Recent record lows of Arctic summer sea ice extent are found to be triggered by the Arctic atmospheric Dipole Anomaly (DA) pattern. This local, second–leading mode of sea–level pressure (SLP) anomaly in the Arctic produced a strong meridional wind anomaly that drove more sea ice out of the Arctic Ocean from the western to the eastern Arctic into the northern Atlantic during the summers of 1995, 1999, 2002, 2005, and 2007. In the 2007 summer, the DA also enhanced anomalous oceanic heat flux into the Arctic Ocean via Bering Strait, which accelerated bottom and lateral melting of sea ice and amplified the ice–albedo feedback. A coupled ice–ocean model was used to confirm the historical record lows of summer sea ice extent.
[1] A 9-km coupled ice-ocean model (CIOM) was implemented in the entire Bering Sea to investigate seasonal cycles of sea ice and ocean circulation under atmospheric forcing. Sea ice cover with a maximum of 0.6 Â 10 6 km 2 in February to late March was reasonably reproduced by the Bering-CIOM and validated by Special Sensor Microwave/Imager (SSM/I) measurements. The model also captures some important spatial variability and downscaling processes such as polynyas and ridging, which the SSM/I measurements cannot reproduce because of their coarse (25 km) resolution. There are two distinct surface ocean circulation patterns in winter and summer on the Bering shelves because of the dominant winds, which are northeasterly in winter and southwesterly in summer. Summer low-temperature, high-salinity water mass (<3°C) on the Bering shelf is formed locally during winter because of strong vertical convection caused by salt injection when ice forms, wind, and wind-wave mixing on the shelf. The northward volume transport across the 62.5°N line, with an annual mean of 0.8 ± 0.33 Sv (1 Sv = 10 6 m 3 s À1 ) that is consistent with the measurements in the Bering Strait, has barotropic structure, which transports heat flux (with an annual mean of 7.74 TW; 1 TW = 10 12 W) northward. The Anadyr Current advects warmer, saltier water northward during summer; nevertheless, it reverses its direction to southward during winter because of predominant northeasterly and northerly wind forcing. Therefore, the Anadyr Current advects cold, salty water southward. The volume transport on the broad midshelf is northward year round, advecting heat (3.3 ± 2.4 TW) and freshwater (À8 ± 10 Â 10 4 practical salinity unit (psu) m 3 s À1 ) northward. One important finding is that the Anadyr Current and the midshelf current are out of phase in volume and heat transports. The Alaskan Coastal Current also transports heat and freshwater northward on an annual basis. The Bering-CIOM also captures the winter dense water formation along the Siberian coast, which is promoted by the downwelling favorable northeasterly wind, and the summer upwelling due to the basin-scale upwelling favorable southwesterly wind, which brings up the cold, salty, and nutrient-rich water from the subsurface to the surface within a narrow strip along the west coast. This upwelling found in the model was also confirmed by satellite measurements in this study.
[1] Eddy-related cross-slope exchange along the Bering Sea shelf break was investigated using a hydrographic observations data set and a numerical model. Results of observations in summer of 2001 showed a shelf break front that formed at a shelf break near an anticyclonic eddy, high nitrate-nitrite concentrations in the subsurface layer, and high chlorophyll a (Chl-a) concentrations (!6 mg m À3 ) in the surface layer. A hydrographic observation in summer of 2002 exhibited relatively high Chl-a concentrations at the surface around the anticyclonic eddy. Tracer experiments revealed two types of cross-slope exchange. Under isopycnals, nutrient-rich water in the basin is transported to the shelf and there is about a 64.53% increase in integrated nitrate-nitrite on-shelf flux (50 m depth $bottom), when mesoscale eddies are formed and propagated along the shelf break. At the surface, high Chl-a waters in the shelf are advected to the deep basin area by eddy transport and propagation. These indicate that (1) mesoscale eddies supply nutrients and sustain primary productivity at the shelf break, and (2) eddies expand the high Chl-a area to the basin, then to the highly productive area, so that the Green Belt is maintained.
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