Warm winds from the Pacific caused extensive Arctic sea-ice melt in summer 2007
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During summer 2007 the Arctic sea-ice shrank to the lowest extent ever observed. The role of the atmospheric energy transport in this extreme melt event is explored using the state-of-the-art ERA-Interim reanalysis data. We find that in summer 2007 there was an anomalous atmospheric flow of warm and humid air into the region that suffered severe melt. This anomaly was larger than during any other year in the data (1989–2008). Convergence of the atmospheric energy transport over this area led to positive anomalies of the downward longwave radiation and turbulent fluxes. In the region that experienced unusual ice melt, the net anomaly of the surface fluxes provided enough extra energy to melt roughly one meter of ice during the melting season. When the ocean successively became ice-free, the surface-albedo decreased causing additional absorption of shortwave radiation, despite the fact that the downwelling solar radiation was smaller than average. We argue that the positive anomalies of net downward longwave radiation and turbulent fluxes played a key role in initiating the 2007 extreme ice melt, whereas the shortwave-radiation changes acted as an amplifying feedback mechanism in response to the melt.
KeywordsArctic Sea ice Energy transport Greenhouse effect Surface-albedo feedback
The authors are thankful to Per Pemberton, SMHI, for providing sea-ice model data and to John Walsh, Mxolisi Shongwe, Frank Selten, and Robert Pickart for useful comments on the manuscript. R. Graversen is funded by Ministry of Transport, Public Works and Water Management, The Netherlands, within the project Abrupt Climate Scenarios, and by International Meteorological Institute (IMI), Sweden. Michael Tjernström is supported by the DAMCOLES EU 6th Framework research program. The ERA-Interim reanalysis data was obtained from the ECMWF data server at http://www.ecmwf.int/, whereas NCEP-Reanalysis 2 data are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their web site at http://www.esrl.noaa.gov/psd/.
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