The Role of the Arctic in Promoting Drought and Heat Waves over Mid-latitude Continents

Climate models typically simulate enhanced heating and pronounced drying during summer over mid-latitude continental regions of the Northern Hemisphere under greenhouse forcing. Various plausible explanations have been offered for this response, including strengthened land-sea temperature contrasts, favorable SST patterns, and locally depleted soil moisture. Changes in the large-scale atmospheric circulation have also been proposed, but these have generally been regarded as secondary mechanisms originating in low and middle latitudes.

Here we present an alternative perspective, by proposing that a major reason for the mid-latitude continental response is an atmospheric circulation change that is regulated by high-latitude processes. Based on an analysis of the RCP8.5 scenario in the Community Climate System Model (CCSM4), we find that the amplified heating ( > 7 K) and drying ( > 20%) of the U. S. Great Plains during summer stems from a shift toward locally weakened westerlies aloft and somewhat stronger northerlies. This circulation change is directly tied to enhanced ridging to the north of the region that is part of a nearly hemispheric-wide band of ridging in high latitudes extending from Eurasia across North America. This band of maximum geopotential height increases aloft is well correlated with the location of greatest summertime snow cover loss in northern Siberia and North America. The circulation pattern appears to be further modulated in high latitudes by residual sea ice coverage around the Canadian Archipelago and by a weakening of the Atlantic Meridional Overturning Circulation. Both of these changes promote troughing locally around northeastern Canada and thus a southward displacement over North America of the high-latitude ridging band to a location where its outflow favors drier and hotter conditions in the continental interior. The resulting circulation shift affects not only the mean summertime climate but also sets up very suitable synoptic conditions for extreme weather events in the form of droughts and heat waves.