Alaska experienced record-setting warmth during the 2015-16 cold season (October-April). Statewide average temperatures exceeded the period-of-record mean by more than 4°C over the seven-month cold season and by more than 6°C over the four-month late-winter period, January-April. The record warmth raises two questions: (1) Why was Alaska so warm during the 2015-16 cold season? (2) At what point in the future might this warmth become typical if greenhouse warming continues? On the basis of circulation analogs computed from sea level pressure and upper-air geopotential fields, the atmospheric circulation explains less than half of the anomalous warmth. The warming signal forced by greenhouse gases in climate models accounts for about 1°C of the of the anomalous warmth. A factor that is consistent with the seasonal and spatial patterns of the warmth is the anomalous surface state. The surface anomalies include (1) above-normal ocean surface temperatures and below-normal sea ice coverage in the surrounding seas from which air advects into Alaska and (2) the deficient snowpack over Alaska itself. The increase of the “excess warmth” in late winter implicates snow cover and its albedo effect, which is supported by observational measurements in the boreal forest and tundra biomes. Climate model simulations indicate that warmth of this magnitude will become the norm by the 2040s and 2015s if greenhouse gas emissions follow their present scenario. We will show how the more recent “normal” winter of 2016-17 fits supports the conclusions.
Publication in the Journal Climate - The Exceptionally Warm Winter of 2015/16 in Alaska