Characteristics Of Sea Ice In Northeast Asia And Its Reasons

Using the daily sea ice concentration(SIC)data from National Snow and Ice Data Center(NSIDC)and the reanalysis of meteorological data from the National Centers for Environmental Prediction–National Center for Atmospheric Research(NCEP/NCAR),the temporal and spatial variation of sea ice in the Northeast Asia and its relationship with atmospheric circulation anomalies was examined.Finally the synchronic change of sea ice in the Sea of Okhotsk and in the Arctic was revealed and discussed.The main conclusions are summarized as follows:(1)The interannual variability of sea ice in spring is consistent with that in winter.Both in winter and in spring,the trend of sea ice is decline dramatically.But the uptrend of sea ice in Bering Sea and Sea of Japan in winter and in spring are not obvious.While the downtrend of sea ice in Bohai is also not obvious.At the same time,there are different periodic signals and turning years between different seasons in different seas.(2)Using the statistical method of empirical orthogonal function(EOF)analysis(including statistical significance),the typical spatial pattern and time coefficient of winter sea ice in Northeast Asia was decomposed.The explained variance of the first EOF(40.2% for EOF1,23.5% for EOF2 and7.6% for EOF3)is much larger than other EOFs.The spatial features of the first EOF mode is a dipole pattern between the Sea of Okhotsk(with the largest decrease of sea ice in the central-north Sea of Okhotsk)and the Bering Sea(with the largest variability in the eastern Bering Sea).And the decrease of sea ice in the Okhotsk Sea is the main feature of the sea ice change in Northeast Asia.The time coefficient showed a significant upward trend,and there was a mutation before and after 2004,which was basically in the positive phase after 2004.This indicate that the first model was the main characteristics of the sea ice change in Northeast Asia from 2004 to 2016.(3)Using regression analysis and synthetic analysis,it is found that the anomalies of sea ice in Northeast Asia have obvious local response by affecting the turbulence heat flux of sea ice and then affecting the temperature field on each level.Meanwhile there is a close relationship between sea ice anomalies and atmospheric circulation.From the top to the bottom,the atmospheric circulation is in a quasi-positive pressure state,indicating that the atmosphere is forcing sea ice changes.The possible mechanism of the action is that the anomalies in the circulation and wind field from the upper troposphere to the middle layer of the circulation field can transport downward,which affects the circulation system of the lower troposphere and the near stratum.The transmission will changes the sea surface temperature in Northeast Asia and eventually making the sea ice anomaly.(4)Using Singular Value Decomposition(SVD)analysis method,correlation analysis and synthetic difference analysis,the influencing factors of sea ice anomaly in Northeast Asia were discussed.It is found that the variation characteristics of sea ice in Northeast Asia are different and the impact factors are different.The influencing factors of sea ice anomaly in Bering Sea are summarized as follows: the transport of sea ice and the effect of cooling water in the Arctic Pacific sector,wind field(closed and transport),ocean currents(Qiandao cold currents,PDO),atmospheric circulation(polar vortex,Siberian high pressure and etc.)and the local cyclone anomaly(Aleutian low pressure).While the changes of sea ice in Sea of Okhotsk has a clear link with Siberian high pressure,polar vortex,East Asia and Ural trough and preSiberia and the middle of the temperature anomalies.Sea ice anomalies in Sea of Okhotsk is mainly controlled by the winter monsoon.But sea ice in Huang Bohai is mainly affected by polar vortex,Siberian high pressure and the impact of the early northern China's temperature.(5)By comparing the characteristics of sea ice in Arctic and in northeast Asia,it is found that there is a synchronous change between sea ice in Arctic and in Okhotsk,which is mainly because both of them are closely related to temperature changes in Arctic.The Arctic temperature can influence the sea ice in the Okhotsk Sea by affecting the temperature change over the Sea of Okhotsk.On the other hand,the mutation of sea ice extent in 2004 in Sea of Okhotsk was due to the anomalous warming of the Arctic and the temperature of the Okhotsk Sea after 2004,while the enhanced northward of the Siberian high pressure was an important factor affecting the Arctic temperature mutation,So the three influencing factors together make the mutation of sea ice in Sea of Okhotsk,eventually sea ice extent was always decreased after 2004.