Upstream environment for SBI - Modeled and observed biophysical conditions in the northern Bering Sea

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1 Upstream environment for SBI - Modeled and observed biophysical conditions in the northern Bering Sea Jaclyn Clement 1, Wieslaw Maslowski 1, Lee Cooper 2, Jacqueline Grebmeier 2, Waldemar Walczowski 3, Jeff Dixon 1 1 Naval Postgraduate School, Department of Oceanography, 833 Dyer Road, Monterey, CA 93943, USA 2 University of Tennessee, Department of Ecology and Evolutionary Biology, Research Dr, Suite 100, Bldg A, Knoxville, TN 37932, USA 3 Institute of Oceanology, Polish Academy of Sciences, ul. Powstanców Warszawy 55, PL Sopot, Poland Introduction Using a high-resolution Pan-Arctic ice-ocean model, the circulation of the northern Bering Sea and transport through Bering Strait are investigated and discussed in relation to their influence on downstream conditions in the Chukchi and Beaufort seas. Model results are compared to observational data including salinity and nutrient concentrations in the Bering Sea and transport measurements in Bering Strait. The high resolution (1/12 or ~9km) and large domain of the model allow for realistic representation of flow through Anadyr, Shpanberg, and Bering straits and calculation of transport estimates. A long-term model estimated mean transport through Bering Strait is ~0.65 Sv. The modeled seasonal pattern of transport is comparable to observational data collected from moorings in Bering Strait, with lower monthly mean transports during winter and higher transports in July and August. The monthly mean transport through Bering Strait is highly correlated with the transport through Anadyr Strait over the model 23-year integration time period (r=0.83), while the correlation coefficient for Bering and Shpanberg Straits is somewhat lower (r=0.64). Observational data in the northern Bering Sea from late spring through summer and fall indicate an eastto-west increase in nitrate concentration, silicate concentration, and salinity. Model results show a similar salinity pattern across the Bering shelf, which represents the characteristics of Alaska Coastal Water to the east and Anadyr Water to the west. However, the model overestimates the salinity near the Alaska coast, which is possibly due to a lack of freshwater input from the Gulf of Alaska via the Alaska Coastal Current. In the Bering Sea, salinity can be used as a proxy for nutrient concentrations, especially in deeper parts of the water column (P. Stabeno, pers. comm.). This allows the model to be used in determining the biologically-relevant characteristics of water moving north through Bering Strait and across the Chukchi shelf. Upstream conditions in the northern Bering Sea are important for developing hypotheses regarding the Shelf-Basin Interaction study region in the Chukchi and Beaufort seas. The model's ability to cross political boundaries and examine high-resolution results over a large scale and long time period is critical for understanding the role of Pacific Water in shelf-basin exchanges, in the arctic general circulation, and in climate change.

2 Summary The Pan-Arctic ice-ocean model agrees rather well with observed measurements of salinity, especially in the northern Bering Sea and in Bering Strait. The model's realistic representation of salinity in this region along with the assumption of high correlation between salinity and nutrient fields (based on observations) will allow the model to provide additional insights, especially in areas lacking observations. The annual climatology of Bering Strait is in agreement with observations (e.g. modeled summer transport up to 0.95 Sv with ~0.55 Sv during winter). In addition the model depicts low-frequency events, such as the winter flow reversal during Shpanberg Strait section, St. Lawrence Island section, and Anadyr Current section all show strong westerly transport just prior to and during the extremely reduced ice concentration measured by satellites. It is important to note that the Bering Strait section and the Anadyr Strait section were not strongly affected by the event. This is likely due to the fact that Anadyr Current flow was westerly away from Bering Strait, with more water moving west through Shpanberg Strait toward Bering Strait. These data indicate that caution is needed when interpreting variability in the Bering Sea and its propagation through Bering Strait and onto the Chukchi/ Beaufort shelves. Acknowledgments We would like to thank the National Science Foundation / Shelf-Basin Interaction (SBI) Program for primary support of this reserch. Additonal support has been provided by the Office of Naval Research (ONR), National Atmospehric and Oceanic Administration (NOAA) and Department of Energy (DOE). Computer resources were provided by the Arctic Region Supercomputing Center (ARSC) through the Department of Defense High Performance Computer Modernization Program (HPCMP) Grand Challenge Project We also thank the U.S. National Ice Center for providing satellite data.

3 Alaska, USA Naval Postgraduate School Arctic Modeling Effort Russia BS AC AS SL SS Figure 1. Map of Bering Sea depicting bathymetry and the location of vertical sections: Bering Strait (BS), Anadyr Strait (AS), Shpanberg Strait (SS), Anadyr Current (AC), and St. Lawrence Island (SL).

4 V olume T ransport (S v) BS Min: Max: Mean: Std: J an Apr J ul Oct AS Min: Max: Mean: Std: J an Apr J ul Oct SS Min: Max: Mean: Std: J an Apr J ul Oct AC Min: Max: Mean: Std: J an Apr J ul Oct SL Min: Max: Mean: Std: J an Apr J ul Oct Month Figure 2. Annual climatological transport through Bering Sea sections. Monthly means are calculated over a 23-year time series from Positive (blue line) fluxes represent flow to the North or East according to the model grid (see Figure 1), while negative (red line) fluxes represent flow to the South or West. Net flow (black line) is the sum of the positive and negative flow components.

5 V olume T ransport (S v) BS SS AS AC SL Min: Max: Mean: Std: Min: Max: Mean: Std: Min: Max: Mean: Std: Min: Max: Mean: Std: Min: Max: Mean: Std: Y ear Figure 3. Monthly mean transport over a 23-year time series from Positive (blue line) fluxes represent flow to the North or East according to the model grid (see Figure 1), while negative (red line) fluxes represent flow to the South or West. Net flow (thin black line) is the sum of the positive and negative flow components and smoothed net flow (thick black line) is calculated as a 13-month running mean.

6 100% 95% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Figure 4. Sea ice concentration on February 19, 1999 (left) and February 19, 2001 (right). Sea ice maps were created using satellite data obtained from the U.S. National Ice Center (Washington, DC). The winter of was the most unusual time period with very little sea ice in the central and eastern Bering Sea for much of winter. This is likely due to strong southeasterly wind forcing during this time. The model also represents this event with extremely high flow reversals across Shpanberg Strait, St. Lawrence Island section, and Anadyr Current section from November 2000 to February 2001.

7 Alaska, USA Russia Reference Vectors 1 cm s cm s -1 25cm s -1 St. Lawrence Island Min Max 3E Mean Std Naval Postgraduate School Arctic Modeling Effort Nunivak Island Figure 5. Twenty-three-year mean velocities averaged over the upper 50m. All vectors are represented. Notice the high velocities (up to 60 cm s -1 ) through Bering and Anadyr straits.

8 Figure 6. Net velocity through a vertical section of Bering Strait calculated from the 23-year mean. The western channel is on the left and the eastern channel on the right with positive values representing northward velocities (cm s -1 ). Velocity varies both vertically and horizontally with higher values typically found in the deeper parts of the channels and lower values near the surface. The highest variation occurs in summer and fall, especially across the western channel.

9 Figure 7. Mean salinity (ppt) through the veritcal section of Bering Strait. The model realistically represents the annual cycle of salinity with higher values in winter and lower values in the surface water during summer.

10 Salinity sppt sppt sppt A Figure 8A. Observed salinity in near-bottom water measured during June Black dots represent station locations with an inverse distance weighted interpolation and extrapolation in color Kilomete rs

11 Salinity sppt sppt sppt B Figure 8B. Modeled salinity at the deepest level at the end of June 1990 interpolated via the same method Kilomete rs

12 Salinity sppt sppt sppt C Figure 8C. Observed salinity in near-bottom water measured during May-June 1994 with interpolation Kilomete rs

13 Salinity sppt sppt sppt D Figure 8D. Salinity at the deepest model level at the end of June 1994 interpolated as before. The model appears to realistically represent salinity, however it likely overestimates salinity in the eastern Bering Sea, possibly due to a lack of freshwater input via the Alaska Coastal Current Kilomete rs

14 A Reference Vectors 1 cm s cm s cm s -1 Min Max Mean Std Figure 9. Bering Sea velocities averaged over the upper 50m. Twenty-five percent of the total vectors are drawn. A: Mean velocity during the year of highest (over the 23-year model integration) northward transport through Bering Strait (1979).

15 B Reference Vectors 1 cm s cm s cm s -1 Min Max Mean Std Figure 9. Bering Sea velocities averaged over the upper 50m. Twenty-five percent of the total vectors are drawn. B: Mean velocity during the year of lowest (over the 23-year model integration) northward transport through Bering Strait (1994).

16 C Reference Vectors 1 cm s cm s cm s -1 Min Max Mean Std Figure 9. Bering Sea velocities averaged over the upper 50m. Twenty-five percent of the total vectors are drawn.c: The difference in the mean velocities of 1979 and The greatest differences occur in Anadyr and Bering straits, with a mean difference of ~0.25 Sv (see Fig. 3) between years.

17 MA3 ADCP MA1 MA2 BS HX174 Sept Kilomete rs Russia Alaska, USA HX139 June 1990 Silicate 0-5 µm 5-10 µm µm µm µm µm µm µm K ilomete rs Figure 10. Observed silicate concentrations in near-bottom water during June 1990 (near St. Lawrence Island) and September 1993 (in and around Bering Strait). Note higher values to the west and lower values to the east. The distributions of silicate (above) and nitrate (not shown) during June 1990 are similar to that of salinity (Fig. 8A), which supports the use of salinity as a proxy for nutrient concentrations. Inset shows the locations of observed measurements, the approximate locations of moorings, and the vertical section of the model.