Passive microwave satellite observations, available from 1979 to present, show declines in Arctic sea ice extent. Decreasing trends, while present in all months, are largest in September when the ice reaches its annual minimum extent. While the downward trend in the minimum ice extent has substantial inter-annual variability, the 2007 minimum extent represents a dramatic departure from the historical trend line (Stroeve et al., 2008).

We found that reduced cloudiness and enhanced downwelling radiation are associated with the unprecedented 2007 Arctic sea ice loss. Over the Western Arctic, total summertime cloud cover estimated from unique spaceborne radar and lidar data (Stephens et al., 2002) decreased by 16% from 2006 to 2007. The clearer skies led to downwelling shortwave (longwave) radiative fluxes increases of +32 Wm-2 (-4 Wm-2) from 2006 to 2007 (Figure 1). Over three months, simple calculations show that these radiation differences alone could enhance surface ice melt by +0.3 m, or warm the surface ocean by 2.4 K, which enhances basal ice melt. If one assumes that all of the additional heat absorbed by the ocean is used to melt sea ice in a marginal ice zone with 50% ice cover and 50% ocean cover, basal melting could be enhanced by +0.7 m.

Increased air temperatures and decreased relative humidity associated with an anti-cyclonic circulation pattern explain the reduced cloudiness. Indeed, the unprecedented 2007 sea ice extent confirms that an anti-cyclonic summertime atmospheric circulation pattern in the Western Arctic is a potent forcing for sea ice loss (Figure 2). Ogi and Wallace (2007) recently identified summertime circulation anomalies as being an important control on sea ice extent minima. Although Ogi and Wallace (2007) invoke anomalous wind stress associated with strong sea level pressure gradients and its affect on ice transport as the mechanism to link anti-cyclonic patterns and ice extent, our analysis points to additional contributing mechanisms. First, in addition to modifying ice transport, southerly wind anomalies enhance poleward atmospheric heat transport (Figure 2). Second, warm and dry conditions associated with the anti-cyclonic circulation lead to reduced cloudiness and enhanced downwelling shortwave radiation (Figure 1). Solar flux increases lead to more melt at the ice surface, but perhaps more importantly, they heat the surface ocean and thus can indirectly enhance basal ice melt. These ice melt factors may have also conspired during 2005, when like 2007, a record sea ice extent minimum was associated with an anti-cyclonic circulation pattern and reduced cloudiness.

Analysis of longer-term satellite and ground-based observations confirm the reduced cloudiness and enhanced downwelling shortwave radiation in 2007. Of particular interest, these records show that the 2007 cloudiness is anomalous in the recent past, but is not unprecedented (Figure 3). Thus, our results suggest that when sea ice is vulnerably thin, natural year-to-year variations in the summertime atmospheric circulation and associated changes in clouds and shortwave radiation can play an increasingly large role in modulating sea ice extent.