The effect of direct anthropogenic aerosol forcing on the Great Plains Low Level Jet

The modification of regional circulations by anthropogenic aerosols has been noted in global model simulations, but aerosol-circulation responses have yet to be explored at very high model resolutions. This study evaluates the role of aerosol forcing over the United States, with a specific focus on the Great Plains Low Level Jet (GPLLJ). The GPLLJ provides the dominant source of moisture to the eastern continental United States that triggers warm season precipitation. Regional climate model simulations indicate that the anthropogenic aerosol loading in the eastern portion of the United States can induce a surface air temperature cooling over the southern Great Plains and a weakening of the GPLLJ. We conduct decadal-length simulations with a regional climate model (the ICTP RegCM4) at 25km resolution with and without anthropogenic aerosols including sulfate, black carbon and organic carbon. The presence of cooling sulfate aerosols on the eastern seaboard reduces the top of the atmosphere (TOA) forcing by about 4-8 W m-2. This reduction in forcing is also noted at the surface with greater magnitude, yet does not cause any significant change in surface air temperatures in this region. The most notable temperature response is centered on the southern Great Plains, which demonstrates a cooling of up to 0.5 K and a weakening of the GPLLJ circulation. This feature is robust across multiple summers and other model members using a different aerosol emission inventory. We hypothesize that the weakening of the GPLLJ is triggered by the aerosol cooling in the eastern half of the continent, reducing the west-to-east thermal gradient and thermal winds.