The Role of Arctic Clouds During Intervals of Rapid Sea Ice Loss

The Arctic climate system appears to be veering quickly toward a much warmer, less icy state. GCMs have long projected this kind of shift, though the simulated rate of change and the amount of polar amplification are model-dependent. Some climate models also produce periods of rapid future climate change superimposed on the long-term warming trend. These intervals coincide with abrupt reductions in sea ice and have been dubbed "rapid ice loss events" (RILEs). Previous work has identified the existence of RILEs in several GCMs and attributed them in part to pulse-like increases in meridional ocean heat transport and the positive surface albedo feedback. Here we present new evidence that clouds may also play an important role in RILEs, based on ensemble simulations using NCAR's Community Climate System Model (CCSM3). Consistent with most GCMs, the CCSM3 projects a gradual increase in future cloudiness over the Arctic, as greenhouse warming takes hold this century. Although the simulated cloud increase occurs in all seasons, it is greatest during autumn and winter (when polar clouds exert the strongest surface warming signal) and appears to be driven mainly by enhanced evaporation from the warming Arctic Ocean. During one simulated RILE in a transient greenhouse simulation, the cloud coverage in CCSM3 varies seasonally to optimize the surface warming and promote ice melt. In this event, cloud anomalies become positive (negative) during autumn-winter (summer), such that both longwave and shortwave radiation at the surface are enhanced. This favorable combination for ice loss results in a sustained interval of positive cloud radiative forcing (CRF) anomalies that hinder ice growth and promote ice melt. We suggest that increased surface evaporation associated with sea ice retreat may be important for initiating and/or maintaining RILEs, in addition to stochastic variations in atmospheric circulation that occasionally favor seasonally optimal combinations of anomalous cloud amount.