Recent changes in winter Arctic clouds and their relationships with sea ice and atmospheric conditions

Abstract

Changes in Arctic clouds during boreal winter (December through February) and their relationship with sea ice and atmospheric conditions in recent decades have been examined using satellite and reanalysis data and they are compared with output data from atmospheric general circulation model (AGCM) experiments. All the datasets from this study consistently show that cloud amount over the Arctic Ocean (north of 67°N) decreased until the late 1990s but rapidly increased thereafter. Cloud increase in recent decade was salient feature in the lower troposphere over a large part of the Arctic Sea, in association with obvious increase of lower tropospheric temperature and moisture. The comparison between the two periods before and after 1997 indicates that interannual-covariability of Arctic clouds and lower tropospheric temperature and moisture was significantly enhanced after the late 1990s. Large reduction of sea ice cover during boreal winter decreased lower tropospheric static stability and deepened the planetary boundary layer. These changes led to an enhanced upward moisture transport and cloud formation. The cloud change exerted considerable longwave radiative forcing to intensify the cloud？moisture？temperature relationship in the lower troposphere. Notice that careful interpretations are needed because spatio-temporal discrepancies of cloud fractions among the reanalyses and satellite products are generally high. Here the AGCM experiments under reduced sea ice conditions successfully reproduced the atmospheric changes over the Arctic including increases in cloud amount and lower tropospheric temperature and their enhanced covariability supporting those results obtained by satellite and reanalysis datasets.