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Friday, November 29, 2013

Jennifer Francis: The where and when of wetter and drier: disappearing Arctic sea ice plays a role

Environmental Research Letter, 8 (2013) 041002; doi:10.1088/1748-9326/8/4/041002

The where and when of wetter and drier: disappearing Arctic sea ice plays a role


Summer precipitation in northern Europe has been above average for each of the past six years (2007–2012), a pattern that is unprecedented in over a century. During these same years, the summer Arctic sea-ice cover has averaged about 40% below its typical extent prior to the 1950s and set two new record minima. Could there be a connection? This is the question that motivated the new study by Dr. James Screen, a Research Fellow at the University of Exeter, UK, that appears in this issue of ERL (2013 Environ. Res. Lett. 8 044015). Adding to the growing body of evidence linking rapid Arctic warming to changing weather patterns in the northern hemisphere mid-latitudes, he concludes that sea-ice loss and associated surface warming lead to large-scale circulation patterns that favor wet summers in northern Europe and dry summers along the northern Mediterranean.

by Jennifer Francis
Dr. Screen used a state-of-the-art atmospheric model to compare the mid-latitude circulation during conditions of extensive sea ice (representative of the late 1970s) to that with much reduced sea ice (representative of present day). Except for the sea-ice extent and ocean temperatures where ice was lost, all surface conditions were fixed at climatological values in the model, thereby isolating the influence of sea-ice loss. Observation-constrained reanalysis fields were composited for the wettest and driest summers in northern Europe and used to verify the model simulations.
The atmospheric responses to reduced sea ice in both real and modeled worlds show not only precipitation patterns in Europe similar to those observed during the past six abnormally wet summers, but they also reveal features in the large-scale circulation that appear coincident with unusual weather patterns experienced elsewhere around the Northern Hemisphere in recent years. For example, the meridional wind anomalies near the jet-stream level (Figure 6a in Screen 2013) suggest that reduced sea ice favors enhanced ridging over the western North Atlantic, which is consistent with increased high-pressure blocking observed during June (Hanna et al. 2013) and expanded melting of the Greenland ice surface (Nghiem et al. 2012). The increased ridging implied by the pair of positive/negative anomalies over North America is consistent with recent drought and heat waves in the western part of the continent. Enhanced troughing in the eastern North Pacific agrees with an observed poleward shift in the Aleutian storm track (Bender et al. 2012). More generally, the patterns of response to sea-ice loss revealed in this investigation further support previous studies linking Arctic warming and ice loss with changes in the large-scale circulation (e.g., Overland et al. 2012, Screen and Simmonds 2013, Francis and Vavrus 2012, Petoukhov et al. 2013 and Cassano et al. 2013 and references therein).
This new study by Dr. Screen contributes additional evidence that Arctic sea-ice loss is partly responsible for shifting weather patterns, and provides new detail about the timing, location, and types of patterns that are expected to emerge as Arctic and global warming continue unabated.

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