Sensitivities of the Lower - Stratospheric Transport and Mixing to 1 Tropical SST Heating

5 The sensitivities of the Brewer-Dobson Circulation (BDC) to different distributions of trop6 ical SST heating are investigated in an idealized aqua-planet model. It is found that an 7 increase in tropical SSTs generally leads to an acceleration of tropical upwelling and an as8 sociated reduction in the Age of Air (AOA) in the polar stratosphere, and that the AOA 9 near the subtropical tropopause is correlated with local isentropic mixing of tropospheric air 10 with stratospheric air. 11 The zonal distribution of SST perturbations has a major impact on the vertical and 12 meridional structure of the BDC as compared with other SST characteristics. Zonally lo13 calized SST heatings tend to generate a shallow acceleration of the stratospheric residual 14 circulation, enhanced isentropic mixing associated with a weakened stratospheric jet, and a 15 reduction in AOA mostly within the polar vortex. By contrast, SST heatings with a zon16 ally symmetric structure tend to produce a deep strengthening of the stratospheric residual 17 circulation, suppressed isentropic mixing associated with a stronger stratospheric jet, and a 18 decrease of AOA in the entire stratosphere. 19 The varied depth of the stratospheric residual circulation change has been linked to 20 wave propagation and dissipation in the subtropical lower stratosphere rather than wave 21 generation in the troposphere. More specifically, the zonal wind response to nearly-zonal SST 22 heating is characterized by an upward shift of the subtropical jet, which in turn refracts the 23 planetary waves equatorward and upward in the lower stratosphere, leading to a deep change 24 in the residual circulation. The stronger upward-shifted subtropical jet also yields a more 25 robust transport barrier associated with weaker isentropic mixing. These results suggest that 26 while the longitudinally localized SST trends under climate change may contribute to the 27 change in the shallow branch of the BDC, the upward shift of the subtropical jet associated 28 with the zonal SST heating can impact the deep branch of the BDC. 29