Attributing the Recent Weakening of the South Asian Subtropical Westerlies
In recent decades, the changes in the southwesterly monsoon circulation have been documented in conjunction with regional changes in temperature. Changes in subtropical westerlies have received less attention, and the roles of external forcing and individual anthropogenic contributions relative to internal variability have not been addressed.
Trends in a range of climate model simulations suggest that the weakening is driven by multiple anthropogenic forcings, including anthropogenic aerosols, greenhouse gases (GHGs), and changes in land-use and land cover (LULC). Over the same period, sea-level pressure has increased by 0.6-1.0 hPa over South Asia's north-western regions, induced by cooling due to aerosol emission and LULC changes, and has decreased over the Arabian Peninsula (AP) mainly due to warming by GHGs. These changes in temperature and pressure act to weaken the regional pressure gradient, deflecting the sub-tropical westerlies from South Asia towards the AP and weakening the winds in the monsoon trough and its adjacent regions.
Our analysis identifies anthropogenic climate forcings as central to the recent weakening of Asian monsoon westerly winds, which has consequences for renewable energy generation and the hydrologic cycle. This study also sets the stage for subsequent work aimed at understanding the forced changes anticipated for the coming decades.