Friday, February 16, 2018

Bering Sea Ice Loss

[Update Feb 17: the Bering Sea ice extent fell another 10,000 km2 yesterday; I've updated the chart.  Also, I added an animation of February 15 ice extent maps and a comment on St. Lawrence Island.]

In Tuesday's post I highlighted the strong ridge over Alaska that produced remarkable temperature inversions across the interior a week ago.  Another effect of the amplified circulation pattern is that the Bering Sea has been subject to strong and persistent southerly flow, and this has really done a number on the sea ice.

Bering Sea ice was already running at record lows for most of this year so far, and now the ice extent has dropped farther below the long-term normal than at any other time in the modern era.  The chart below illustrates this: the gray shading indicates the historical range of daily ice extent anomalies (departure from a 1979-2016 mean), and the red curve is this year's anomaly; as of yesterday it was more than 400,000 km2 below the mean for the first time.


I've annotated four previous years in which sea ice extent also dropped to very low levels in late winter and early spring, and it turns out that each of these was a La Niña winter (like this winter).  I'll do some more analysis later, but it seems clear that La Niña favors the kind of high-amplitude trough-ridge pattern that can bring disruptive southerly flow to the Bering Sea ice.

As an aside, it's interesting to note that the current ice loss episode has produced a decrease of 133,000 km2 in ice extent, or over 30%, in just 8 days.  As extreme as this seems, especially for mid-winter, it's not unprecedented; in the early February 1985 episode, the Bering Sea lost 161,000 km2 of ice in 10 days - but then gained it back, and more, in the next 10 days.  The difference this time, of course, is that we're starting from a very low point owing to the season-long shortfall in ice.

Here are the daily ice analyses from February 7 (left) and today (right), courtesy of NOAA.



Here's a simple animation of the February 15/16 sea ice extent maps from NSIDC (clipped to a suitable domain).  This gives a bit of context on interannual variability.  The maps for 1985, 1989, 2001, and 2018 are included separately below.







Note that St. Lawrence Island was only partially surrounded by ice in mid-February 2001, and this is similar to the current situation; webcam images from Savoonga and Gambell today both clearly showed open water.



5 comments:

  1. I updated the post with a few more graphics and a look at webcams on St. Lawrence Island.

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  2. Thanks Richard. This is very helpful. In 2001, note the much larger area with ice on the Russian coast. Rick

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  3. Presumably the lack of ice is mostly due to strong southerly winds bringing warm air and water into the strait. What relationship is there between "cumulative southerly wind days" and the lack of ice? Is there a threshold needed to get the big melts like this year and 2001?

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    1. Yes, warmth and strong winds. I plan to do any analysis on the weather pattern once the ice extent stabilizes... it was still falling as of yesterday, now 75% below the 1981-2010 normal!

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