[A-CG46] Science in the Arctic Region

[ACG46-P04] Interannual variability of sea ice melt in the southern Sea of Okhotsk, inferred from spring salinity data

*Mariko Honda¹, Kay I. Ohshima^2,3, Vigan Mensah², Jun Nishioka^2,3, Stephen Riser⁴, Masatoshi Sato⁵ (1.Graduate School of Environmental Science, Hokkaido University, 2.Institute of Low Temperature Science, Hokkaido University, 3.Arctic Research Center, Hokkaido University, 4.University of Washington, 5.Wakkanai Fisheries Research Institute, Hokkaido Research Organization)

Keywords:sea ice melt, global warming, the Sea of Okhotsk

The Sea of Okhotsk is the southernmost seasonal sea ice zone in the Northern Hemisphere. Sea ice is mainly produced in the coastal polynya in the northwestern part, and advected to the south by the prevailing northernly wind and the East Sakhalin Current. In spring, melting sea ice supplies freshwater and negative heat to the ocean surface in the southern part of the Sea of Okhotsk. There have been no quantitative estimations of sea ice melt from observations. In this study, we estimate the amount of sea ice melt using the spring hydrographic data in the Sea of Okhotsk. When sea ice melt, the ocean surface is considerably freshened. We estimate the amount of ice melt by vertically integrating the salinity deficit between the surface and the top of the winter water layer. Only 20 % of the data that actually exhibit a salinity deficit and can be used for the estimation. The result shows that the average ice melt amount is 62 cm thick in the southern part (south of 48 deg. N) of the Sea of Okhotsk. The associated freshwater flux is comparable to the discharge of the Amur River. An analysis of time series of yearly ice melt amounts in the southern part of the Sea of Okhotsk reveals a decreasing trend of -15 cm in ice thickness for the past 40 years. In addition to the estimation of sea ice melt from the salinity deficit, we independently created a gridded data set of April-May salinity for two periods, before and after 1990. The comparison of these two data sets shows that the salinity from the surface to the depth of 50 m has been increasing in the southern part of the Sea of Okhotsk. This may be due to the decrease in freshwater supply by the reduced sea ice melt. This salinity change corresponds to a decrease in ice melt amount of 14 cm in thickness. The two different methods of ice melt estimation provide similar values for the decreasing trend, which enhances the reliability of the estimation.