Impact of Reduced Arctic Sea Ice on Northern Hemisphere Climate and Weather in Autumn and Winter

The Northern Hemisphere transient atmospheric response to Arctic sea decline is investigated in autumn

and winter, using sensitivity experiments performed with the CNRM-CM6-1 high-top climate model. Arctic sea ice albedo is

reduced to the ocean value, yielding ice-free conditions during summer and a more moderate sea ice reduction during the

following months. A strong amplification of temperatures over the Arctic is induced by sea ice loss, with values reaching up

to 25°C near the surface in autumn. Significant surface temperature anomalies are also found over the midlatitudes, with a

warming reaching 1°C over North America and Europe, and a cooling reaching 1°C over central Asia. Using a dynamical

adjustment method based on a regional reconstruction of circulation analogs, we show that the warming over North

America and Europe can be explained both by changes in the atmospheric circulation and by the advection of warmer

oceanic air by the climatological flow. In contrast, we demonstrate that the sea ice–induced cooling over central Asia is

solely due to dynamical changes, involving an intensification of the Siberian high and a cyclonic anomaly over the Sea of

Okhotsk. In the troposphere, the abrupt Arctic sea ice decline favors a narrowing of the subtropical jet stream and a slight

weakening of the lower part of the polar vortex that is explained by a weak enhancement of upward wave activity toward the

stratosphere. We further show that reduced Arctic sea ice in our experiments is mainly associated with less severe cold

extremes in the midlatitudes.