Arctic Cloud Changes During Intervals of Rapid Sea Ice Loss

We investigate the role of clouds during rapid sea ice loss events (RILEs) in the Arctic, as simulated by multiple ensemble projections of the 21st century in the Community Climate System Model (CCSM3). Trends in cloud properties and sea ice coverage during RILEs are compared with their secular trends between 2000–2049 during summer, autumn, and winter. The results suggest that clouds promote abrupt arctic climate change during RILEs through increased (decreased) cloudiness in autumn (summer) relative to the changes over the first half of the 21st century. The trends in cloud characteristics (cloud amount, water content, and radiative forcing) during RILEs are most strongly and consistently an amplifying effect during autumn, whereas the trends are more highly variable among RILEs during winter. The total cloud trends in every season are primarily attributable to low clouds, which show a more robust response across RILEs than do middle and high clouds. Lead-lag correlations of monthly sea ice concentration and cloud cover during autumn demonstrate that the relationship between less ice and more clouds is enhanced during RILEs, but we find no evidence on monthly time scales that either variable is leading the other. Given that Arctic cloud projections in CCSM3 are similar to those from other state-of-the-art GCMs and that observations show increased autumn cloudiness associated with the extreme 2007 and 2008 sea ice minima, this study suggests that the ongoing decline in arctic sea ice will be enhanced by changes in polar clouds.