[A-CG33] Extratropical oceans and atmosphere

[ACG33-05] Atmospheric circulations associated with rapid sea-ice reductions in the Okhotsk Sea

*Youichi Kamae¹, Hiroaki Ueda¹, Tomoshige Inoue¹, Humio Mitsudera² (1.Faculty of Life and Environmental Sciences, University of Tsukuba, 2. Institute of Low Temperature Science, Hokkaido University)

Keywords:sea ice, extratropical cyclone, the Okhotsk Sea, Western Pacific pattern

The Okhotsk Sea (OS) is characterized by a large fraction of sea ice cover from November to June. Spatial distribution of the sea ice concentration (SIC) is contributed by wind stress, ocean current, and river discharge. The interannual variability of SIC in OS (OSSIC) has an important role in the atmospheric circulation through modulations of the surface heat budget. Previous studies pointed out that the interannual variability of OSSIC is contributed by the East Asian winter monsoon, Aleutian Low, Arctic Oscillation, North Atlantic Oscillation, and Western Pacific (WP) pattern, through the analyses of monthly or seasonal-mean OSSIC. In this study, we examine sea ice reduction events based on daily SIC data obtained from GLORYS12V1. We also obtained similar results if we use SIC data from OISST version 2. We found the rapid reduction events based on OSSIC time series. On 25 December 2017, as an example, twin extratropical cyclones over Japan were found at the surface with a ridge and a trough at the upper troposphere east and west of Japan. From 26 to 30, the WP pattern with a north-high south-low geopotential pattern was dominant. A surface extratropical cyclone developed rapidly and brought strong easterly over the OS. We detected similar rapid reduction events from 1993/94 winter to 2018/19 winter. Results of composite analyses based on the detected 21 events indicate that a surface low pressure over northern Japan and a high pressure over the OS, Siberia, and Aleutian Islands and easterly winds over the OS are found during these events. All the 21 events were accompanied with the north-high south-low geopotential height pattern sustained for more than two days. The results of this study indicate that the surface easterly associated with the geopotential height pattern pushes the sea ice westward, resulting in the rapid OSSIC reduction events.