The Influence of Arctic Amplification on Mid-latitude Atmospheric Circulation and Extreme Events

We expand on our recent work that provided evidence for a mechanism linking Arctic Amplification to changes in mid-latitude circulation patterns that favor extreme weather events (Francis and Vavrus, 2012). Here we analyze greenhouse-forced projections from the Community Climate System Model (CCSM4) to assess the future evolution of Arctic change on Northern Hemisphere weather patterns. We hypothesize that remote impacts of changes in the energy budget of the Arctic surface will influence the atmospheric circulation in middle latitudes through changes in large-scale, deep-tropospheric, meridional thickness gradients that induce generally weaker zonal flow and higher amplitude large-scale waves aloft. Because such features are slow-moving and associated with persistent weather conditions, they favor more frequent and severe extreme weather episodes resulting from prolonged cold-air outbreaks, heat waves, droughts, and heavy precipitation. The primary physical mechanism driving this change is an enhanced and seasonally varying Arctic heating: in fall/winter it is ocean-based due to substantial sea ice loss, while in warmer months it is land-based due to earlier snow melt and reduced soil moisture.

Simulations by CCSM4 support our hypothesized linkages, as the projected future climate changes depict a seasonally varying circulation response hinging on the enhanced warming and resulting geopotential height increases aloft in the Arctic. During boreal autumn and winter, sea ice loss leads to upper-air height increases mainly over the Arctic Ocean with compensating decreases over mid-latitudes, which reduces the poleward gradient. During spring and summer, however, the band of maximum anomalous ridging shifts southward over high-latitude land. This behavior resembles the upper-air circulation changes induced by prescribed reductions in sea ice and snow cover in previous versions of the model. The associated seasonal changes in mid-tropospheric zonal winds exhibit a nearly symmetrical reduction in mid-latitude westerlies in all seasons, suggesting a combination of weaker winds and perhaps a poleward shift in the flow. Weaker zonal winds slow the eastward progression of planetary waves, favoring more persistent weather conditions, thermal isolation of mid-latitude continents, and thus greater seasonality superimposed on an overall warming world. During summer, changes in upper-air geopotential heights are expected to act synergistically with projected reductions in soil moisture in middle latitudes to foster more frequent and intense heat waves and droughts. During winter a slowing and meridional elongation of planetary waves caused by Arctic Amplification favors cold-air outbreaks over the continents, thereby tempering the greenhouse-forced decline in extreme cold events and favoring regionally alternating snowy patterns and abnormal winter warmth that vary from year to year.