Midlatitude Jet Position and Shift Linked to Atmospheric Convective Types

Abstract:

Midlatitude weather is largely governed by bands of strong westerly winds known as the extratropical jets, but what controls the jet properties, particularly their latitudes, remains poorly understood. Indeed, climate models show a spread of about eight degrees in the predicted present-day latitude of the Southern Hemisphere jet, and a related spread in its predicted poleward shift under global warming. Here we show that much of this disagreement can be explained by differences in the behavior of atmospheric moist convection in the midlatitudes, where shallow and deep convective types compete with one another. Shallow convection produces a moister midlatitude lower troposphere, leading to a warmer upper troposphere and a poleward jet position, particularly in summer. This also accounts for a link between today’s summertime jet and the climate-driven shift in the wintertime jet when the convective role is most temperature sensitive. These results show an unexpected role for atmospheric moist convection in the midlatitude circulation relevant to its future change under global warming. Observational constraints suggest that the wintertime poleward jet shift will be on the low side of past estimates from models.