Atmospheric Circulations Associated with Sea-Ice Reduction Events in the Okhotsk Sea

Wintertime sea ice cover in the Okhotsk Sea (OS) exhibits strong interaction with the atmosphere over the Far East and the North Pacific. As per previous studies, it has been determined that interannual variability of sea ice cover in the OS is associated with large-scale atmospheric circulations. However, the atmospheric processes responsible for rapid changes in sea ice cover in the OS on subweekly-to-weekly timescales are yet to be determined. Thus, in this study, we aim to investigate the atmospheric circulations that contribute to rapid reduction events of OS sea ice concentration (OSSIC) using daily high-resolution ocean reanalysis data. In total, we detected 21 rapid reduction events of OSSIC from 1993 to 2019. The reduction events shared common features in terms of atmospheric circulation, i.e., a developing extratropical cyclone over the southern OS and anomalous high pressure over the northern Bering Sea with strong surface southeasterly winds between the two. The strong southeasterlies, which blow in the opposite direction to the surface westerlies that regulate the seasonal development of sea ice cover, often result in the rapid reduction of OSSIC. Substantial reduction in sea ice was noted to occur in the northern and central OS owing to sea ice advection and sea ice melt associated with the easterly winds. The eastward-moving extratropical cyclone contributes both to rapid reduction of OSSIC and to reduction of sea level pressure over the northern North Pacific, resulting in a lagged relationship between OSSIC and the Aleutian Low.