Epochal Westerly Changes Driven by Shifts in Anthropogenic Aerosol Emissions across Eurasia since 1940

Abstract The Eurasian subtropical westerly jet (ESWJ) is closely related to regional climate changes, such as heat waves over Eurasia. While the weakening of the ESWJ in summer since 1979 has been documented, earlier variations prior to the satellite era remain poorly understood. Here, using both reanalysis datasets and radiosonde observations, along with climate model simulations, we examine ESWJ changes from 1940 to 2020 and find that its post-1979 weakening is only part of a broader interdecadal shift driven by the time-evolving sulfate aerosol distributions. This epochal shift in the ESWJ further led to a transition from decreasing to increasing trends in heat wave durations across Eurasia in the late 1970s. Before 1979, extensive sulfate emissions in Eurasia modulated the tropospheric temperature structure, leading to increased wind speeds along and south of the jet axis (∼40°N). After 1979, reductions in sulfate aerosols over Europe and increased concentrations in Asia contributed to the weakening of the ESWJ. Additionally, moist adiabatic processes in the tropics, driven by rising greenhouse gas concentrations, favored an increase in ESWJ strength during both periods, partially offsetting the weakening induced by anthropogenic aerosol forcing after 1979. Our results highlight the crucial role of the time-evolving distribution of sulfate aerosols associated with different paces of industrial development across the Eurasian countries in driving the epochal westerly changes since the early twentieth century. Significance Statement The Eurasian subtropical westerly jet (ESWJ) plays a critical role in modulating climate and extreme events, but current studies on the ESWJ are limited to the satellite era. This study extends the analysis to 1940 and reveals that the observed weakening of the ESWJ since the beginning of the satellite era is only part of broader interdecadal changes driven by time-evolving sulfate aerosol distributions. Before 1979, extensive sulfate emissions strengthened the ESWJ, whereas the reduction of sulfate aerosols in Europe and increased aerosols in Asia contributed to its weakening since 1979. These findings underscore the complex interactions between anthropogenic aerosol distributions over different regions of Eurasia, westerly jet variability, and heat wave persistence.