Sensitivity of the stratospheric circulation to the latitude of thermal surface forcing

Using the Chemistry Climate Model IGCM-FASTOC, we analyze the response in the Northern Hemisphere winter stratosphere to idealized thermal forcing imposed at the surface. The forcing is a 2K temperature anomaly added to the control surface temperature at all gridpoints within a latitudinal window of 10 or 30 degrees. The band-wise forcing is applied systematically throughout all latitudes of the Northern Hemisphere. Thermal forcing applied anywhere equatorwards of 20N, or continuously from the equator to 30N, increases the baroclinicity in the troposphere, leading to greater wave generation, and enhancing the flux of wave activity propagating vertically from the troposphere. Consequently, a greater flux of wave activity enters the mid- to high latitude stratosphere and breaks in the polar vortex, increasing the Brewer-Dobson circulation and leading to a warm anomaly in the polar stratosphere. Ozone concentration increases at high latitudes and decreases at low latitudes. Thermal surface forcing imposed between 30N and 60N has the reverse effect - decreased tropospheric baroclinicity together with reduced poleward deflection of vertically propagating wave flux - and leads to a stronger and colder vortex. Thermal forcing applied polewards of 60N has little effect on tropospheric baroclinicity, but results in a sufficient decrease of the vertical flux of wave activity for the vortex to be anomalously strong and cold. In all cases when surface forcing is imposed only polewards of 30N, ozone concentration decreases at high latitudes but is not affected at low latitudes. Combining the forcing in an equatorial and a mid-latitude band leads to a response similar to that of the equatorial forcing, demonstrating that the subtropical surface temperature changes determine the sign of the surface-driven response in the vortex.