Identifying Dynamical Forcing and Cloud-Radiative Feedbacks Critical to the Formation of Extreme Arctic Sea-Ice Extent in the Summers of 2007 and 1996

Abstract:

Along with significant changes in Arctic climate system, the largest year-to-year variation in sea-ice extent has occurred in the Laptev, East Siberian, and Chukchi seas (define it here as the Area Of Focus, AOF), where two extremes of opposite signs were observed in the summers of 2007 and 1996. To untangle underlying forcing and feedbacks critical to the formation of the minimum (maximum) Arctic sea-ice extent of 2007 (1996), we examined in details the corresponding 2007 and 1996 anomalies of the large-scale atmospheric circulation and atmospheric physical parameters relevant to sea-ice variation utilizing satellite-derived sea-ice products and the NASA MERRA reanalysis. Our results indicate that, in addition to a triggering role of spring large-scale atmospheric circulation anomaly, a positive cloud-radiation-precipitable water vapor (PWV) feedback played the most important role in driving the sea-ice extent to a record low in the summer of 2007. Specially, atmospheric circulation change in spring induced anomalous southerly winds from the North Pacific not only advected more warm air, but also brought more water vapor to the AOF and formed more clouds. When cloud fraction (CF) was high and Arctic surfaces were covered by snow and ice, particularly during the onset of sea-ice melting (May-June), the cloud-greenhouse (LW) effect overwhelmed the cloud-albedo (SW) effect, producing a positive cloud radiative effect on surface radiation budget. Rising surface temperature subsequently enhanced evaporation and elevated atmospheric PWV, which drastically increased downwelling LW flux activating another positive feedback to the surface temperature. As sea-ice melting continued, additional SW (and LW) radiation was absorbed by open seas to increase surface temperature, and more water vapor evaporated to form more clouds, further accelerating sea-ice retreat through the positive cloud-radiation-PWV feedback. By contrast, when sea-ice reached an anomalously high extent in the summer of 1996, anomalous northerly winds and negative anomalies of surface temperature, CF, PWV, surface LW and total energy fluxes were reached. The findings reported here have major implications for improving the simulation of Arctic sea-ice variability and the related extremes of sea-ice extent in a climate model.