The tropospheric response to zonally asymmetric momentum torques: implications for the downward response to wave reflection and SSW events

The role of zonal structure in the stratospheric polar vortex for the surface response to weak vortex states and concurrently occurring wave reflection events is isolated using an intermediate-complexity moist general circulation model. Zonally asymmetric wave-1 momentum torques with varying longitudinal phases are transiently imposed in the stratosphere to induce stratospheric sudden warmings (SSWs) and downward wave propagation, and the subsequent tropospheric and surface response is diagnosed. The response in these torque-induced SSWs is compared to 48 spontaneous SSWs in the control experiments. Wave1 forcings with opposite phases induce contrasting influences in the stratosphere and troposphere, including oppositely shifted polar vortex and opposite structures of wave reflection. Notably, downward wave propagation predominantly occurs over North America in the phase-90 ensemble, while primarily over North Eurasia in the phase-270 ensemble. These differences extend to surface responses: the phase90 ensemble features pronounced cooling over Alaska and eastern Eurasia, along with enhanced rainfall concentrated over the North Pacific and North Atlantic, extending to northwest Europe. In contrast, the phase-270 ensemble exhibits significant cooling over central North America and North Eurasia, accompanied by enhanced rainfall over the North Pacific and North Atlantic, stretching into subtropical Eurasia. By analyzing the mass streamfunction of the divergent component of the meridional wind, we observe oppositesigned zonal dipole patterns between the stratosphere and free troposphere, which further elucidates the pathway of stratosphere-troposphere coupling associated with SSWs and concurrent downward wave propagation events. The tropospheric jet begins to shift equatorward in the forcing stage and helps bridge the forced stratospheric signal with the nearsurface response. In the lower troposphere, the meridional mass streamfunction is linked to the surface cooling and warming responses to both SSWs and downward wave propagation events, as reported in previous studies, via the meridional advection of cold and warm air masses. Overall, this study indicates that the surface response following SSWs and concurrently occurring stratospheric wave reflection events is a genuine signal arising from, and causally forced by, stratospheric perturbations.