Quantifying nutrient supply to the Southern Ocean



[1] Using nutrient concentrations on neutral density surfaces in the Southern Ocean and climatological winds, both of which are fairly well constrained, we have estimated the long-term average of net export from the upper ocean (i.e., the net supply of nutrients in Circumpolar Deep Water and Antarctic Intermediate Water to the surface layer) over the whole of the Southern Ocean south of the wind stress maximum at an average latitude of 50°S. We calculate net new production (equivalent to export production) values of 51 ± 3 Tmol Si yr−1 and 14 ± 3 Tmol N yr−1. The latter is equivalent to 1.1 ± 0.2 Pg C yr−1 when scaled with the Redfield ratio of C to N. These values are in good agreement with recent observational and modeling estimates and are reasonably in line with inverse model calculations. Interpolating the high-quality data from meridional World Ocean Circulation Experiment sections onto the neutral density surfaces revealed remarkable constancy of nutrient concentrations at all longitudes and depths below about 340 ± 100 m, which we call the surface-influenced depth. This indicates that isopycnic stirring by eddies in the Southern Ocean is efficient at homogenizing nutrient concentrations and removing any signature of remineralization. The large depth of the surface-influenced depth, over twice the winter mixed layer depth, also indicates that eddies must be responsible for transferring nutrient deficits resulting from drawdown in the surface layer across the pycnocline to several hundred meters deep.

1. Introduction

[2] The Southern Ocean plays a major role in the global circulation of nutrients. High values of orthosilicic acid (“silicate”) and nitrate are found particularly in the deep Pacific Ocean, where remineralization of nitrate and dissolution of biogenic silica have gradually increased the inorganic values of these nutrients in very “old” Deep Water masses which have not been in the surface (atmospherically affected) layer of the oceans for decades or centuries. The presence of the Antarctic Circumpolar Current (ACC) requires isopycnals (here we shall use neutral density surfaces [Jackett and McDougall, 1997]) to rise southward across the ACC [Pollard et al., 2002], and divergence of the Ekman flux south of the wind stress maximum [Sloyan and Rintoul, 2001] leads to upwelling of properties along neutral density surfaces which outcrop south of the Polar Front. Thus the Southern Ocean is a major area where nutrients in the surface layers can be replenished by the upwelling of Circumpolar Deep Water (CDW) [Callahan, 1972]. Indeed, it has been argued recently [Sarmiento et al., 2004] that the Southern Ocean nutrient supply can account for up to three-quarters of the biological production north of 30°S.

[3] Numerous estimates of nutrient supply to the surface layer south of the Polar Front have been made, mostly by extrapolation in space and time of in situ measurements [DeMaster, 2002; Nelson et al., 2002; Pondaven et al., 2000; Tréguer and Jacques, 1992; Tréguer and van Bennekom, 1991], but also by modeling [Gnanadesikan, 1999; Schlitzer, 2002a]. Here we present a different and potentially more robust approach, calculating the nutrient supply as the product of upwelling rates and dissolved nutrient concentrations.

2. Methods

[4] Depth, temperature, salinity, and nutrients were extracted from the Ocean Data View (ODV) [Schlitzer, 2002b] database for 11 meridional sections occupied as part of the World Ocean Circulation Experiment (WOCE) and more or less evenly distributed around the Southern Ocean (Figure 1). Neutral density [Jackett and McDougall, 1997] was calculated and all parameters were interpolated onto neutral density surfaces. On each section and on each neutral density the nutrient values were remarkably constant below a few hundred meters. The mean values and the depth at which the nutrients begin to diminish toward the surface are shown in Table 1.

Figure 1.

All conductivity-temperature-depth (CTD) stations from the Ocean Data View (ODV) database (Schlitzer, 2000, Ocean Data View, available at http://www.awi-bremerhaven.de/GEO/ODV) used in creating Southern Ocean property maps are shown as dots. The 11 World Ocean Circulation Experiment (WOCE) sections used in this study are marked as bold lines together with their WOCE designation (P11–19, AR1, A12, A23, and I6–9).

Table 1. Silicate and Nitrate Below the Surface-Influenced Deptha
WOCE LineLongitudeSurface-Influenced DepthNitrateSilicate
  • a

    Mean values in μmol kg−1 for each World Ocean Circulation Experiment section and each neutral density surface (27.4 to 28.0 kg m−3) along with the depth (surface-influenced depth, m) at which nutrient values begin to decrease toward the surface.

P15A170W328309294177 30.033.735.132.627.349.469.988.9
P17S135W453250   30.233.5  29.847.7  
P18S103W340341   29.532.4  23.147.9  
P19S88W488404462 30.452.273.7 
AR1S69W545531297  30.033.935.1 27.753.174.5 
A23S35W553278252422 30.232.834.232.027.551.974.088.7
A12S0E406320223298 30.732.734.232.725.544.374.394.2
I06S30E414296272261 30.534.234.633.627.454.876.299.4
I08S90E519385285272 30.434.034.832.430.050.671.186.8
I09S115E426240162241 30.332.534.631.730.347.169.383.4
P11S155E244247275276 29.833.135.233.728.
Average 42932728027833630.1533.2734.7032.6727.949.772.289.3
Std dev 978680741040.330.630.380.752.

[5] Temperature and salinity were extracted from the Levitus 1° × 1° database (Levitus, 1994, World Ocean Atlas 1994, available at http://ingrid.ldeo.columbia.edu/SOURCES/.LEVITUS94/) at a depth of 100 m for winter (July to September) and neutral density was calculated. The circumpolar paths of particular neutral density values (26.8, 27.0, 27.2, 27.4, 27.6, 27.8, and 27.9 kg m−3) were calculated. Winter values were used to be most representative of the geographical positions of the neutral density surfaces at the base of the seasonal surface layer.

[6] Wind climatologies derived from in situ observations undersample the Southern Ocean because of lack of data, so tend to underestimate wind stress there because of extrapolation of lower wind stress values from farther north [Josey et al., 2002]. We therefore used the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis climatology [Kallberg, 1997] which is very similar in latitude dependence to the National Centers for Environmental Prediction (NCEP) climatology [Josey et al., 2002]. Annual means are required to give the temporally integrated effect of the Ekman transport. We therefore extracted annual mean wind stress vectors from the ECMWF wind climatology along the neutral density pathways, taking the component of the stress vector parallel to the path. Integrating along each path allowed the Ekman flux perpendicular to the path to be calculated. Table 2 lists mean silicate and nitrate concentrations averaged around the Southern Ocean on each neutral density surface (from Table 1), the mean latitude of each surface and the Ekman flux across each surface.

Table 2. Mean Nutrient Concentrations on Neutral Density Surfaces, Mean Latitude of Neutral Density Surfaces, and Mean Ekman Flux Across Neutral Density Surfacesa
Neutral Density, kg m−3Mean Silicate, μmol kg−1Mean Nitrate, μmol kg−1Mean Latitude, °SEkman Flux, Svb
  • a

    Nutrient concentrations are averaged around the Southern Ocean at all depths beneath the surface-influenced depth (Table 1). Mean latitude and Ekman flux are calculated for winter neutral density surfaces (July to September) at a depth of 100 m (Levitus, 1994, World Ocean Atlas 1994, available at http://ingrid.ldeo.columbia.edu/SOURCES/.LEVITUS94/).

  • b

    Sv = sverdrup; 1 sverdrup = 106 m3 s−1.

[7] Multiplying the Ekman flux divergence between neutral density surfaces by the mean nutrient values on those surfaces, summing over all divergent pairs of neutral densities, and subtracting nutrient flux out of the Southern Ocean across the Polar Front (Table 3) then gives the net supply of nutrients to the surface layer of the Southern Ocean. In Table 3 we have assumed that the residual nutrient leaving the Southern Ocean across the Polar Front is 5 μmol kg−1 of silicate and 20 μmol kg−1 of nitrate. The effect of changes in these residual assumptions on the nutrient supply is discussed in section 3.3.

Table 3. Net Supply of Nutrients to the Surface Layer Between Neutral Density Surfaces 27.0 and 27.9 kg m−3
Water MassaNeutral Density Layer, kg m−3Ekman Divergence, SvSilicate-5, μmol kg−1Nitrate-20, μmol kg−1Silicate Supply, Tmol Si yr−1Nitrate Supply, Tmol N yr−1
  • a

    SAMW is Subantarctic Mode Water; AAIW is Antarctic Intermediate Water; and CDW is Circumpolar Deep Water.

  • b

    The equivalent carbon export when converted using Redfield ratios is 1.1 Pg C yr−1.

Total    50.813.7b

3. Results

3.1. Nutrient Distributions

[8] An example of the meridional and depth distribution of nutrients is illustrated in Figure 2, along the WOCE line P15 at 170°W in the Pacific Ocean (Figure 1). Nutrient concentrations are highest at depths of 2000–4000 m in the low-latitude South Pacific, from where they rise in Circumpolar Deep Water (CDW) centered on the neutral density surface 27.8 kg m−3 across the ACC [Pollard et al., 2002] to outcrop south of the Polar Front. CDW, easily recognized by its high nutrients and low oxygen values, is the source of nutrients to the Southern Ocean [Callahan, 1972], as is graphically shown by mapping nitrate on the 27.8 kg m−3 neutral density surface (Figure 3). Nitrate concentrations over 35 μmol kg−1 are confined to the deep Pacific and Indian Oceans, but can be seen rising southward along the west coast of South America, to be transported eastward across the South Atlantic sector of the Southern Ocean and onward by the ACC.

Figure 2.

Sections of (a) nitrate and (b) silicate along 170°W are shown for the WOCE P15 section. Dots show all data points. Black lines are nutrient contours. These contours are at 2, 5, then in steps of 2.5 to 30, then in steps of 1 to 36 μmol kg−1 for nitrate and at 2, 5, then in steps of 5 to 25, then in steps of 25 to 125 μmol kg−1 for silicate. White lines are superimposed neutral density contours.

Figure 3.

Nitrate is mapped on the 27.8 neutral density surface.

[9] Let us first examine by how much nutrient concentrations vary on neutral density surfaces. For each conductivity-temperature-depth (CTD) profile along WOCE section P15 the oxygen and nutrient concentrations interpolated onto 27.4 kg m−3 are plotted on the side and bottom panels of Figure 4. For nitrate in particular, below 328 m (dashed line) the concentrations are remarkably constant (29.7–30.7 μmol kg−1) down to at least 1000 m and everywhere north of 60°S. Above 328 m, nitrate values drop to below 27 μmol kg−1. The rise in oxygen from 250 to 300 μmol kg−1 that accompanies this nitrate decrease shows that the decrease is the result of biological uptake of nutrients in the relatively recently ventilated surface layer. Just below this recently ventilated layer, between 56 and 60°S (Figure 4c), in the depth range 330 to 540 m, six out of seven values of nitrate lie between 29.6 and 30.0 μmol kg−1 with oxygen between 243 and 254 μmol kg−1. This indicates a water mass that has been influenced by surface drawdown at some time in the past more recent than all deeper (farther north) waters on the neutral density surface, whose nitrate and oxygen values are greater than 30.3 μmol kg−1 and less than 232 μmol kg−1 respectively.

Figure 4.

(a) Potential temperature is contoured for the upper 1000 m of the WOCE P15 section south of 45°S with neutral density contours overlaid (white lines). Nitrate, silicate, and oxygen along the neutral density line (27.4) are plotted against (b) depth and (c) latitude. In 4c, potential temperature and salinity are also plotted.

[10] Above 328 m, silicate values also fall, from 26.9 μmol kg−1 to under 23.1 μmol kg−1, but they rise again farther south. This rise is caused by winter mixing across neutral density surfaces (>27.4 kg m−3) that have much larger silicate values (Table 1). Below 328 m, silicate concentrations rise gradually with depth. Silicate lies in the range 26.6–26.9 μmol kg−1 between 56 and 60°S, jumps to 29.2–29.9 μmol kg−1 between 54 and 56°S (depths 530–630 m), then jumps to over 31.8 μmol kg−1 north of 54°S (and deeper than 800 m). This steplike structure indicates, as for nitrate, the influence of water masses with slightly different properties. Nevertheless, the variation of silicate below 328 m along the neutral density surface 27.4 kg m−3 is much less than the variation between neutral density surfaces. On the adjacent tabulated neutral density surface 27.6 kg m−3, for example, silicate is greater than 49 μmol kg−1 (Table 1).

[11] Structure similar to that just described is apparent along every neutral density surface for every section, so the nutrient values in Table 1 are those just below the tabulated depth above which oxygen rises and nutrients fall. We shall call this the surface-influenced depth (SID). Several striking conclusions can be drawn. First, below the SID there is remarkably little variation in nutrient concentrations on a given neutral density surface right around the Southern Ocean and over all depths below the SID. Variations along a neutral density surface (measured by the standard deviations in Table 1) are an order of magnitude less than variations between adjacent tabulated neutral density surfaces 0.2 kg m−3 apart, equivalent to a few hundred meters apart in the vertical (Figure 4a). We infer from this lack of variation that advection right around Antarctica and isopycnic stirring by mesoscale eddies homogenizes nutrient distributions on neutral density surfaces faster than they can be modified by diapycnic mixing with adjacent neutral density surfaces. Likewise, any signal of remineralization of nitrate or dissolution of silicate is greatly weakened by these physical processes, as we can find only weak hints of silicate or nitrate values slightly higher just below the SID than they are deeper on the upwelling neutral density surfaces. Finally, it is intriguing to note that the SID to which nutrient depletion extends is significantly deeper on all sections than the winter mixed layer depth, which can be identified in Figure 4a as the depth of the temperature minimum. This indicates that nutrient depletion in the surface layers is carried across the near-surface pycnocline by mesoscale isopycnic mixing (upwelling and downwelling along sloping neutral density surfaces), which can be large in small-scale ageostrophic motions [Hales and Takahashi, 2004; Pollard and Regier, 1990; Pollard and Regier, 1992]. These remarks all show that mesoscale eddies play a major role in setting nutrient distributions in the Southern Ocean.

3.2. Nutrient Supply

[12] Having established that nutrients are constant on neutral density surfaces below the SID, it is logical to use neutral density surfaces to calculate nutrient supply to the surface layer (Tables 2 and 3). The Ekman flux between pairs of neutral density surfaces has been previously calculated [Sloyan and Rintoul, 2001], but we have recalculated it using annual average winds from the ECMWF climatology combined with winter (July, August, and September) neutral densities from Levitus (1994, World Ocean Atlas 1994, available at http://ingrid.ldeo.columbia.edu/SOURCES/.LEVITUS94/). Table 2 shows that the Ekman flux is divergent for neutral densities greater than 27.0 kg m−3. The wind stress is nearly zero along neutral density 27.9 kg m−3 (mean latitude 67°S) and westward south of that. Thus water that upwells with neutral densities between 27.0 and 27.9 kg m−3 will be carried northward until it eventually crosses 27.0 kg m−3. The relationship between the wind stress maximum (south of which the Ekman flux will be divergent) and the winter neutral density surfaces is shown graphically in Figure 5. Around most of the Southern Ocean the wind stress maximum lies close to neutral density 27.2 kg m−3. The core of CDW lies along neutral density 27.8 kg m−3 (Figure 2) and CDW spans the range 27.4–28.0 kg m−3. Similarly, the low salinities identifying Antarctic Intermediate Water (AAIW) span the neutral density range 27.0–27.4 kg m−3 (Figure 2b) [Sloyan and Rintoul, 2001]. Table 3 confirms that it is CDW that is subject to the greatest upwelling, over 30 sverdrup (Sv; 1 sverdrup = 106m3s−1). However, we note in passing that AAIW is also upwelling (8 Sv) and it is Subantarctic Mode Water (SAMW) at neutral densities less than 27.0 kg m−3 which is downwelling, driven by Ekman convergence. Thus AAIW is actually an “old” water mass, as may be inferred from the gradients of oxygen and nutrients across the neutral density range 27.0–27.4 kg m−3 in Figure 2. This conclusion supports the hypothesis that AAIW is formed from SAMW [McCartney, 1977].

Figure 5.

The annual mean wind stress amplitude (shaded) from the European Centre for Medium-Range Weather Forecasts (ECMWF) climatology highlights the wind stress maximum in the Southern Ocean. Winds are primarily eastward, and the Ekman divergence is positive south of the wind stress maximum, leading to upwelling. Lines of constant winter neutral density at 100 m are superimposed. Neutral density 27.2 kg m−3 (bold solid line) aligns reasonably closely with the path of the Subantarctic Front (SAF) (bold dashed line) [Orsi et al., 1995] and with the wind stress maximum.

[13] During passage northward in the surface layer, which may take several years, the upwelled nutrients are taken up by phytoplankton growth in the surface layer, so that the nutrients remaining by the time the surface water leaves the Southern Ocean are much reduced. How do we define the "northern boundary of the Southern Ocean"? Strictly, in this analysis we mean the latitude of maximum wind stress, and Figure 5 shows that this is reasonably approximated by the 27.2 kg m−3 winter mixed layer isopycnal (at 52°S, Table 2) and by the Subantarctic Front (SAF). However, Table 2 shows the strongest zonally averaged Ekman flux at 27.0 kg m−3 (48.7°S). It is certainly north of the Antarctic Polar Front (APF) which lies close to the 27.4 kg m−3 isopycnal (at 56°S) and marks the transition from CDW to AAIW. Thus our calculations of nutrient supply (Table 3) include nutrients upwelling in AAIW as well as in CDW, although it makes little difference whether the cutoff is taken as 27.0 or 27.2 kg m−3 (say, 50°S).

[14] Maps of nutrients at 50 m depth (Figure 6) from the ODV database (nearly all summer stations in the Southern Ocean) show that silicate is reduced to about 5 μmol kg−1 and nitrate to 20 μmol kg−1 along neutral density 27.2 kg m−3, so we have used these values to determine the nutrients remaining when the Ekman transport crosses the Polar Front. The effect of varying these estimates is discussed in section 3.3, but we note that this work confirms that, despite the high-nutrient low-chlorophyll nature of the Southern Ocean, silicate is reduced to near limiting values for diatom growth by the time the upwelled water exits the Southern Ocean.

Figure 6.

Maps at a depth of 50 m of (a) nitrate and (b) silicate show their equatorward decrease. White contour line is 5 μmol kg−1 silicate. Winter neutral density lines at 100 m (Levitus, 1994, World Ocean Atlas 1994, available at http://ingrid.ldeo.columbia.edu/SOURCES/.LEVITUS94/) are superimposed.

3.3. Sources of Error

[15] While our calculation of net nutrient supply to the surface layer does not suffer from errors resulting from extrapolation of local in situ measurements, there are still several possible sources of error. We consider variation of nutrients on density surfaces, reliability of neutral density distributions in the Levitus database, reliability of the wind climatology, the assumptions of nutrient drawdown, and the assumption that upwelling is balanced by divergence of the wind stress.

[16] We believe that negligible errors arise from the calculation of nutrient concentrations on neutral density surfaces because of the remarkable constancy shown in Table 1 for WOCE sections at longitudes right around the globe, each spanning a wide range of latitudes and depths. Isopycnic mixing in the Southern Ocean has homogenized nutrients on each density surface to a remarkable degree.

[17] To estimate sensitivity resulting from changes in the spatial distribution of neutral density surfaces, we compared nutrient fluxes calculated using the June and September positions of the Levitus neutral density surfaces with those in Table 3, calculated using the winter (July to September) mean. Between June and September the mean latitude of neutral density surfaces shifted northward by 0.4° (ND 27.9) to 3.9° (ND 26.8) and the resulting flux estimates increased from 47.8 to 53.5 Tmol Si yr−1 (compare with 50.8 Tmol Si yr−1 in Table 3) and from 12.8 to 13.8 Tmol N yr−1 (compare with 13.7 Tmol N yr−1 in Table 3). Thus our estimates are not seriously sensitive to changes of several degrees in the latitudes of the neutral density surfaces and we suggest that 51 ± 3 Tmol Si yr−1 and 14 ± 1 Tmol N yr−1 includes reasonable error bars.

[18] Initially we used a wind climatology based on in situ observations [Hellerman and Rosenstein, 1983] combined with published neutral density calculations [Sloyan and Rintoul, 2001]. This gave lower nutrient fluxes by a factor of 3 (15.6 Tmol Si yr−1 and 4.0 Tmol N yr−1). Thus our results are sensitive to the wind climatology. It is clear from Table 3 how the factor of 3 arises. The Hellerman and Rosenstein climatological wind stress peaks well north of the ECMWF climatology [Josey et al., 2002] so that contributions to the Ekman [Sloyan and Rintoul, 2001] and hence nutrient fluxes from the Hellerman and Rosenstein winds tail off rapidly for neutral densities greater than 27.6 kg m−3. For the ECMWF climatology (Table 3) in contrast, the Ekman divergence remains significant for the neutral density classes 27.6–27.8 and 27.8–27.9 kg m−3, so that the contributions to nutrient fluxes from those two classes are as large as for 27.4–27.6 kg m−3. Indeed they are larger, particularly for silicate, because of the substantial rise in silicate values with neutral density. Thus substantial contributions to the nutrient fluxes arise across the whole range of CDW densities (27.4–27.9 kg m−3).

[19] Having argued [Josey et al., 2002] that the Hellerman and Rosenstein climatology is particularly poor in the Southern Ocean, a more useful estimate of nutrient flux variability would be obtained by comparing results using the NCEP-NCAR climatology with the ECMWF results. We have not repeated the calculation, but note [Josey et al., 2002, Figure 5] that these two climatologies have very similar latitudinal dependence but the NCEP wind stresses are smaller by about 5%. A 5% reduction in the nutrient fluxes from the ECMWF winds is less than 3 Tmol Si yr−1 and 1 Tmol N yr−1, so less than half the range estimated above from latitudinal variations in the Levitus data.

[20] The net nutrient supply to the surface layer depends on how much residual nutrient leaves the Southern Ocean in the surface layer. We have used drawdown to 5 μmol kg−1 for silicate and 20 μmol kg−1 for nitrate in Table 3. From Figure 6, silicate may be even more reduced, to perhaps 2 μmol kg−1, and residual nitrate could be higher, say, 22 μmol kg−1, or possibly lower, say, 18 μmol kg−1. Lowering the residual silicate by 3 μmol kg−1 would increase the net silicate supply by 3.5 Tmol Si yr−1 to 54.3 Tmol Si yr−1. Changing the residual nitrate by 2 μmol kg−1 would change the net nitrate supply by 2.4 Tmol N yr−1.

[21] In a recent paper [Hoppema et al., 2003] it has been argued that a significant fraction of upwelled nutrients (in their case, iron) leaves the Southern Ocean in less than a year, before it can be taken up by phytoplankton. Our analysis shows that this is unlikely. Although the maximum Ekman flux is 39 Sv near 50°S (Table 2), recent wind climatologies indicate that the major upwelling is well south of that, in CDW between 56°S and 67°S, where the Ekman flux ranges from 0–30 Sv (Table 2). Hence the mean Ekman flux, and hence northward transport velocity, of much of the nutrient rich upwelled water is less than half the 34 Sv used by Hoppema et al. Thus water, once it has reached the surface, will likely take several years to advect out of the Southern Ocean across 50°S, giving more than one summer season in which nutrients, whether nitrate, silicate or iron, can be taken up by phytoplankton growth.

[22] Finally, the dynamical balance of the Southern Ocean is an active research area. For example, the eddy transports derived by Karsten and Marshall [2002] would, if applied directly to the surface layer, reduce the upward mass divergence and hence the nutrient fluxes by 20–30%. The internal dynamics are still a matter of debate, and there is great uncertainty in Karsten and Marshall's estimates, but it is likely that our nutrients fluxes are upper limits, which will be somewhat reduced by eddy transports.

[23] In summary, we believe that our estimates of nutrient fluxes are robust, with a range estimate of ±6% for silicate and 20% for nitrate, thus 51 ± 3 Tmol Si yr−1 and 14 ± 3 Tmol N yr−1.

4. Discussion

[24] Our estimates for net nutrient supply to the surface layer are estimates of new production [Eppley and Peterson, 1979]. On an annual basis the new production of a given biogenic element should be balanced by export production. So, our net nutrient supply fluxes can be compared to estimates of export production for a given element.

[25] Estimates of total production of biogenic silica around the Antarctic Ocean in the early 1990s of 50 Tmol Si yr−1 [Tréguer and van Bennekom, 1991] and 11–32 Tmol Si yr−1 [Leynaert et al., 1993] are lower than our net production estimate of 51 Tmol Si yr−1. However, a more recent study [Pondaven et al., 2000] has estimated the annual production of biogenic silica in the Indian sector of the Southern Ocean from sections along 62°E. From their data [Pondaven et al., 2000, Table 1] for the spring-summer period, which is the appropriate period for export of biogenic silica to depth (compare with Pondaven et al.'s Figure 3), we calculate estimates of net silica production by vertical integration of winter minus summer nutrient depletion across the mixed layer down to the temperature minimum at 100–200 m, corrected for diffusion and advection. These estimates are 1.76 ± 0.38, 2.67 ± 0.52 and 1.14 ± 0.34 mol Si m−2 yr−1 for the Polar Frontal Zone (PFZ), Permanently Open Ocean Zone (POOZ) and Seasonal Ice Zone (SIZ), respectively. Extrapolating to the whole Southern Ocean, taking into account the areas of those zones (3, 14, and 16 M km2) [Tréguer and Jacques, 1992]), gives an estimate of silica net production south of the Subantarctic Front of 61 ± 14 Tmol Si yr−1 exported across 150 m, which compares well with our estimate of 51 Tmol Si yr−1 net export down to 340 m (Table 3). Similarly, from Pondaven et al.'s data we also get an estimate of nitrate new production of 12.7 ± 3.3 Tmol N yr−1, which again compares well with our estimate of 13.7 Tmol N yr−1.

[26] In a summary of the Antarctic Environment and Southern Ocean Process Study (AESOPS) [Nelson et al., 2002] along 170°W (the same longitude as the WOCE P15 section, Figures 2 and 4), particulate export of biogenic silica across 100 m was estimated from 234Th deficit as 1.4 mol Si m−2 yr−1 in both the Polar Frontal Region and southern ACC (spanning 59°–65.5°S and equivalent to PFZ and POOZ) which would integrate up to 53.2 Tmol Si yr−1 over the Southern Ocean (38 M km2), close to our estimate. Nelson et al found that the main factor controlling the southward increase in biogenic silica production was the standing stock of Si(OH)4 in surface waters at the end of winter, which ranged from 10 μM in the northern ACC to 35–55 μM in the southern ACC and was drawn down to <2 μM by the bloom. This finding supports our conclusion that there is large biogenic silica export as far south as 67°S, driven by upwelling of silicate rich water (as large as 80 μM on neutral density 27.9 kg m−3, Table 2). Nelson et al also found that 70–80% of the initial standing stock of Si(OH)4 was exported as biogenic silica, and that opal fluxes decrease by much less than POC fluxes between 100 and 1000 m.

[27] A modeling study [Gnanadesikan, 1999] has determined a silica export production rate of 38 Tmol Si yr−1 for the Southern Ocean euphotic zone. All three of these papers [Gnanadesikan, 1999; Nelson et al., 2002; Pondaven et al., 2000] support higher silica production rates than the earlier studies, and are consistent with the calculations of this paper [see also DeMaster, 2002]. Another modeling study [Schlitzer, 2002a] uses a global circulation and biogeochemical model to estimate export fluxes out of the Southern Ocean averaged over long periods. Schlitzer's approach is similar to ours in making use of the large volume of data in the Ocean Data View database (Schlitzer, 2000, Ocean Data View, available at http://www.awi-bremerhaven.de/GEO/ODV) (compare with Figure 1) to determine globally integrated estimates of export flux. However, where we use wind climatology to directly calculate nutrient upwelling rates, Schlitzer uses an adjoint model to optimally fit export parameters.

[28] Schlitzer's [2002a] best estimate of the total (particulate plus dissolved) organic carbon flux out of the euphotic zone for the entire Southern Ocean south of 50°S is 1.0 ± 0.2 Pg C yr−1, which at first sight compares well with our estimate of 1.1 Pg C yr−1 (Table 3) (calculated by converting nitrate to carbon using the Redfield ratio). However, Schlitzer used 133 m (the first two model layers) as the euphotic zone, whereas our estimate is for the SID, 340 ± 100 m (Table 1), which is significantly deeper. From his fitted remineralization scale heights, Schlitzer estimated net export flux of 0.59 Pg C yr−1 (POC + DOC) across 320 m, just over half our estimate. Schlitzer does note that his boundary of 50°S is arbitrary, and that his total export fluxes would be doubled if that boundary was shifted to 40°S. The mean latitude of our northern boundary, at a neutral density of 27.0 kg m−3, is 48.7°S (Table 2) which includes significantly more of Schlitzer's high-export region (his Figure 5). This reduces the difference between our estimates.

[29] Probably more important is the shape of profile chosen for the remineralization parameterization. Schlitzer uses the usual form [Suess, 1980], in which export fluxes below the euphotic depth E decay as (z/E)−b where b is close to 1. This leads to halving of the flux when z doubles from E to 2E. Our analysis suggests that vertical motion associated with mesoscale motions can transport water from the surface layer across the euphotic depth and even the winter mixed layer depth to the SID (Table 1), so that the remineralization scale is not well defined above the SID.

5. Conclusions

[30] We have examined the distributions of the major nutrients silicate and nitrate on neutral density surfaces in the Southern Ocean, and find that they are remarkably constant on a given surface at all longitudes and at all depths below a surface-influenced depth. This SID has a depth of 340 ± 100 m, much deeper than the winter mixed layer depths south of the Polar Front. Both the constancy of the isopycnic values and the large SID indicate efficient isopycnic mixing by eddies in the Southern Ocean.

[31] Using the ECMWF reanalysis wind climatology to calculate Ekman divergence between neutral densities in the Southern Ocean, we find that AAIW as well as CDW are upwelling, and the net annual mean supply of nutrients to the surface layer south of about 50°S is found to be 51 ± 3 Tmol Si yr−1 and 14 ± 3 Tmol N yr−1. The latter converts using classic Redfield ratios to 1.1 ± 0.2 Pg C yr−1. These values are in good agreement with recent estimates from several in situ observational campaigns and a recent model.


[32] RTP acknowledges helpful discussions with many colleagues, but particularly with Bernadette Sloyan, Reiner Schlitzer, and Simon Josey. This work is a contribution to the Core Strategic Project Biophysical Interactions and Controls on Export Production (BICEP) of the George Deacon Division of the National Oceanography Centre, Southampton, U.K., funded by the Natural Environmental Research Council. This work is contribution number 986 of the European Institute for Marine Studies.