Wednesday, March 7, 2018

Sea Ice and Southerly Winds

Last month's look at ice loss in the Bering Sea provoked a comment from reader Eric about the impact of southerly winds on ice extent, so I thought it would be interesting to make a quick comparison of this winter to previous years in this respect.

The chart below shows some November-February average data from the north-central part of the Bering Sea.  Based on reanalysis data from a grid point near St. Lawrence Island, the black line shows mean temperature at 925 mb (about 2000' above sea level), and the blue line shows the southerly component of the wind at the same height - so negative wind values indicate northerly flow (the usual situation).  The red line shows the southerly wind component observed at the surface at Gambell and Savoonga, based on hourly observations since 1996.


It's interesting to note that this winter so far has produced the record highest mean temperature and southerly wind component at 925mb, according to the reanalysis; the average 925mb wind still had a northerly component, but barely.  The surface winds from the two observing sites on the island are highly correlated with the 925mb winds, and the surface data reveal a positive mean southerly component for the first time on record.  It's no wonder that ice extent has suffered with such an absence of cold northerly flow.

Here's an update of the chart from the last post, showing that the ice extent staged a modest recovery after its collapse in February, but in recent days it has turned down again.


Obviously the ice situation is now well outside the historical range from the satellite era, and this is partly because there was such little ice cover prior to the recent episode of ice loss.  The magnitude of the recent loss is not in itself unprecedented, as 6 previous years since 1979 saw at least as much ice loss over a 15-day period - although presumably it's "easier" to lose a lot of ice when it's being removed from warmer locations in the southern Bering Sea.

The maps below show sea-level pressure anomalies from the aforementioned 15-day periods in which ice loss was greater than the recent episode, and the last map shows this year's anomaly.  The common feature is obviously an enhanced southerly flow between high pressure over the Gulf of Alaska and strong low pressure over the western Bering Sea or far eastern Russia.


The unusual flow pattern appears to be partly related to the modest La Niña anomaly that continues to be evident in the tropical Pacific; the map below shows the average February-March sea-level pressure anomaly for 10 previous years when weak-moderate (but not strong) La Niña conditions were observed in late winter.  Strong La Niña episodes tend not to be as favorable for ice loss, because the PDO is more often negative in those winters and the Bering Sea tends to be more dominated by high pressure (and thus it's colder in Alaska).


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