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

Abstract. The role of zonal structure in the stratospheric polar vortex for the surface response to weak vortex states 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 wave reflection events, 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. Wave-1 forcings with opposite phases (centered at 90E vs. 270E) induce contrasting influences in the stratosphere and troposphere, including oppositely shifted polar vortex and opposite structures of zonal wind anomalies. 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 phase-90 ensemble features pronounced cooling over Alaska and eastern Eurasia, along with enhanced rainfall concentrated over the North Pacific and the 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 the North Atlantic, stretching into subtropical Eurasia. By analyzing the mass streamfunction of the divergent component of the meridional wind, we observe opposite-signed zonal dipole patterns between the stratosphere and free troposphere, which further elucidates the pathway of stratosphere-troposphere coupling associated with SSWs and downward wave propagation events. In the lower troposphere, the meridional mass streamfunction is linked to the surface cooling and warming responses to both SSWs and wave reflection events, as reported in previous studies, by illustrating the meridional advection of cold and warm air masses. Overall, this study indicates that the observed surface response following SSWs and stratospheric wave reflection events is a genuine signal arising from, and causally forced by, stratospheric perturbations.