Relative Roles of Surface Boundary Forcings in Weakening Summer Circulation and Increasing Extreme Heat Waves since 1979

North Hemisphere (NH) summer circulation has recently significantly weakened, and made weather more persistent and hence favored the occurrence of prolonged heat extremes. Arctic sea ice, snow cover decline and SST warming have coincided with weakening summer circulation and an increasing number of extreme heat waves and other destructive weather events in the NH midlatitudes in summer. It is important to examine how such high-impact events are related to changes of boundary forcing. We use atmospheric model ensemble simulations to attribute effects of surface boundary forcing on observed summer climate trends since 1979. The ongoing greenhouse gas buildup and resulting sea surface temperature warming outside the Arctic explains nearly all land warming and a significant portion of observed weakening zonal winds in the NH midlatitudes. Meanwhile, spring and summer snow cover decline also significantly contribute to surface air temperature increases and zonal wind decreases in the NH midlatitudes. In contrast, sea ice loss has induced a negative Arctic Oscillation(AO)-type circulation with summer surface and tropospheric cooling trends over large portions of the NH midlatitudes, and with strengthening zonal winds in the NH subtropics.