Simulated decadal variations of fresh water content and sea surface height in the Beaufort Sea

Our objective is to investigate the decadal variations of fresh water content (FWC) and sea surface height (SSH) in the Beaufort Sea, particularly their increases since 2003. Changes in the Arctic fresh water balance play an important role in the decadal variations of the North Atlantic circulation. A 25% increase in freshwater discharge through the Fram Strait maintained for two years can account for the salinity deficit observed in the North Atlantic during the “Great Salinity Anomaly” of the 1970s (Aagaard and Carmack 1989; Dickson et al. 1988; McPhee et al. 2009). Similar studies suggest that salinity anomalies can have profound impacts on the intensity of the Atlantic Ocean meridional overturning circulation (Curry and Mauritzen 2005). A significant change of FWC in the central Beaufort Sea has been detected since 2003 (Proshutinsky et al. 2009; McPhee et al. 2009). In 2008, the FWC increased by as much as 11 m at some stations, compared to the FWC climatology, and the location of the maximum shifted to the southeast. Measurements in the Canada and Markarov Basins suggest a FWC increase in the surveyed area by about 8500 cubic kilometres. Accordingly, the steric sea level increased about 75% (McPhee et al. 2009). Several factors could be responsible for the FWC increase, which include melting, river runoff, surface moisture flux, and surface wind stress. Proshutinsky et al. (2009) speculated that one of the major causes for the variability in the FWC could be related to the changes in the atmospheric circulation and Ekman pumping. In the Arctic Ocean, the largest fresh water storage is located in the Beaufort Gyre (BG), a dominant anticyclonic circulation in the Beaufort Sea (Aagaard and Carmack 1989). On decadal time-scales, the wind-driven circulation alternates between cyclonic and anticyclonic circulation regimes, with each regime persisting for 5-7 years (Proshutinsky and Johnson 1997). The BG accumulates significant amounts of fresh water during the anticyclonic regime and releases it during the cyclonic regime (Proshutinsky et al. 2002; Hakkinen and Proshutinsky 2004). However, to fully understand the causes for the FWC increase, a model simulation has to be conducted to estimate the fresh water budget and to understand the causes for FWC variations. To understand decadal variations in FWC and SSH, particularly their sharp increases since 2003, we implemented a coupled ice-ocean model (CIOM) in the Arctic Ocean and conducted a 40-year simulation, validating the model with available observations. We investigate the decadal variations of FWC and SSH in the Beaufort Sea, particularly their increases since 2003, using the CIOM model, implemented for the Arctic Ocean to simulate the decadal variations. The CIOM model is forced with NCEP daily reanalysis (1970-2009); the initial and lateral boundary conditions for salinity and temperature are provided by the Polar Hydrographic Climatology (PHC). Neither surface flux adjustment nor surface temperature and salinity restoration are applied. The CIOM simulation exhibits a salinity minimum in the Beaufort Sea and a warm Atlantic water layer in the Arctic Ocean, which is similar to the PHC climatology. CIOM also captures the observed maximum FWC in the central Beaufort Sea and simulates very well the observed decadal variation of total ice concentration, including the rapid decrease of sea ice in recent years. CIOM also reproduces the observed interannual variations of SSH and FWC during 2003-2009. The model simulations of SSH and FWC suggest positive trends in the central Beaufort Sea in recent years. During the last decade, the simulated SSH increase is about 8 cm while the FWC increase is about 2.5m, with most of these increases occurring in the center of Beaufort Gyre. We show that these trends are due to an increased surface wind stress curl during 2003-2009, which converged and increased the FWC in the Beaufort Sea by about 0.7 m/year through Ekman pumping. In addition, enhanced ice melting also contributes to the FWC increase by about 0.3 m/year.

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