Transient extratropical response to solar ultraviolet radiation in the Northern Hemisphere winter

An intermediate complexity general circulation model is used to investigate the transient response of the NH winter stratosphere to modulated ultraviolet (UV) radiation by imposing a stepwise, deliberately exaggerated UV perturbation and analyzing the lagged response. Enhanced UV radiation is accompanied by an immediate warming of the tropical upper stratosphere. The warming then spreads into the winter subtropics due to an accelerated Brewer-Dobson circulation in the tropical upper stratosphere. The poleward meridional velocity in the subtropics leads to an increase in zonal wind in midlatitudes between 208 and 508N due to Coriolis torque. The increase in midlatitude zonal wind is accompanied by a dipole in Eliassen-Palm flux convergence, with decreased convergence near the winter pole and increased convergence in midlatitudes (where winds are strengthening due to the Coriolis torque); this dipole subsequently extends the anomalous westerlies to subpolar latitudes within the first 10 days. The initial radiatively driven acceleration of the Brewer-Dobson circulation due to enhanced shortwave absorption is replaced in the subpolar winter stratosphere by a wave-driven deceleration of the Brewer-Dobson circulation, and after a month the wave-driven deceleration of the Brewer-Dobson circulation encompasses most of the winter stratosphere. Approximately a month after UV is first modified, a significant poleward jet shift is evident in the troposphere. The results of this study may have implications for the observed stratospheric and tropospheric responses to solar variability associated with the 27-day solar rotation period, and also to solar variability on longer time scales.