Sensitivity of the Stratospheric Circulation to the Latitude of Thermal Surface Forcing

• Published in: Journal of climate 2011-10, Vol.24 (20), p.5397-5415
• Main Authors: Winter, Barbara, Bourqui, Michel S.
• Format: Article
• Language: English
• Subjects: Atmosphere; Atmospheric chemistry; Atmospheric circulation; Climate change; Climate models; Earth, ocean, space; Equator; Exact sciences and technology; External geophysics; Fluctuations; Heat flux; Latitude; Meteorology; Northern hemisphere; Ozone; Stratosphere; Surface temperature; Temperature; Temperature control; Tropopause; Troposphere; Winter
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/2011JCLI4006.1

Using the chemistry climate model Intermediate General Circulation Model–Fast Stratospheric Ozone Chemistry (IGCM-FASTOC), the authors analyze the response in the Northern Hemisphere winter stratosphere to idealized thermal forcing imposed at the surface. The forcing is a 2-K temperature anomaly added to the control surface temperature at all grid points within a latitudinal window of 10° or 30°. The bandwise forcing is applied systematically throughout all latitudes of the Northern Hemisphere. Thermal forcing applied anywhere equatorward of 20°N, or continuously from the equator to 30°N, increases planetary-wave generation in the troposphere and enhances the flux of wave activity propagating vertically into the stratosphere. Consequently, a greater flux of wave activity 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 30° and 60°N has the reverse effect—decreased planetary-wave generation in the lower troposphere and reduced vertically propagating wave flux entering the stratosphere—and leads to a stronger and colder vortex. Thermal forcing applied poleward of 60°N has little effect on the tropospheric mean state but nonetheless decreases the planetary-scale eddy heat flux from the surface to the tropopause, resulting in a sufficient decrease of the vertical flux of wave activity for the vortex to be anomalously strong and cold. When surface forcing is imposed only poleward of 30°N, ozone concentration decreases at high latitudes but is not affected at low latitudes. Combining the forcing in an equatorial and an extratropical 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.