Causes of Sea ice Extremes in the Pacific Arctic During Winter and Spring 2018

An extremely early sea ice retreat in the spring 2018 in the Bering Sea followed a late sea ice freeze up in the Chukchi Sea in the Pacific Arctic. The duration (defined as >20% areal coverage) of the sea ice in a 50km by 50km box centered around the M8 mooring in the SE Bering Sea has 0 days for the first time in the satellite era, where historically it has at least 100 days of ice cover and 150days in the 1990s. From November 2017 to April 2018 an unusually warm atmosphere lingered above the Chukchi Sea and Bering Sea domain. A strong south-southeast flow, contrary to its climatological northerly flow pattern, dominated the northern Bering Sea in February 2018 limiting the sea ice from advancing farther into the Bering Sea. Late Chukchi freeze up and February SSE winds are the two major factors contribute to this extreme low ice conditions in the Bering Sea. These large scale weather conditions prolonged impacts on the ocean conditions such as warmer near-bottom temperatures and further, on the components of ecosystem, such as the timing of reproduction of copepods (Calanus spp). This case study shows changes of sea ice resulting from unusual atmospheric conditions in the context of historical uniqueness and future probability. In particular, the timing of sea-ice freeze-up in the Chukchi Sea may be a precursor for continued unusual Pacific Arctic conditions.