structure in the central regime of weakly stratified estuaries, and a reverse two-layer structure in highly stratified estuaries. The changes in vertical distribution of ATM-induced flows result from the influence of stratification on the covariance of eddy viscosity and vertical shear. Such covariance represents the driving force of ATM-induced flow in the tidally averaged momentum equation. Compared to density-driven flow, ATM-induced flow dominates in periodically stratified estuaries with strong tides, has the same order of magnitude in weakly stratified estuaries with moderate tides, and is less important in highly stratified estuaries with weak tides. In contrast to density-driven flow that always increases estuarine stratification and transports salt landward, the ATM-induced flow exhibits different behaviors because of its varying vertical structure. In estuaries with strong tides, ATM-induced flow tends to enhance stratification and to transport salt landward, similar to density-driven flow. In estuaries with weak tides, ATM-induced flow tends to reduce stratification and to transport salt seaward.

[1] Recent dramatic changes of freshwater systems in high latitudes will allow Submarine Fresh Groundwater Discharge (SFGD) to play a more important role in the coastal environment; especially in that SFGD will directly effect heat flux. Toyama Bay, along the central western Japan coast, is a suitable and representative case study area to estimate SFGD flux using hydrographic properties since multiple high-flow rate SFGD sites exist there. Salinities averaged over the water column (depth range 10–100 m) measured during research cruises in June 2003 and May 2005 show lower levels in the eastern than the western area in this bay. Together with monthly hydrographic properties over a 10 year period (1987–1998), the low-salinity water mass in the eastern area exists consistently but distinctly and varies systematically, as does nutrient flux, affected by SFGD more than riverine input. SFGD fluxes in June 2003 (1 × 10⁸ m³ month⁻¹) and May 2005 (<1 × 10⁸ m³ month⁻¹) were estimated using a box model, which is divided into a shallow box (0–40 m) and a deeper box (40–100 m). The monthly flux ratio between the SFGD and the river inputs is 13% in June, comparable to higher values reported in other global studies. Our results demonstrate that the box model analysis, based on hydrographic observations in coastal areas, is an efficient approach that can be used to estimate SFGD fluxes between the land and ocean.

[1] The timing, duration, and intensity of wind-driven upwelling and downwelling along the North American Pacific coast play an integral role in coastal circulation and basinwide ecosystem composition. It has been suggested that global warming will cause changes in these winds. Here we develop a new set of objective criteria to unambiguously determine the onset, duration, and intensity of upwelling and downwelling seasons due to local wind forcing. We use these criteria to examine and better characterize temporal trends in wind-driven coastal currents over the previous 60 years and relate them to global warming and large-scale climate oscillations in the coastal ocean between northern California and Vancouver Island (37°N and 51°N). We find an exceptionally variable onset of upwelling at all locations. Some significant temporal trends are found in summer onset and upwelling intensity time series near the Juan de Fuca Strait and off the coast of Oregon. Positive phases of the Pacific Decadal Oscillation are correlated to later and shorter upwelling seasons with weaker upwelling. Warm phases of the El Niño Southern Oscillation are associated with a later onset of summer upwelling south of Oregon and with more intense downwelling throughout the study area. Our analysis identifies strong interannual to interdecadal variability, and emphasizes the importance of time series length when isolating physical temporal trends influenced by large-scale oscillatory behavior of the climate.

[1] A series of controlled laboratory experiments have been conducted to investigate the backscatter of high frequency sound (3–5 MHz) from suspensions of fine sediment in its unflocculated (primary) state and at various levels of flocculation. The size and fall-velocity distributions of the flocs were determined using an optical system and a settling tube, thus allowing floc density to be determined. The measurements have conclusively demonstrated that the acoustic properties of the flocculated particles are not solely controlled by the primary particles; some aspect of the floc structure is influencing the scattering characteristics. The overall trend is for the form function (K_s) to increase as the degree of flocculation increases. This trend was also observed in the total scattering cross section ( ) but this result is dependent on the assumption that viscous absorption for flocculated particles is negligible. The measured scattering properties are compared to the predicted values from two theoretical models, the elastic (ES) and fluid sphere (FS) models. While the results show that, in their current form, neither model is capable of adequately representing the scattering characteristics of a suspension of flocculated particles, the two models did provide upper (ES) and lower (FS) bounds to the measurements. In terms of the operational use of acoustics to measure the concentration of flocculated sediments, empirical relationships could be fitted to the observations but, until a better theoretical understanding of how sound interacts with flocculated particles is achieved, the fitting of such empirical relations may be somewhat premature.

[1] A three-part algorithm is described and tested to provide robust bathymetry maps based solely on long time series observations of surface wave motions. The first phase consists of frequency-dependent characterization of the wave field in which dominant frequencies are estimated by Fourier transform while corresponding wave numbers are derived from spatial gradients in cross-spectral phase over analysis tiles that can be small, allowing high-spatial resolution. Coherent spatial structures at each frequency are extracted by frequency-dependent empirical orthogonal function (EOF). In phase two, depths are found that best fit weighted sets of frequency-wave number pairs. These are subsequently smoothed in time in phase 3 using a Kalman filter that fills gaps in coverage and objectively averages new estimates of variable quality with prior estimates. Objective confidence intervals are returned. Tests at Duck, NC, using 16 surveys collected over 2 years showed a bias and root-mean-square (RMS) error of 0.19 and 0.51 m, respectively but were largest near the offshore limits of analysis (roughly 500 m from the camera) and near the steep shoreline where analysis tiles mix information from waves, swash and static dry sand. Performance was excellent for small waves but degraded somewhat with increasing wave height. Sand bars and their small-scale alongshore variability were well resolved. A single ground truth survey from a dissipative, low-sloping beach (Agate Beach, OR) showed similar errors over a region that extended several kilometers from the camera and reached depths of 14 m. Vector wave number estimates can also be incorporated into data assimilation models of nearshore dynamics.

[1] We present a data-assimilated model of the ocean's phosphorus cycle that is constrained by climatological phosphate, temperature, salinity, sea-surface height, surface heat and freshwater fluxes, as well as chlorofluorocarbon-11(CFC-11) and natural Δ¹⁴C. Export production is estimated to be 5.8±2.0×10¹² mol P/yr of which (26±6)% originates in the Southern Ocean (SO) south of 40°S. The biological pump efficiency, defined as the proportion of the ocean's phosphate inventory that is regenerated, is (39±7)%. Dividing the SO south of 40°S into a sub-Antarctic zone (SANTZ) and an Antarctic zone (ANTZ) separated by the latitude of maximum Ekman divergence, we estimate that the SANTZ and ANTZ account, respectively, for (23±5)% and (3±1)% of global export production, (17±4)% and (3±1)% of the regenerated nutrient inventory, and (31±1)% and (43±5)% of the preformed nutrient inventory. Idealized SO nutrient depletion experiments reveal a large-scale transfer of nutrients into circumpolar and deep waters and from the preformed to the regenerated pool. In accord with the concept of the biogeochemical divide, we find that nutrient drawdown in the ANTZ is more effective than in the SANTZ for increasing the efficiency of the biological pump, while having a smaller impact on production in regions north of 40°S. Complete SO nutrient drawdown would allow the biological pump to operate at 94% efficiency by short circuiting the transport of nutrients in northward Ekman currents, leading to a trapping of nutrients in circumpolar and deep waters that would decrease production outside the SO by approximately 44% while increasing it in the SO by more than 725%.

[1] Hourly time series from a quasi-global set of 145 tide gauges are used to investigate annual maximum water levels at each station. High water levels are deconstructed into (1) a predicted tidal component, (2) a seasonal component, (3) a low-frequency nontidal residual that accounts for sea level variability at time scales greater than a month but less than a year, and (4) a high-frequency nontidal residual that captures variability particularly associated with storms at time scales greater than a month. The time-averaged annual maximum water level correlates significantly with, and scales as 2.5 times, the water level standard deviation at the tide gauge stations. This relationship is used to estimate time-averaged annual maximum water level on a nearly continuous global scale (excluding ice-covered polar regions) by specifying variance maps of the tides from a tide model, the seasonal and low-frequency residual components from satellite altimetry sea surface height, and the high-frequency residual component from an atmospheric reanalysis product. The variance fields are combined to estimate time-averaged annual maximum water levels that compare well with observed values at the tide gauge stations. Spatial patterns of annual maximum water levels and relative contributions from the tides and nontidal residual components are considered.

[1] The circulation in a glacial fjord driven by a large tidewater glacier is investigated using a nonhydrostatic ocean general circulation model with a melt rate parameterization at the vertical glacier front. The model configuration and water properties are based on data collected in Sermilik Fjord near Helheim Glacier, a major Greenland outlet glacier. The approximately two‒layer stratification of the fjord's ambient waters causes the meltwater plume at the glacier front to drive a “double cell” circulation with two distinct outflows, one at the free surface and one at the layers' interface. In summer, the discharge of surface runoff at the base of the glacier (subglacial discharge) causes the circulation to be much more vigorous and associated with a larger melt rate than in winter. The simulated “double cell” circulation is consistent, in both seasons, with observations from Sermilik Fjord. Seasonal differences are also present in the vertical structure of the melt rate, which is maximum at the base of the glacier in summer and at the layers' interface in winter. Simulated submarine melt rates are strongly sensitive to the amount of subglacial discharge, to changes in water temperature, and to the height of the layers. They are also consistent with those inferred from simplified one‒dimensional models based on the theory of buoyant plumes. Our results also indicate that to correctly represent the dynamics of the meltwater plume, care must be taken in the choice of viscosity and diffusivity values in the model.

[1] Basic constraints on the dense water formation rate and circulation resulting from cooling around an island are discussed. The domain under consideration consists of an island surrounded by a shelf, a continental slope, and a stratified ocean. Atmospheric cooling over the shelf forms a dense water that penetrates down the sloping bottom into the stratified basin. Strong azimuthal flows are generated over the sloping bottom as a result of thermal wind. Thermally direct and indirect mean overturning cells are also forced over the slope as a result of bands of convergent and divergent Reynolds stresses associated with the jets. The Coriolis force associated with the net mass flux into the downwelling region over the slope is balanced by these nonlinear terms, giving rise to a fundamentally different momentum budget than arises in semienclosed marginal seas subject to cooling. A similar momentum balance is found for cases with canyons and ridges around the island provided that the terms are considered in a coordinate system that follows the topography. Both eddy fluxes and the mean overturning cells are important for the radial heat flux, although the eddy fluxes typically dominate. The properties of the dense water formed over the shelf (temperature, diapycnal mass flux) are predicted well by application of baroclinic instability theory and simple heat and mass budgets. It is shown that each of these quantities depends only on a nondimensional number derived from environmental parameters such as the shelf depth, Coriolis parameter, offshore temperature field, and atmospheric forcing.

[1] We investigate basal melting of all Antarctic ice shelves by a circumpolar ice shelf-sea ice-ocean coupled model and estimate the total basal melting of 770–944 Gt/yr under present-day climate conditions. We present a comparison of the basal melting with previous observational and modeling estimates for each ice shelf. Heat sources for basal melting are largely different among the ice shelves. Sensitivities of the basal melting to surface air warming and to enhanced westerly winds over the Antarctic Circumpolar Current are investigated from a series of numerical experiments. In this model the total basal melting strongly depends on the surface air warming but is hardly affected by the change of westerly winds. The magnitude of the basal melting response to the warming varies widely from one ice shelf to another. The largest response is found at ice shelves in the Bellingshausen Sea, followed by those in the Eastern Weddell Sea and the Indian sector. These increases of basal melting are caused by increases of Circumpolar Deep Water and/or Antarctic Surface Water into ice shelf cavities. By contrast, basal melting of ice shelves in the Ross and Weddell Seas is insensitive to the surface air warming, because even in the warming experiments there is high sea ice production at the front of the ice shelves that keeps the water temperature to the surface freezing point. Weakening of the thermohaline circulation driven by Antarctic dense water formation under warming climate conditions is enhanced by basal melting of ice shelves.

[1] Sea ice thicknesses derived from NASA's Ice, Cloud, and Land Elevation Satellite (ICESat) altimetry data are examined using two different approaches, buoyancy and empirical equations, and at two spatial scales—ICESat footprint size (70 m diameter spot) and Advanced Microwave Scanning Radiometer (AMSR-E) pixel size (12.5 km by 12.5 km) for the Bellingshausen and Amundsen Seas of west Antarctica. Ice thickness from the empirical equation shows reasonable spatial and temporal distribution of ice thickness from 2003 to 2009. Ice thickness from the buoyancy equation, however, additionally needing snow depth information derived from the AMSR-E, shows an overestimation in terms of maximum, mean (+63% to 75%), and standard deviation while underestimation in modal thickness (−20%) as compared with those from the empirical equation approach. When ICESat snow freeboard is used as the snow depth in the buoyancy equation, i.e., the zero ice freeboard assumption, the derived ice thicknesses match well with those from the empirical equation approach, within 5% overall. The AMSR-E, therefore, may underestimate snow depth and accounts for ~95% of the ice thickness overestimation as compared with the buoyancy approach. The empirical equation derived ice thickness shows a consistent asymmetrical distribution with a long tail to high values, and seasonal median values ranging from 0.8 to 1.4 m over the 2003–2009 period that are always larger than the corresponding modal values (0.6–1.1 m) and lower than the mean values (1.0–1.6 m), with standard deviation of 0.6–1.0 m. An overall increasing trend of 0.03 m/year of mean ice thickness is found from 2003 to 2009, although statistically insignificant (p = 0.11) at the 95% confidence level. Starting from autumn, a general picture of seasonal mean, modal, and median ice thickness increases progressively from autumn to spring and decreases from spring to the following autumn, when new thin ice dominates the ice thickness distribution. The asymmetric shape of the thickness distribution reflects the key role of ice deformation processes in the evolution of the thickness distribution. The statistical properties of the thickness distribution interannually (high range of mean thickness and standard deviation) indicate the variability of deformation processes. However, spring ice volume, the product of ice mean thickness and areal extent computed for the spring maximum, shows variability year to year but is primarily dominated by ice extent variability, with no increasing or decreasing trend over this record length. The dependence of the volume on the ice extent primarily suggests that ice thickness changes have also not covaried with the ice extent losses seen over the satellite record in this region, unlike the Arctic. These properties reflect the interactive processes of ice advection, thermodynamic growth and ice deformation that all substantially influence ice mass balance in the Bellingshausen-Amundsen Seas region.

[1] We investigate the momentum balance in the surf zone, in a setting which is weakly varying in the alongshore direction. Our focus is on the role of nonlinear advective terms. Using numerical experiments, we find that advection tends to counteract alongshore variations in momentum flux, resulting in more uniform kinematics. Additionally, advection causes a shifting of the kinematic response in the direction of flow. These effects are strongest at short alongshore length scales, and/or strong alongshore-mean velocity. The length-scale dependence is investigated using spectral analysis, where the effect of advective terms is treated as a transfer function applied to the solution to the linear (advection-free) equations of motion. The transfer function is then shown to be governed by a nondimensional parameter which quantifies the relative scales of advection and bottom stress, analogous to a Reynolds Number. Hence, this parameter can be used to quantify the length scales at which advective terms, and the resulting effects described above, are important. We also introduce an approximate functional form for the transfer function, which is valid asymptotically within a restricted range of length scales.

[1] Parameterizations of near-bed sediment processes are commonly associated with the poor predictive skill of coastal sediment transport models. We implement a two-dimensional Reynolds-averaged Navier-Stokes model to directly assess these parameterizations by reproducing measurements obtained in large-scale wave flume experiments. A sediment transport model has been coupled to wave hydrodynamics and turbulence, and numerical experiments provide temporal and spatial variations of free surface, flow velocity, sediment concentration, and turbulence quantities. Model-data comparisons enable the direct assessment of how key suspension processes are represented and of the inherent variability of the sediment transport model. We focus on the different processes occurring above rippled beds versus dynamically flat beds. Numerical results show that increasing roughness alone is not sufficient to have good predictive capability above steep ripples. Some parameterization of the vortex entrainment process is necessary and a simple modification, which leads to constant sediment diffusivity above steep-rippled beds, is sufficient to obtain good predictions of wave-averaged suspended concentrations. Model-data comparisons for the turbulent kinetic energy are also presented and highlight the need to account for the effect of vortex entrainment on near-bed turbulence and transfer of momentum.

[1] Data from drifters of the surface velocity program and tropical cyclones (TCs) of the Joint Typhoon Warning Center during 1985–2009 were analyzed to demonstrate strong currents under various storm intensities such as category-4 to −5, category-2 to −3, and tropical storm to category-1 TCs in the northwestern Pacific. Current speeds over 2.0 m s⁻¹ are observed under major TCs with the strongest mean currents to the right of the storm track. This study provides the characterization of the near-surface velocity response to all recorded TCs, and agrees roughly with Geisler's theory (1970). Our observations also verify earlier modeling results of Price (1983).

[1] Time series measurements of temperature, salinity and surface meteorological parameters recorded at 8°N, 90°E in the southern central Bay of Bengal (BoB) from a Research Moored Array for African-Asian-Australian Monsoon Analysis and predication (RAMA) buoy are used to document temperature inversions and their influence on the mixed layer heat budget during the winters, defined as October to March, of 2006–2007 (W67) and 2007–2008 (W78). There is a marked difference in the frequency and amplitude of temperature inversion between these two winters, with variations much stronger in W78 compared to W67. The formation of temperature inversions is favored by the existence of thick barrier layers, which are also more prominent in W78 compared to W67. Inversions occur when heating in the barrier layer below the mixed layer by penetrative shortwave radiation is greater than heating of the mixed layer by net surface heat flux and horizontal advection. Our analysis further demonstrates that intraseasonal and year-to-year variability in the frequency and magnitude of temperature inversions during winter have substantial influence on mixed layer temperature through the modulation of vertical heat flux at the base of mixed layer.

[1] We examine the basinwide trends in sea ice circulation and drift speed and highlight the changes between 1982 and 2009 in connection to regional winds, multiyear sea ice coverage, ice export, and the thinning of the ice cover. The polarity of the Arctic Oscillation (AO) is used as a backdrop for summarizing the variance and shifts in decadal drift patterns. The 28-year circulation fields show a net strengthening of the Beaufort Gyre and the Transpolar Drift, especially during the last decade. The imprint of the arctic dipole anomaly on the mean summer circulation is evident (2001–2009) and enhances summer ice area export at the Fram Strait. Between 2001 and 2009, the large spatially averaged trends in drift speeds (winter: +23.6%/decade, summer: +17.7%/decade) are not explained by the much smaller trends in wind speeds (winter: 1.46%/decade, summer: −3.42%/decade). Notably, positive trends in drift speed are found in regions with reduced multiyear sea ice coverage. Over 90% of the Arctic Ocean has positive trends in drift speed and negative trends in multiyear sea ice coverage. The increased responsiveness of ice drift to geostrophic wind is consistent with a thinner and weaker seasonal ice cover and suggests large-scale changes in the air-ice-ocean momentum balance. The retrieved mean ocean current field from decadal-scale average ice motion captures a steady drift from Siberia to the Fram Strait, an inflow north of the Bering Strait, and a westward drift along coastal Alaska. This mean current is comparable to geostrophic currents from satellite-derived dynamic topography.

[1] The Pacific Decadal Oscillation (PDO) has been shown to have significant climatic and environmental impacts across the Pan-Pacific Basin; however, there are no records of PDO activity from the South China Sea (SCS), the largest marginal sea in the northwest Pacific Ocean. This study suggests that a series of geochemical profiles obtained from a modern coral in the northern SCS records annual PDO activity dating back to 1853. These geochemical data are significantly correlated with the PDO index, and their patterns of variation closely match those of the PDO index over the last century. The relationship between the PDO and coral geochemistry may be related to the influence of the PDO on rainfall on Hainan Island. Rainfall patterns influence the volume of terrestrial runoff, which, in turn, is a primary determinant of δ¹⁸O and Δδ¹⁸O values in coral; however, coral δ¹³C values are also influenced by the ¹³C Suess effect. The results indicate that Sr/Ca ratios in coral are affected by a combination of sea surface temperature and terrestrial runoff.

[1] Phytoplankton bloom phenology has important consequences for marine ecosystems and fisheries. Recent studies have used remotely sensed ocean color data to calculate metrics associated with the phenological cycle, such as the phytoplankton bloom initiation date, on regional and global scales. These metrics are often linked to physical or biological forcings. Most studies choose one of several common methods for calculating bloom initiation, leading to questions about whether bloom initiation dates calculated with different methods yield comparable results. Here we compare three methods for finding the date of phytoplankton bloom initiation in the North Atlantic: a biomass-based threshold method, a rate of change method, and a cumulative biomass-based threshold method. We use these methods to examine whether the onset of positive ocean-atmosphere heat fluxes coincides with subpolar bloom initiation. In several coherent locations, we find differences in the patterns of bloom initiation created by each method and differences in the synchrony between bloom initiation and positive heat fluxes, which likely indicate various physical processes at play in the study region. We also assess the effect of missing data on the chosen methods.

[1] Seawater concentrations and distributions of brominated very short-lived substances (BrVSLS), including bromoform (CHBr₃), dibromomethane (CH₂Br₂), bromodichloromethane (CHBrCl₂), chlorodibromomethane (CHClBr₂), were measured in the upper water column (5–750 m) in the eastern Pacific. Inorganic nutrient, pigment concentrations, and picoplankton cell counts were measured to determine biogeochemical factors that affect the production and distribution of these BrVSLS. Elevated concentrations of BrVSLS were observed in coastal and tropical seawater. Concentration maxima for CHBr₃, CH₂Br₂, and CHClBr₂ were observed below the mixed layer, near the subsurface chlorophyll a maxima, which suggest BrVSLS production may be related to photosynthetic biomass production. Our results also suggest that heterotrophic bacteria may also contribute to CH₂Br₂ and CHBrCl₂ production in the water column. The maximum CHBrCl₂ concentration was observed at a depth much deeper than the euphotic zone, which suggests sources other than photosynthetic biomass. Elevated CHBrCl₂ concentrations in deeper waters were coincident with elevated CHCl₃ concentrations, which may be an evidence for successive chlorine substitution of CHBr₃ in deeper and older water masses.

[1] We present a new one-dimensional parameterization of gravity drainage implemented in an all-new thermodynamic component of the Los Alamos Sea Ice Model (CICE), based on mushy layer theory. We solve a set of coupled, nonlinear equations for sea-ice temperature (enthalpy) and salinity using an implicit Jacobian-free Newton-Krylov method. Time resolved observations of gravity drainage show two modes of desalination during growth. Rapid drainage occurs in a thin region just above the ice/ocean interface, while slower drainage occurs throughout the ice. Parameterizations are designed to represent each of these modes and work simultaneously. Near the interface, desalination occurs primarily via the fast drainage, while slow drainage continues to desalinate ice above the interface. The rapid desalination is convectively driven and is parameterized based on a consideration of flow driven upward within the mush and downward in chimneys, modified by the Rayleigh number. The slow desalination is represented as a simple relaxation of bulk salinity to a value based on a critical porosity for sea-ice permeability. It is shown that these parameterizations can adequately reproduce observational data from laboratory experiments and field measurements.

[1] The attenuation of downward planar irradiance can be quantified by , the diffuse attenuation coefficient calculated from the surface to the depth where the irradiance E_d at wavelength λ falls to 10% of its surface value. Theoretical studies by Gordon (1989) and Lee et al. (2005a) suggest that can be derived from the absorption coefficient, a(λ) and the backscattering coefficient, b_b(λ), using equations incorporating either the solar zenith angle (θ_a) or the subsurface distribution function (D₀) and empirical coefficients derived by radiative transfer modeling. These results have not, however, been validated against in situ measurements. We have therefore assessed the performance of both models using measurements of a(λ), b_b(λ), and for 100 stations in UK coastal waters. Best results were obtained from the Lee et al. (2005a) model, for which over 90% of the predicted values in the 440 nm to 665 nm range were within ±0.1 m⁻¹ of those measured in situ. A strong linear relationship (R²> 0.95, mean relative difference 5.4%) was found between at 490 nm and the reciprocal of the depth of the midpoint of the euphotic zone (z_10%, PAR). This allowed (z_10%, PAR) to be predicted from measured values of a(490 nm), b_b(490 nm) and θ_a, using the Lee et al. model as an intermediate step, with an RMS error of 1.25 m over the 2.5–25.0 m range covered by our data set.

[1] This paper presents an experimental analysis of the salinity distribution, the salt balance, and the variation of the saline intrusion in comparison to the freshwater discharge in the Guadalquivir estuary, which is a mesotidal system regulated and normally subjected to extremely low river flows. In such low-flow conditions, it is positive, well-mixed, and tidally dominated. The estuary is also characterized by a nonstationary, effective longitudinal dispersion coefficient, whose probability density becomes increasingly narrower and whose mean value is higher further upstream. The tidal-averaged salt flux is controlled by the following mechanisms (in order of importance): the nontidal transport, the Stokes transport, and the tidal pumping induced by the covariance between the current and salinity. These three factors account for more than 98% of the flux variation. In high river-flow conditions, the subtidal response and recovery of the estuary to changes in the river flow is analyzed. The increase in the tidal-averaged salinity during the first 2 weeks of the post-riverflood recovery in the middle and upper sections of the estuary is found to be linear in time. During that time, the celerity of the salt intrusion front was 4cm/s. The 2 psu isohaline salt intrusion X₂ exhibits a complex dependence on the river flow Q_d, including the effects of human interventions in the estuary. Three regimes are identified for the intrusion: X₂=57.0± 2.1km for discharges of less than 20m ³/s, X₂ proportional to between 20 and 1000m ³/s, and X₂proportional to for larger discharges.

[1] A monthly, isopycnal/mixed-layer ocean climatology (MIMOC), global from 0 to 1950 dbar, is compared with other monthly ocean climatologies. All available quality-controlled profiles of temperature (T) and salinity (S) versus pressure (P) collected by conductivity-temperature-depth (CTD) instruments from the Argo Program, Ice-Tethered Profilers, and archived in the World Ocean Database are used. MIMOC provides maps of mixed layer properties (conservative temperature, Θ, absolute salinity, S_A, and maximum P) as well as maps of interior ocean properties (Θ, S_A, and P) to 1950 dbar on isopycnal surfaces. A third product merges the two onto a pressure grid spanning the upper 1950 dbar, adding more familiar potential temperature (θ) and practical salinity (S) maps. All maps are at monthly 0.5° × 0.5° resolution, spanning from 80°S to 90°N. Objective mapping routines used and described here incorporate an isobath-following component using a “Fast Marching” algorithm, as well as front-sharpening components in both the mixed layer and on interior isopycnals. Recent data are emphasized in the mapping. The goal is to compute a climatology that looks as much as possible like synoptic surveys sampled circa 2007–2011 during all phases of the seasonal cycle, minimizing transient eddy and wave signatures. MIMOC preserves a surface mixed layer, minimizes both diapycnal and isopycnal smoothing of θ-S, as well as preserves density structure in the vertical (pycnoclines and pycnostads) and the horizontal (fronts and their associated currents). It is statically stable and resolves water mass features, fronts, and currents with a high level of detail and fidelity.

[1] A high-resolution global ocean/sea ice model is used to investigate the modes of Arctic Ocean heat content variability for the period 1968–2007. A rotated empirical orthogonal function analysis is performed on the monthly mean vertically integrated heat content to investigate the mechanisms governing its spatiotemporal variations. In the model, 28% of the heat content variability is driven by the seasonal and interannual fluctuations of the atmospheric heat flux in the seasonally ice free regions. The heat flux variability associated with Atlantic Water advected through Fram Strait drives 31% of the heat content variability. Changes of temperature and circulation drive Fram Strait heat transport variability, and these two effects project on different modes and thus drive heat content variations in different parts of the Eurasian Basin. A second branch of Atlantic Water is modified in the Barents Sea and the variations of the heat flux associated with the Barents Sea water branch penetrating the deep Arctic yield heat content variations in the Eurasian Basin. The effect of the Bering Strait heat flux variations remains limited to the Chukchi Sea. Autonomous observing system may be able to capture the Arctic heat content variability. Sea surface temperature satellite observations combined with temperature profiles of the top 800 m in the deep Arctic covered by sea ice are sufficient to capture most of the variability signal. The results emphasize the crucial need for measurements in the Eurasian Basin.

[1] Satellite remote sensing offers one of the best spatial and temporal observational approaches. However, well-validated satellite imagery has remained elusive for Lake Superior. Lake Superior's optical properties are highly influenced by colored dissolved organic matter (CDOM), which has hindered the retrieval of chlorophyll concentration through band-ratio algorithms. This study evaluated seven existing inversion algorithms. The top-performing inversion algorithm was tuned to a Lake Superior optical data set and applied to satellite imagery. The retrieval of chlorophyll concentration via inversion algorithms was not possible due to errors in derived CDOM absorption being greater than phytoplankton absorption values and the very small contribution of phytoplankton absorption to the overall absorption budget. However, the retrieval of absorption due to CDOM from satellite imagery was encouraging. To ensure that the best satellite remotely sensed reflectance estimates were used in the retrieval of absorption due to CDOM, several atmospheric correction schemes were evaluated. The absorption due to CDOM was greatest in the western arm of Lake Superior and near river mouths and decreased with distance offshore. The absorption due to CDOM had a bimodal distribution over the annual cycle with the greatest peak in fall and a smaller peak in spring.

[1] Direct numerical simulation of a wind-driven gravity-capillary wave and the underlying turbulent flow is conducted to identify the characteristic signatures of various surface parameters, including temperature, gas flux, velocities, and roughness, and to reveal the impacts of the nonbreaking surface waves on these surface flow and tracer parameters. Three characteristic surface signatures and the corresponding flow processes are identified: the carrier gravity wave, the parasitic capillary wavelets, and the elongated streaks. The elongated streaks are induced by both the coherent streamwise vortices formed within the turbulent shear layer and the Langmuir circulations arising from nonlinear interaction between the carrier gravity wave and the drift current. All three surface signatures can be observed in the distributions of various quantities, although some are more apparent than the others. Image-processing techniques, employing empirical mode decomposition and phase averaging, are developed to decompose the distinct signatures thus to quantify the contributions by the responsible flow processes. It is found that elongated streaks prevail the distribution of surface temperature and gas flux, indicating that Langmuir cells and the coherent eddies contribute to the major interfacial heat and gas transport. These eddies also induced strong cross-stream velocity divergence at the water surface, which exhibits resemblant elongated distribution as that of gas flux (correlation coefficient ≈ 0.6). High correlation between the surface distributions of temperature and gas flux is observed (correlation coefficient ≈ 0.8 to 0.9), suggesting that the spatial and temporal distribution of surface temperature is a good proxy tracer of interfacial gas transfer.

[1] In shallow, fetch-limited estuaries, variations in current and wave energy promote heterogeneous surface water-groundwater mixing (benthic exchange), which influences biogeochemical activity. Here, we characterize heterogeneity in benthic exchange within the subtidal zone of the Delaware Inland Bays by linking hydrodynamic circulation models with mathematical solutions for benthic exchange forced by current-bedform interactions, tides, and waves. Benthic fluxes oscillate over tidal cycles as fluctuating water depths alter fluid interactions with the bed. Maximum current-driven fluxes (~1–10 cm/d) occur in channels with strong tidal currents. Maximum wave-driven fluxes (~1–10 cm/d) occur in downwind shoals. During high-energy storms, simulated wave pumping rates increase by orders of magnitude, demonstrating the importance of storms in solute transfer through the benthic layer. Under moderate wind conditions (~5 m/s), integrated benthic exchange rates due to wave, current, and tidal pumping are each ~1–10 m³/s, on the order of fluid contributions from runoff and fresh groundwater discharge to the estuary. Benthic exchange is thus a significant and dynamic component of an estuary's fluid budget that may influence estuarine geochemistry and ecology.

[1] A three-dimensional eddy census data set was obtained from a global ocean simulation with one-tenth degree resolution and a duration of 7 years. The census includes 6.7 million eddies in daily data, which comprise 152,000 eddies tracked over their lifetimes, using a minimum lifetime cutoff of 28 days. Variables of interest include eddy diameter, thickness (vertical extent), minimum and maximum depth, location, rotational direction, lifetime, and translational speed. Distributions of these traits show a predominance of small, thin, short-lived, and slow eddies. Still, a significant number of eddies possess traits at the opposite extreme; thousands of eddies larger than 200 km in diameter appeared in daily data each year. A tracking algorithm found hundreds of eddies with lifetimes longer than 200 days. A third of the eddies are at least 1000 m tall and many penetrate the full depth of the water column. The Antarctic Circumpolar Current contains the thickest and highest density of eddies. Thick eddies are also common in the Gulf Stream, Kuroshio Current, and Agulhas ring pathway. The great majority of eddies extend all the way to the surface, confirming that eddy censuses from surface observations are a good proxy for the full-depth ocean. Correlations between variables show that larger-diameter eddies tend to be thicker and longer lived than small eddies.

[1] Layering of ocean velocity “fine structure” has been coherently observed across the entire extent of a Gulf Stream warm-core ring using a shipboard acoustic Doppler current profiler system in September 2009 and independently sampled as the ring transited a moored array. Lines of constant velocity phase generally followed isopycnals as they deepened within the ring center. We also observed a clear separation of the vertical structure of the flows associated with the ring (of order 0.5 m/s) with the shorter (200 m) and less energetic (~0.2 m/s) flows of the velocity fine structure, which was further observed to rotate clockwise with increasing depth, consistent with downward propagating near-inertial waves (NIWs). Observations are consistent with a ring-scale NIW packet, probably wind forced, that shows enhanced NIW energy within the sloping pycnocline at depths of 300–700 m. Evidence of wind-forced NIWs within anticylonic eddies in a numerical simulation shows some similar features to our observations, which we try to understand physically with basic WKB-type wave/current dynamics along the lines of previously published work and a new calculation of NIW trapping within an isolated, baroclinic vortex.

[1] We constructed annual cycles of National Centers for Environmental Prediction air-sea fluxes and temporal oceanic heat content change from Seward Line hydrographic surveys to quantify the different contributions to the oceanic heat budget within the Alaska Coastal Current (ACC) on the northern Gulf of Alaska shelf. The deficit between air-sea fluxes and the temporal change in oceanic heat content throughout the cooling season (October–April) varies from ~40 to 110 W m⁻² and is balanced by ocean heat flux convergence. Cross-shelf heat flux convergence is insignificant on annual average, and the nearshore heat budget is likely entirely balanced by the ACC, which resupplies ~15%–50% of the heat removed by air-sea fluxes during the cooling season. Furthermore, we estimated spatial heat flux gradients and conclude that air-sea fluxes increase from east to west and from offshore to onshore. The cross-shore gradients are governed by wind speed gradients, likely due to ageostrophic nearshore wind events during the cooling season, while the along-shelf heat flux gradients are governed by the occurrence of low-pressure systems in the northern GOA that result in cold northerly winds over the northwestern GOA. These results underline the ACC's role as the dominant oceanic heat source to the northern GOA shelf and further imply an increased cooling rate of the ACC west of the Seward Line. Furthermore, our analysis showed that nearshore regions, particularly waters in the ACC, are subjected to stronger winter cooling than the middle and outer shelves.

[1] The Dead Sea, located in the rift valley between Jordan and Israel, is a hypersaline lake, resulting in unique biogeochemistry and optical properties. In the spring of 2004 we conducted two days of physical and optical measurements in the lake. Because of the significant effect of dissolved salts on the optical properties of water, our analysis required a novel processing approach to obtain dissolved and total inherent optical properties from the measurements. In addition, we show that the lake's salinity can be estimated from measurements of hyper-spectral absorption or attenuation spectra in the red and infrared parts of the spectrum, using published values of specific absorption of dissolved NaCl, despite the fact that the lake's salt chemistry is complex. In situ observations demonstrated that the lake has a two-layer structure with a warm and more turbid layer at the top 20–30 m and a clearer colder layer below. Both the particulate and dissolved absorption are well approximated by exponentially decreasing functions with the spectral slope of the particulate absorption about half that of the dissolved fraction and consistent with other aquatic environments. Both have relatively low and similar magnitudes in the blue (O(0.15 m^–1)). Mean particle size was observed to increase with depth, consistent with precipitating salt crystals (observed in past campaigns) shown here to play a major role in the lake's optical properties.

[1] The ability of the models contributing to the fifth Coupled Models Intercomparison Project (CMIP5) to represent the Southern Ocean hydrological properties and its overturning is investigated in a water mass framework. Models have a consistent warm and light bias spread over the entire water column. The greatest bias occurs in the ventilated layers, which are volumetrically dominated by mode and intermediate layers. The ventilated layers have been observed to have a strong fingerprint of climate change and to impact climate by sequestrating a significant amount of heat and carbon dioxide. The mode water layer is poorly represented in the models and both mode and intermediate water have a significant fresh bias. Under increased radiative forcing, models simulate a warming and lightening of the entire water column, which is again greatest in the ventilated layers, highlighting the importance of these layers for propagating the climate signal into the deep ocean. While the intensity of the water mass overturning is relatively consistent between models, when compared to observation-based reconstructions, they exhibit a slightly larger rate of overturning at shallow to intermediate depths, and a slower rate of overturning deeper in the water column. Under increased radiative forcing, atmospheric fluxes increase the rate of simulated upper cell overturning, but this increase is counterbalanced by diapycnal fluxes, including mixed-layer horizontal mixing, and mostly vanishes.

[1] The development of the deep Southern Ocean winter mixed layer in the climate models participating in the fifth Coupled Models Intercomparison Project (CMIP5) is assessed. The deep winter convection regions are key to the ventilation of the ocean interior, and changes in their properties have been related to climate change in numerous studies. Their simulation in climate models is consistently too shallow, too light and shifted equatorward compared to observations. The shallow bias is mostly associated with an excess annual-mean freshwater input at the sea surface that over-stratifies the surface layer and prevents deep convection from developing in winter. In contrast, modeled future changes are mostly associated with a reduced heat loss in winter that leads to even shallower winter mixed layers. The mixed layers shallow most strongly in the Pacific basin under future scenarios, and this is associated with a reduction of the ventilated water volume in the interior. We find a strong state dependency for the future change of mixed-layer depth, with larger future shallowing being simulated by models with larger historical mixed-layer depths. Given that most models are biased shallow, we expect that most CMIP5 climate models might underestimate the future winter mixed-layer shallowing, with important implications for the sequestration of heat, and gases such as carbon dioxide, and therefore for climate.

[1] This paper examines the net transport through a multiple-inlet bay under a combined force of strong wind and tide, with observations and a model experiment. The observations were made in central Georgia in Sapelo and Altamaha Sounds between 13 and 17 September 2000. Wind was weak in the beginning of the survey. An air pressure trough (as a weak cold front) passed the area on 16 September, when the wind changed to the northeast and increased in magnitude. This front was associated with a midlatitude cyclone in the New England area. This weather event with an episode of strong northeasterly winds prompted a numerical model experiment on an idealized three-inlet bay, with a set of nonlinear 2-D shallow water equations on an f plane, which provides some insight to the wind-driven circulation under the presence of tidal forcing. It is found that tidally induced currents are small compared to wind-induced flows. When the wind direction is not perpendicular to the alignment of the three inlets, the net outward flow tends to occur at the inlet farther away in the downwind direction. This is associated with a net inward transport in the inlet opposite of the downwind direction. As a result, the middle inlet has the minimum of the net flow. When the wind is perpendicular to the barrier islands, and if the three inlets have different maximum depth values, the deeper inlet tends to have a net flow against the wind, while the shallower inlet tends to have a net flow in the direction of the wind. Offshore (onshore) currents may develop outside of the inlet with outward (inward) flow, as an effect of fluxes through the inlets on the coastal ocean.

[1] The magnitude of geothermal heating in the East/Japan Sea is about 100 mW/m², twice that of a typical abyssal plain. In addition, bottom stratification in the East/Japan Sea is much weaker than that typical of the open ocean. Thus, geothermal heating could have more prominent effects in the East/Japan Sea than in the open ocean, and we tested this hypothesis via numerical modeling. With less than 100 mW/m² bottom heat flux, we were able to reproduce bottom mixed layers that are thicker than ~1000 m as observed. Previously, no numerical model has been successful in reproducing such bottom mixed layers. Geothermal heating intensifies the bottom flows but the simulated flows are not as strong as the observed ones. Over the northern part of the East/Japan Sea, reduction in deep stratification strengthens deep water mass formation, intensifying cyclonic circulations located over this area, so the effects of the heating extend to the surface. As the cyclonic circulation becomes stronger, the water at the center of the gyre is trapped and more exposed to cold air, so it becomes cooler, and colder deep water is produced. When the geothermal heating is strong enough, the surface cooling effect dominates the bottom heating and the deep layer becomes cooler showing that the nonlinear effects of geothermal heating are far reaching. Thus, to account for the observed dynamics, the full three-dimensional circulation at the basin scale is needed.

[1] Mesoscale eddy properties in the northwestern subtropical Pacific Ocean are investigated by analyzing 22,567 cyclonic eddies (CEs) and 26,365 anticyclonic eddies (AEs) detected from 19 year altimetric sea level records. Eddy occurrence frequency and kinetic energy are prevailingly high in the Subtropical Countercurrent zonal band between 19°N and 26°N and further elevated near the Luzon-Taiwan coast. A general superiority of AEs is observed at most latitudes except between 19°N and 22°N, where the CE number is larger. The modal radius and mean lifespan of the CEs (AEs) are 134 km and 11.2 weeks (121 km and 10.9 weeks), respectively. After generation, most eddies propagate westward with a mean speed of 7.2 cm s⁻¹ and deflect northward following the Kuroshio along the Luzon-Taiwan coast. Three-dimensional eddy structures are further explored with composite eddy images in five subregions constructed by surfacing Argo temperature/salinity data into altimeter-detected eddy areas. Due to the existence of mode waters in the main thermocline, eddy-induced temperature anomaly exhibits a double-core vertical structure which is especially evident in CE images. Because of the vertical water mass distribution, salinity anomaly features a sandwich-like pattern which is more evident in AE images. Also revealed is the significant structure difference in these five subregions. Eddies are greatly intensified as they approach the western boundary, inducing larger temperature and salinity anomalies and influencing deeper ocean. Along the Luzon-Taiwan coast, AEs are preferentially strengthened by the northward background flow.

[1] We have tested a dissolved oxygen (DO) transport model based on large-eddy simulation (LES) of a transitional oscillatory flow observed in the bottom boundary layer of Lake Alpnach, Switzerland. The transition from a quasi-laminar to a fully turbulent state makes this flow difficult to study with a Reynolds-averaged Navier-Stokes equation (RANSE) model. By resolving the full range of governing transport processes, LES provides a reliable prediction of the sediment-oxygen uptake (SOU). The model biogeochemical and flow parameters have been calibrated against DO and velocity measurements from published in situ data at the earliest phase available in the cycle. The fully developed flow thus obtained is used as an initial condition for the imposed oscillatory forcing. Numerical predictions show that transport in the outer layer is in equilibrium with the main current throughout most of the cycle and that nonequilibrium effects are limited to the diffusive sublayer response to the external forcing. During flow deceleration, the concentration boundary layer slowly expands as turbulence decays; later, during re-transition, mixing is restored by rapid and intense turbulent production events enhancing the SOU with a well-defined time lag. An algebraic model for the SOU is proposed for eventual inclusion in RANSE biogeochemical management-type models developed based on parameterizations used in turbulent mass transfer and with the support of published numerical data and the present simulation. The only input parameters required are the sediment oxidation rate, bulk temperature and DO concentration, and friction velocity.

[1] This study deals with observations and simulations of the evolution of coastal polynias focusing on the Ronne Polynia. We compare differences in polynia extent and ice drift patterns derived from satellite radar images and from simulations with the Finite Element Sea Ice Ocean Model, employing three atmospheric forcing data sets that differ in spatial and temporal resolution. Two polynia events are analyzed, one from austral summer and one from late fall 2008. The open water area in the polynia is of similar size in the satellite images and in the model simulations, but its temporal evolution differs depending on katabatic winds being resolved in the atmospheric forcing data sets. Modeled ice drift is slower than the observed and reveals greater turning angles relative to the wind direction in many cases. For the summer event, model results obtained with high-resolution forcing are closer to the drift field derived from radar imagery than those from coarse resolution forcing. For the late fall event, none of the forcing data yields outstanding results. Our study demonstrates that a dense (1–3 km) model grid and atmospheric forcing provided at high spatial resolution ( < 50 km) are critical to correctly simulate coastal polynias with a coupled sea-ice ocean model.

[1] This study develops a strategy for tracing a target water mass, and applies it to analyzing the pathway of the North Pacific Intermediate Water (NPIW) from the subarctic gyre to the northwestern part of the subtropical gyre south of Japan in a simulation of an ocean general circulation model. This strategy estimates the pathway of the water mass that travels from an origin to a destination area during a specific period using a conservation property concerning tangent linear and adjoint models. In our analysis, a large fraction of the low salinity origin water mass of NPIW initially comes from the Okhotsk or Bering Sea, flows through the southeastern side of the Kuril Islands, and is advected to the Mixed Water Region (MWR) by the Oyashio current. It then enters the Kuroshio Extension (KE) at the first KE ridge, and is advected eastward by the KE current. However, it deviates southward from the KE axis around 158°E over the Shatsky Rise, or around 170°E on the western side of the Emperor Seamount Chain, and enters the subtropical gyre. It is finally transported westward by the recirculation flow. This pathway corresponds well to the shortcut route of NPIW from MWR to the region south of Japan inferred from analysis of the long-term freshening trend of NPIW observation. Copyright © 2013 John Wiley & Sons, Ltd.

[1] This study considered cross-frontal exchange as a possible mechanism for the observed along-front freshening and cooling between the 27.0 and 27.3 kg m^− 3 isopycnals north of the Subantarctic Front (SAF) in the southeast Pacific Ocean. This isopycnal range, which includes the densest Subantarctic Mode Water (SAMW) formed in this region, is mostly below the mixed layer, and so experiences little direct air-sea forcing. Data from two cruises in the southeast Pacific were examined for evidence of cross-frontal exchange; numerous eddies and intrusions containing Polar Frontal Zone (PFZ) water were observed north of the SAF, as well as a fresh surface layer during the summer cruise that was likely due to Ekman transport. These features penetrated north of the SAF, even though the potential vorticity structure of the SAF should have acted as a barrier to exchange. An optimum multiparameter (OMP) analysis incorporating a range of observed properties was used to estimate the cumulative cross-frontal exchange. The OMP analysis revealed an along-front increase in PFZ water fractional content in the region north of the SAF between the 27.1 and 27.3 kg m^− 3 isopycnals; the increase was approximately 0.13 for every 15° of longitude. Between the 27.0 and 27.1 kg m^− 3 isopycnals, the increase was approximately 0.15 for every 15° of longitude. A simple bulk calculation revealed that this magnitude of cross-frontal exchange could have caused the downstream evolution of SAMW temperature and salinity properties observed by Argo profiling floats.

[1] The bottom boundary layer above sloping topography can be highly turbulent, even in deep seas. This is demonstrated here using high-resolution 1-Hz sampling temperature sensors that were moored for 5 months every 0.5 m between 6.5 and 61 m above a 572 m deep seafloor promontory on the continental slope off Valencia, Spain. Using these data, turbulence parameters have been estimated. With time and in the vertical, values vary over four orders of magnitude. They have a dominant local inertial period which is modulated by an about 11 day periodicity associated with variations in a baroclinic unstable boundary current. When this current is strong and Eastward, the upslope phase of inertial wave generates convective turbulence which reaches closest to the bottom and therefore can effect sediment dispersal. In late winter, equally strong shear-induced turbulence in 50 m high Kelvin-Helmholtz (K-H) overturns is forced by 0.2 m s⁻¹ off/downslope motions, which are preceded by periods of warming of a few 0.01°C before the cooler near-bottom water is suddenly flushed over the promontory into the basin. Such anomalously large K-H overturns occurred 6 times in the investigated winter period.

[1] In situ monitoring of deep water formation in the Labrador Sea is severely hampered by the harsh winter conditions in this area. Furthermore, the ongoing monitoring programs do not cover the entire Labrador Sea and are often summer observations. The network of satellite altimeters does not suffer from these limitations and could therefore give valuable additional information. Altimeters can in theory detect deep water formation, because the water column becomes denser during convection and therefore the sea surface becomes lower. This signal is small compared to variability in sea surface height induced by other sources, but when properly filtered and appropriately averaged in time and space, all four winters with Labrador Sea Water formation or renewal in the 1994–2009 period (1994, 1995, 2000, and 2008) have a clear large negative anomaly. The magnitude of this anomaly compares favorably with the range predicted by theory and in situ data analysis. Out of 16 winters, only one winter (2006) would be falsely identified as a deep convection winter based on its sea surface height anomaly signal, while the method did not miss a single deep convection winter. For most deep‒water‒formation winters even the spatial structure of the mixed layer depth distribution can be inferred.

[1] An anticyclonic eddy in the Balearic Sea (western Mediterranean) was described using data from a mooring line deployed at the northern slope of Mallorca Island at about 900 m deep. Its surface signature was investigated using sea surface height and sea surface temperature images. The eddy, which lasted around 1 month, modified the thermohaline characteristics and the currents of the entire water column. Levantine Intermediate Waters, usually resident in the region, were displaced by colder and fresher Western Mediterranean Intermediate Waters associated with the eddy. Along-slope main currents (toward NE) were completely reversed at 500 m and significantly deviated at 900 m. Interestingly, near-bottom velocities were found to be systematically larger than those at intermediate depths. Furthermore, during the eddy, velocities reached values up to 26 cm/s at the bottom, 5 times larger than the bottom average speed. The recurrence of the phenomenon was explored with an eddy detection tool applied to satellite observations. Results indicated that anticyclonic eddies are common structures in the Balearic Current.

[1] The velocity structure of the Malvinas Current is described based on the analysis of high-resolution hydrographic data and direct current observations. The data show that though the current width exceeds 150 km, the flow is concentrated in two relatively narrow (~10–20 km) jets. Within these cores, the direct observations indicate surface velocities exceeding 0.5 m.s⁻¹. Surface drifter, satellite-derived mean dynamic topography, and sea surface temperature data suggest that the high-velocity jets are also ubiquitous features of the time mean circulation. Both jets appear to be continuous features extending more than 900 km along the western slope of the Argentine Basin. These jets closely follow the 200 and 1400 m isobaths. Additional high-velocity cores are apparent in direct current measurements and hydrographic observations, but these features are weaker and not continuous along the slope. Though the Malvinas Current transport is mostly barotropic, baroclinic jets are also identified in relative geostrophic velocity sections. The baroclinic jets are colocated with the barotropic jets. Our results suggest that the main Malvinas Current core is located over a relatively flat portion of the bottom, referred to as the Perito Moreno terrace. This observation is in agreement with recent seismic and geological evidence suggesting that in geological time scales the Malvinas Current played a key role in the configuration of the bottom sediments over the western slope of the Argentine Basin.

[1] A new equation was developed to relate the size and settling velocity of particulate matter commonly recurring in aqueous ecosystems. This equation explicitly balanced the gravitational, buoyancy, viscous, and inertial forces as in Rubey () but was amended to describe in one instance both individual particles and granular aggregates with an internal fractal architecture. This approach allowed for an algebraic solution of the settling velocity, thus overcoming earlier approaches that required iterative numerical solutions. The equation was tested with mineral, biomineral, and biological suspended particles and granular aggregates from 52 existing experimental data sets, and resulted in average correlation coefficients R between 71% and 93.9%, and normilized residuals between 14.3% and 24.8% over Reynolds numbers ranging within 10⁻⁷ and 10². Accuracy of these results was generally better than for the Stokes' law, the Stokes' law modified with the Schiller-Naumann drag coefficient, and Rubey's equation. Estimated parameters ranged within observed ones, thus suggesting that the equation was robust. An analysis of the drag showed that inertial force was negligible only for biological cells (isolated cysts), whereas it contributed by not less than 5% to the drag on large mineral particles and up to 20% for biomineral and biological aggregates. Finally, a correlation was found between the organic matter content and fractal properties of granular aggregates, which were described by empirical equations proposed here for the first time. The hypothesis that the settling velocity is a function of linear and nonlinear drag, and is ultimately determined by physical characteristics as much as biological composition and internal aggregate geometry, is supported here by quantitative analyses.