Effects Of Pacific Inflow On Water Mass And Sea Ice In The Chukchi Sea

Pacific inflow through the Bering Strait varies interannually and increases in the changing Arctic.As a dominant nutrient source for the western Arctic Ocean,Pacific Winter Water(PWW)within the inflow has attracted much attention on its distribution and variation in the Chukchi Sea.The response of the Chukchi sea ice to the Pacific heat inflow has also been brought into focus.In this study,the influences of Pacific inflow on water masses and sea ice in the Chukchi Sea are first analyzed at seasonal and interannual scales,based on a coupled ocean and sea ice model covering the North Atlantic-North Pacific-Arctic Oceans with a nominal horizontal resolution of(1/4°)in latitude/longitude(NAPA1/4).Secondly,the formation mechanism and predictability of open-water in Bering Strait are explored.The main conclusions are as follows:(I)Pacific Summer Water(PSW)enters into the Chukchi Sea mainly in summer and autumn,and regulates the interannual variability in Pacific inflow(correlation coefficient r=0.88,p value <0.01).While PWW enters into the Chukchi Sea in winter and spring with slight interannual variation in annual volume(0.52±0.03 Sv).In summer and autumn,PWW distributes in the northern Chukchi Sea,with the upper boundary deepening and the vertical thickness decreasing.From 1994 to 2015,the annual mean volume of PWW shows a downward trend in the southern Chukchi Sea,which is affected by the interannual variation of PSW(r=-0.74,p<0.01).While the annual variation of PWW in the northern part is relatively small except for2002-2004.(II)Sea ice volume variation in the Chukchi Sea during the ice-melting season is mainly attributed to the thermodynamics-induced ice loss,with the ice basal melting more than twice the surface melting.Heat budget estimation reveals that the heat for ice basal melting is composed of the heat flux at the sea-water surface and that with Pacific inflow.In early and peak ice-melting stages(from mid-May to early August),the contribution of net heat flux at sea water surface is dominant;in late and final ice-melting stages(from mid-August to mid-October),Pacific inflow plays a major role,and continues to provide heat in the early freezing stage,slowing down the freezing rate.During the entire ice-melting season,the total heat entering the study area water is mainly contributed by the Pacific inflow(～67%).Compared with the heat flux at the sea-water surface,Pacific heat inflow is of more importance in regulating the interannual variation of the ice coverage in the Chukchi Sea(r=-0.71,p<0.01).Additionally,Pacific inflow dominates the interannual variations of the heat content variation(r=0.85,p<0.01)and the heat flux towards downstream(r=0.88,p<0.01).(III)Open-water onset in Bering Strait is the precursor of sea ice melting in the Chukchi Sea.In the Bering Strait,analysis of observed sea ice concentration during1989-2019 from satellite remote sensing shows that the dates of open-water onset vary from late April to early June.Analyses of the simulation results of a coupled ocean and sea ice model show that the onset dates have a significant correlation(r=0.88,p<0.01)with sea ice concentration in the Cape Navarin area during the ice formation period of January 16 to February 15.In the Cape Navarin area,the ocean and sea ice conditions during the sea ice formation period impact open-water onset locally due to the closeness to the marginal ice zone.Once sea ice melts widely near Cape Navarin,the open-water absorbs solar heat input,and the warmed surface water is advected by the Anadyr Current to melt the sea ice downstream.From Cape Navarin area to Bering Strait,the progressive formation of open-water happens rapidly within 20 days.Thus,the four-month lead prediction of open-water onset in Bering Strait is established through the correlation in ice conditions between the ice formation and melting periods upstream of the Anadyr Current near Cape Navarin.