Abstract
Abstract. This study investigates the effect of sea ice reduction on Arctic cloud cover in historical simulations with the coupled atmosphere–ocean general circulation model MIROC5. Arctic sea ice has been substantially retreating since the 1980s, particularly in September, under simulated global warming conditions. The simulated sea ice reduction is consistent with satellite observations. On the other hand, Arctic cloud cover has been increasing in October, with about a 1-month lag behind the sea ice reduction. The delayed response leads to extensive sea ice reductions because the heat and moisture fluxes from the underlying open ocean into the atmosphere are enhanced. Sensitivity experiments with the atmospheric part of MIROC5 clearly show that sea ice reduction causes increases in cloud cover. Arctic cloud cover increases primarily in the lower troposphere, but it decreases in the near-surface layers just above the ocean; predominant temperature rises in these near-surface layers cause drying (i.e., decreases in relative humidity), despite increasing moisture flux. Cloud radiative forcing due to increases in cloud cover in autumn brings an increase in the surface downward longwave radiation (DLR) by approximately 40–60 % compared to changes in clear-sky surface DLR in fall. These results suggest that an increase in Arctic cloud cover as a result of reduced sea ice coverage may bring further sea ice retreat and enhance the feedback processes of Arctic warming.
Highlights
Satellite observations have shown that Arctic sea ice has decreased gradually since the 1980s (Comiso et al, 2008)
3 Results 3.1 Simulated change of Arctic sea ice and clouds decrease from the 1970s was common in all ensemble members
The seasonal minimum sea ice area (SIA) occurs in September, and Arctic sea ice cover generally begins to recover in October
Summary
Satellite observations have shown that Arctic sea ice has decreased gradually since the 1980s (Comiso et al, 2008). A major feedback process in climate change is the ice–albedo feedback, in which reduced sea ice decreases the global albedo and increases shortwave radiation entering the climate system (e.g.,Curry et al, 1995; Dickinson et al, 1987; Manabe and Stouffer, 1980; Perovich et al, 2007). This feedback is likely to occur in high-latitude regions, where snow cover and sea ice are seasonally extended. As Yoshimori et al (2014) mentioned with regard to the climate model results that Arctic surface warming in autumn-winter is attributed to a seasonal reduction in ocean heat storage and an increased cloud greenhouse effect, other processes such as ocean heat uptake, atmospheric stability, and low-level cloud response may require further attention to better understand the Arctic warming mechanism
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