Rapid autumn Arctic warming and boundary-layer cloud increases in the early 21st century

The Arctic warming reported in surface air temperature (SAT) occurs non-uniformly with season with the strongest increase in autumn. Studying the data acquired by MISR during 2000-2009, we find that boundary-layer cloudiness in October has been increasing rapidly over the Arctic Ocean in the past decade, at a striking rate of 2.18 % per year over the Beaufort and East Siberian Seas. The distribution of the increased cloudiness corresponds well to the strongest warming in this region. Because low clouds can effectively trap longwave radiation in cold seasons, the new observations provide strong evidence for the suggested positive cloud-temperature feedback over the Arctic Ocean. Further analysis with NCEP/NCAR reanalysis data suggests that the boundary-layer cloud increases likely contribute more than water vapor to the intense warming seen in 2000-2009.