Response of the Gulf of Alaska 3D winter circulation to oceanic climate shifts: Ecosystem implications

Authors


Abstract

[1] Recent measurements of zooplankton biomass and biological productivity in the Gulf of Alaska have raised a number of questions regarding possible linkages between climate and availability of renewable resources. In this article we compare 3 abrupt oceanic regime shifts in the Gulf of Alaska, the 1976–1977 warming shift, the 1998 cooling episode, and the 1998 to 1999 El Niño to La Niña transition, against concomitant changes in biological conditions reported in the literature. After the 1976–1977 warming shift, changes in the Gulf's 3D circulation, i.e., interior upwelling, onshore transport and coastal downwelling, had the same sign as their climatological means, thus providing a candidate explanation to the observed increased productivity of the upper ocean. Warming and cooling transitions have associated with them very different patterns of both horizontal and vertical circulation, where the latter is confirmed to be linked to the wind stress curl variability. Important shifts in the local biology have been reported in the literature (warming of 1976–1997 and El Niño to La Niña transition of 1998–1999) when climatological shifts in our simulated vertical velocities were large; in turn, when they were small, the ecosystem did not show significant changes and this was in part due to the resilience set by the 1976–1977 shift through the strengthening of the GOA's mean 3D circulation.

1. Introduction

[2] The Gulf of Alaska (GOA) is a complex physical oceanographic system that supports economically important fisheries and ecologically important wildlife areas [Stabeno et al., 2004]. The Alaska Current, which runs northwestward along the northwest shelf-slope boundary, and the Alaska Stream, which runs southwestward along the western shelf-slope boundary both generate and feed the open-ocean with an energetic mesoscale circulation [Kelly et al., 1993, Hermann et al., 2002]. The GOA exhibits strong variability from the interannual to decadal/interdecadal timescales [Royer, 1989, Lagerloef, 1995]. Auad et al. [2003] estimated the pycnocline variability induced by the 1976–1977 rapid warming shift [Ebbesmeyer et al., 1991; Miller et al., 1994] in the North Pacific ocean. They found that south of about 50°N and west of 140°W, pycnocline depths got shallower, while along coastal areas of the GOA they deepened [Auad et al., 2003]. On the western coast this was found to be due to the propagation of pycnocline depth anomalies along the coast [Capotondi et al., 2005].

[3] The ocean's productivity and the intensity of its exploitation by human activity are sensitive to upwelling events taking place in the interior of the GOA and ultimately to the horizontal transport of nutrients (and iron) upwelled in the interior. This transport can be generated by mean advection and/or the cross-shelf transport due to eddies [Crawford et al., 2007]. Thus, paradoxically, the areas of highest primary productivity are located near the coasts [Stabeno et al., 2004, Miller et al., 2004]. This can be better understood by closely studying the structure and intensity of the acrosshore transport and mixing although many aspects of this are still poorly known [Francis et al., 1998]. Lagerloef [1995] demonstrated that significant dynamic height variations could be largely explained as a static response to local Ekman pumping variations. Most importantly, Ekman pumping mean positive values increased over the GOA interior as part of the 1976–1977 shift [Lagerloef, 1995]. Benson and Trites [2002] noted that rapid shifts in biological productivity generally correspond to the timing of climate shifts. The 1976–1977 also led to the increase in zooplankton biomass across the subarctic North Pacific from the late 50's to the late 80's [Brodeur and Ware, 1992], while Anderson and Piatt [1999] showed that part of the catch biomass increased (e.g., Pacific Cod), decreased (e.g., shrimp) or did not show a specific response (e.g., sandfish) to the 1976–1977 warming shift in the GOA. However, after the 1998–1999 cold shift [Peterson and Schwing, 2003] there was no evidence of community reorganization [Litzow, 2006] in the GOA. The picture is more complicated because fish species in the GOA may respond on many timescales in the atmosphere-ocean system [Francis et al., 1998]. As an example, Salmon stock variations along the entire North American west coast are being connected to low frequency climatological changes [Beamish and Bouillon, 1993; Mantua et al., 1997], while warming transitions have the potential to affect not only several environmental variables which, in turn, might not respond with the same lags, but also the overall budget of the local economy. The goal of this paper is to characterize the circulation changes of the 1969–1976 to 1979–1986, 1990–1997 to 1999–2003, 1998 to 1999 climate shifts (RWS, RCS and N2N hereafter, respectively) and offer a possible explanation for the different biological responses of the GOA to those events. The motivation for chossing the overlapping periods of RCS is N2N is twofold: climate changes in these two scenarios have different amplitudes, and also their timescales are different. We will see that only in one of those climate shifts, important ecosystem changes were reported.

2. Model

[4] The Regional Ocean Model System (ROMS) is an s-coordinate, terrain following, primitive equation model. It is an optimized version of SCRUM [Song and Haidvogel, 1994], modified to run in multi-processor servers. The present quasi-global (80°S–80°N) configuration uses a horizontal resolution of 0.3° at the equator slowly changing to 0.5° at 35° and 0.7 degrees at 62°. In the zonal direction the horizontal resolution is 0.8°. In the vertical direction the model has 20 s-levels with increased resolution at the surface and near the bottom, while the northern boundary is set to a vertical wall.

[5] The quasi-global ROMS was first spun up from rest and Levitus climatology for 100 years under climatological wind stress and heat flux forcing. Then it was spun up for another 8 years by cycling twice through the 1948–1953 time period for additional stability, and continued through 2003. In this article we analyze the timeframe 1967–2003 (37 years). In the GOA the global model has zonal and meridional resolutions of 0.8° and 0.65° respectively. NCEP [Kalnay et al., 1996] wind stresses and heat fluxes were used to force our simulations while sea surface temperatures (SST) and sea surface salinities (SSS) are relaxed to their climatological values, both with a timescale of 60 days. The ROMS code was modified to realistically scale the small (500 m2/s) horizontal viscosity coefficient as a function of the variable grid resolution.

3. Results

[6] We have defined the RWS, RCS and N2N shifts as the difference, for any of the variables analyzed below, between the climatological means of the epoch after the shift minus that one from before the shift (epochs' timeframes were defined above at the end of the Introduction). Figure 1 shows the winter (JFM) changes attained by the wind stress field, its associated WSC (left column), and SST (right column) for all three scenarios defined above. Winter climatologies exhibit the largest amplitudes among all 4 seasons, in agreement with previous findings [e.g., Auad et al., 2003; Peterson and Schwing, 2003; Schwing et al., 2002] and are in line with previous reports [Miller et al., 1994; Schwing and Medelssohn, 1997]. The negative WSC anomaly after a RWS along the eastern margin of the gulf and the positive values on the west were described in Capotondi et al. [2005], and are associated with the overall weakening of the Aleutian Low system after the RWS of 1976–1977. Net heat fluxes over the GOA are also characterized by stronger changes in winter than in summer with maximum amplitudes located in the head of the gulf (not shown). Positive winter anomalies (i.e., a climatological difference hereafter) do not exceed 120 W m−2 (i.e., a warming for the ocean) in the NE corner of the gulf and decreasing toward the SW to no less than about −30 W m−2.

Figure 1.

The left column displays climatological differences for the wind stress vectors and wind stress curl for the (a) RWS, (b) RCS and (c) N2N events, while the right column shows SST changes induced by the (d) RWS, (e) RCS, and (f) N2N events.

[7] The wind patterns from the cooling shifts of Figures 1b and 1c (RCS, N2N) are different from the warming shift of Figure 1a and also show the extreme atmospheric values of the N2N event (Figure 1c). Figures 1a and 1d show the typical atmospheric and oceanic patterns for the warm phase of the Pacific Decadal Oscillation (PDO) in the GOA [e.g., Mantua et al., 1997], whi1e Figure 1e resembles the Victoria Pattern [Bond et al., 2003], typically the second most energetic SST mode, after the PDO. Both cooling scenarios (Figures 1e and 1f) show a cooler GOA in its interior and eastern flank while its western part presents mild warm anomalies. However, maximum cooling takes place at different locations: off Vancouver Island for the RCS and in the GOA's interior for the N2N shift, while the latter exhibits the largest cold anomalies of both scenarios. The largest positive RWS anomalies are located off the Alaska's eastern coast, in Bristol Bay and off the Seward Peninsula, while these signals, atmospheric and oceanic, have been shown to be significantly larger in winter than in any other season [e.g., Auad et al., 2003; Hartmann and Wendler, 2005]. While the SST pattern of Figure 1d resembles the shift in heat flux climatologies (not shown) for the RWS scenario [e.g., Miller et al., 1994; Auad et al., 2003], below the 70–75 m level, and in agreement with the recent study of Barnett et al. [2005], cold anomalies are present in the model's ocean temperature trends (not shown) over the last 40 years. This is important since many species have their habitat reaching at least that depth and it is not yet clear how these climate scenarios affect the subsurface environment.

[8] After the RWS, RCS and N2N, the local winter circulation in the GOA adjusted in the manner shown in Figure 2 for the 20 m currents (left column) and Ekman transport (right column). Twenty meter currents for the RWS change (Figure 2a) strengthening the mean circulation (Figure 2g), while anomalous Ekman and 20 m flows for the RCS and N2N scenarios, favor the export of nutrients and tracers to the south and into the California Current (CC) area [Peterson and Schwing, 2003]. The N2N event was a short-lived relaxation event from the 1998 El Niño to the 1999 La Niña shifting southward the habitat of several species due to the advection of food in that direction plus their need for a more favorable, warmer, temperature range. On the other hand, there is compelling evidence that the RCS flows (Figures 2c and 2d) had no major impact on the community reorganization of the Gulf of Alaska [Litzow, 2006], as they had in the CC area [Schwing and Moore, 2000; Peterson and Schwing, 2003]. These surface anomalous currents are in line with those reported by Wu and Hsieh [1999] for the RWS.

Figure 2.

The left column displays climatological winter changes for the 20 m currents induced by the (a) RWS, (c) RCS, and (e) N2N events, while the right column shows analogous changes in Ekman Transport for the same scenarios, respectively (b, d and f). For reference, also shown are the winter climatologies of 20 m currents (g) and winter climatologies of Ekman Transport (h) corresponding to the 1969–1976 timeframe.

[9] Strong wind stress and wind stress gradients characterize the winter atmospheric circulation in the GOA. Changes in their climatological values are important since they are known drivers of important changes in the magnitude of Ekman pumping velocities [Lagerloef, 1995; Capotondi et al., 2005], which in turn, are key factors affecting habitat quality of many species. Figures 3b, 3c, and 3d display winter vertical velocities at the 20 m depth for all three scenarios. The similitude between the spatial patterns of Figures 3b, 3c, and 3d and those of the WSC in Figure 1abc is in line with their findings. Similarly to our earlier description of the GOA anomalous flows of the RWS scenario, vertical velocity anomalies (Figure 3b) are also and in general of the same sign as their mean climatologies (Figure 3a), thus suggesting an overall strengthening of the mean horizontal and mean vertical circulations after the RWS [Miller et al., 2004; Capotondi et al., 2005]. Unlike the mild similarity shown above (Figures 2c and 2e versus Figures 2d and 2f) for the horizontal flows and transports of both cooling scenarios (RCS and N2N), their 20 m vertical velocities display very different spatial patterns (Figures 3c and 3d). The N2N scenario is closely responding to the WSC forcing of Figure 1c, which is about 3–5 times larger than that of the RCS scenario (Figure 1b). The latter not only confirms the importance of the WSC forcing of vertical velocities in the GOA, but also points to the sensitivity of the vertical flow, given the larger WSC and vertical velocities of the N2N event than those of the RCS scenario. Also different between both cooling events (RCS and N2N), was the biological response of the GOA. While no major biological productivity changes were detected in the detailed analysis of Litzow [2006] for the RCS, increased productivity due to increased coastal upwelling, with respect to 1998, characterized the N2N shift [Whitney and Welch, 2002]. According to them, the N2N event brought most nutrient concentrations close to their long term mean values.

Figure 3.

Winter vertical velocities for (a) climatology from the 1969–1976 timeframe, (b) climatological change after the RWS, (c) climatological change after the RCS, (d) climatological change after the N2N event, (e) climatological change from the 1969–1976 era to the 1990–1997 era and (f) climatology change from the 1969–1976 era to the 1999–2003 era.

[10] Figures 3e and 3f allow to track down the persistence of the RWS in vertical velocities, a key physical variable in the GOA's ecosystem, by estimating the 1997–1990 minus 1976–1969, and the 2003–1999 minus 1976–1969 climatological differences. By comparing Figures 3b, 3e, and 3f, not only we can appreciate the similarity between Figures 3b and 3f but also that one between Figures 3f and 3a, which overall shows that the RWS has resisted the counter action of other climate shifts by strengthening its mean 3D circulation from the late 60's up to at least 2003. We have also computed the WSC change, i.e., a proxy for Ekman pumping, from the 1969–1976 epoch to the 1999–2003 (not shown). This pattern does not resemble the WSC change of Figure 1a or the vertical velocity change of Figure 3b. This is important as it suggests that the resiliency of the RWS of 1976–1977 lies in the ocean rather than in the atmosphere.

4. Discussions and Conclusions

[11] Three different regime shift events were compared in their forcing and circulation and these results were contrasted against recent findings on biological variability. The RWS in the GOA circulation was characterized by changes in vertical and horizontal circulation of the same sign as the mean circulation. This likely led to increased interior upwelling, increased onshore transport and warmer coastal waters. While the intensification of this 3D cell is a reasonable candidate to explain the observed increased nutrients along the periphery of the GOA and after the RWS [Brodeur and Ware, 1992], opposing physical conditions characterizing the N2N shift (reduced coastal SSTs and coastal downwelling, and a slowdown of the Alaskan gyre) did not have the opposing effect in the GOA's productivity. The latter plus the facts that (a) most but not all communities reorganized after the RWS [Anderson and Piatt, 1999], and (b) the RCS did not have a major impact on the reorganization of communities in the GOA [Litzow, 2006], calls for a more detailed analysis of physical-biological interactions in the GOA, and it further suggests that ecosystem changes observed during 1999 were mostly due to the large N2N transition. The lack of a dramatic community reorganization after either the Victoria Pattern shift of 1998 [Litzow, 2006], or the 1989 North Pacific atmospheric shift [Anderson and Piatt, 1999], has a candidate explanation in that also minor were the changes experienced by upwelling and downwelling velocities after those two events.

[12] We conclude that the WSC, vertical velocities and biology are three intimately related players in the GOA. The weakest WSC of all three scenarios considered corresponds to the RCS, which in turn, have no major biological changes associated with it [e.g., Litzow, 2006] even though this same shift did have a biological component in the California Current area [Peterson and Schwing, 2003]. This dichotomy between both areas could be related to the fact that the Victoria Pattern [e.g., Bond et al., 2003] has an amplitude 3 to 5 times larger in the California Current area than in the GOA. In turn, this event (RCS) was clearly visible in their (and others) modal time series because their first SST mode (PDO-like) stayed with a small amplitude after 1998. This reinforces our claim that these two changes were not large enough to shift the GOA back to a pre-1977 state.

[13] Our findings focused on the transport system, vertical and horizontal, sustained by direct wind forcing and mass distribution in the GOA. During spring, anomalous vertical velocities and Ekman transport, exhibited patterns (not shown) similar to those of winter (Figure 3) but with smaller (20%–40%) amplitudes. This winter-spring feature is consistent with the findings of Stabeno et al. [2004] who described mechanisms for controlling the concentration of nutrients in the central GOA. The problem in practice is very complex because once that the three-dimensional change in temperature takes place, a given sensitive-to-temperature species will tend to remain in its favorable range, but it will also need to adjust to the geographical reallocation of its food due to changes in currents and temperatures. The current experiments have shown that only intermediate model resolution and wind stress and heat flux forcing suffice to (a) decently model the basic circulation climatological cell, and (b) describe its changes, which offers an explanation to the observed increase in primary productivity that characterized the post 1976 era. We further conclude that if the current global warming scenario is to persist, embedded in a post-1976 resilient state of the North Pacific ocean, high productivity conditions in the coastal areas of the GOA will continue to be favored.

Acknowledgments

[14] Financial support was provided by the Department of Energy (FG02-04ER63857), by the National Science Foundation (OCE04-52692), by the National Oceanographic and Atmospheric Administration (NA17RJ1231) through the Environmental Climate Prediction Center (ECPC), and by the Eastern Pacific Consortium of the Inter-American Institute for Global Change Research (IAI-EPCOR, CRN-062). I especially thank Art Miller for many useful discussions during the preparation of this manuscript. The views herein are those of the authors and do not necessarily reflect the views of NOAA or any of its sub agencies.