Abstract
The Eurasian subtropical westerly jet (ESWJ) is a major feature of the summertime atmospheric circulation in the Northern Hemisphere. Here, we demonstrate a robust weakening trend in the summer ESWJ over the last four decades, linked to significant impacts on extreme weather. Analysis of climate model simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) suggests that anthropogenic aerosols were likely the primary driver of the weakening ESWJ. Warming over mid-high latitudes due to aerosol reductions in Europe, and cooling in the tropics and subtropics due to aerosol increases over South and East Asia acted to reduce the meridional temperature gradient at the surface and in the lower and middle troposphere, leading to reduced vertical shear of the zonal wind and a weaker ESWJ in the upper troposphere. If, as expected, Asian anthropogenic aerosol precursor emissions decline in future, our results imply a renewed strengthening of the summer ESWJ.
Highlights
The Eurasian subtropical westerly jet (ESWJ) is a major feature of the summertime atmospheric circulation in the Northern Hemisphere
Accompanying the weakening of the Eurasian subtropical westerly jet is an enhancement of the East Asian subpolar jet at about 60°N, indicating concurrent variation of the subtropical and subpolar jets over East Asia[12,13,16]
Our results have demonstrated a robust weakening of the summer ESWJ over the last four decades from 1979 to 2019
Summary
The Eurasian subtropical westerly jet (ESWJ) is a major feature of the summertime atmospheric circulation in the Northern Hemisphere. Warming over mid-high latitudes due to aerosol reductions in Europe, and cooling in the tropics and subtropics due to aerosol increases over South and East Asia acted to reduce the meridional temperature gradient at the surface and in the lower and middle troposphere, leading to reduced vertical shear of the zonal wind and a weaker ESWJ in the upper troposphere. Anthropogenic aerosols affect global and regional climate through aerosol-radiation and aerosol-cloud interactions[26] Because of their inhomogeneous spatial distributions, aerosols can cause changes in horizontal and vertical temperature gradients[27–35], which in turn affect atmospheric circulation, potentially including the strength and position of subtropical jet streams in the Northern[27,30–34] and Southern[34,35] Hemispheres. We use multimodel simulations from the coupled model intercomparison project phase 6 (CMIP6)[42] to identify the causes of this weakening trend (see Methods)
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