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Sea Ice and Freshwater Ice Assessments

2018 Sea Ice

Rick Thoman
20 May 2018

Note: This is an early assessment for late 2017 and early 2018. It will be updated and expanded periodically througout 2018.

2017-18 Bering Sea Ice Summary

The 2017-18 Bering Sea ice season was unprecedented in the lack of ice. The November 2017 through April 2018 mean extent from the National Snow and Ice Data Center’s passive microwave database (Regional Sea Ice Index Version 3) of 180,702 km2 is less than 35% of the 1981-2010 median (Figure 1). Based on the University of Alaska-Fairbanks Historical Sea Atlas, ice extent this low in the Bering Sea is unprecedented since 1850. While the causes of this extreme anomaly are under active research, preliminary work suggests contributions by an atmospheric flow pattern that was persistently stormy through most of the cold season and preconditioning of the Bering Sea, as sea surface temperatures have been above normal for much of the past several years.

Fig. 1. Bering Sea ice extent in the 2017-2018 winter season compared to the 1981-2010 median and the 2011-2012 winter.

The start of the ice season was quite late in the Bering Sea, coincident with record late ice-over of the Chukchi Sea. Sea ice was largely confined to protected bays and river delta regions until the last week of the November. Thereafter Norton Sound largely iced over during the first week of December, but ice was slow to form in the Bering Strait and St. Lawrence Island waters, and there was little ice in the Gulf of Anadyr. The second half of January into early February 2018 brought a comparative lull in storminess, allowing ice to form along the Alaska coast as far south as Bristol Bay and across Shpanberg Strait, though the ice failed to reach St. Matthew Island. However, storminess returned to the region starting the second week of February, causing more the half the areal extent of ice to be lost in less than two weeks due to melting and northward push of ice from the persistent southerly winds. The outcome of this included several days in mid-February when there was connected open water from Bering northward through the Bering Strait and into the Chukchi Sea. This also coincided with a strong storm that brought unheard-of late winter coastal flooding to Little Diomede, Alaska, in middle of the Strait, on February 20. The ice extent expanded and contracted a couple of times during March and early April before the final meltout during late April and early May. On May 4 the ice extent fell to less than 5% of the normal maximum, effectively marking “meltout”. This is again by far the earliest meltout of record, and six weeks earlier than the 1981-2010 mean meltout date of June 19.

The maximum ice extent in the Bering Sea for the season was reached on February 7 at just under 412,000 km2, less than half of normal and by far the lowest seasonal maximum in the 40 year passive microwave record. This is also unusually early for the Bering Sea ice extent maximum to occur (median date of maximum extent is March 15). Figure 2 shows the average (not maximum) daily ice extent over the months November - April in the Bering Sea since 1978.

Harder to quantify but of great importance to communities in the region, the quality of ice that was present was often difficult or impossible to work on. Stable shore fast ice failed to form or formed late and did not last nearly as long as usual. Many communities were forced to make changes in subsistence strategies, often at significantly increased cost and risk. In some places mobile ice was repeated pushed onshore, resulting in difficulty in getting boats to the ocean or hindered beach oriented gathering activities.

Fig. 2. Bering Sea average daily ice extent over the months November through April, for each year from 1978-79 to 2017-18.