Midlatitude jet position spread linked to atmospheric convective types

Abstract

Abstract Midlatitude weather is largely governed by bands of strong westerly winds known as the midlatitude jets, but what controls the jet properties, particularly their latitudes, remains poorly understood. Climate models show a spread of about ten degrees in their simulated present-day latitude of the Southern Hemisphere (SH) jet, and a related spread in its predicted poleward shift under global warming. We find that models with more poleward jets simulate more low-level moisture, a warmer upper troposphere, and different precipitation patterns than those with equatorward jets, potentially implicating inter-model differences in moist convection and microphysics. Accordingly, a suite of atmospheric model runs is performed where the deep or shallow convective parametrizations are individually turned off either globally or in specific latitude bands. These experiments suggest that models that produce more shallow convection in the midlatitudes tend to position the jet relatively poleward in SH summer, whereas those which favor deep convection tend to position it equatorward. This accounts for a spread 60% as large as that of AMIP during the Austral summer. Our results suggest that, in the Boreal summer, similar biases appear in the northern hemisphere. The presence of shallow convection in the Northern Hemisphere midlatitudes reduces SH jet shift in a warmer climate in accordance to the correlation between jet positions and shift seen in this season. These results can help explain inter-model differences in the position and shift of the jet, and point to an unexpected role for atmospheric moist convection in the midlatitude circulation.