Interhemispheric asymmetry in the seasonal variation of the zonal mean tropopause

Abstract

The SUNYA Atmospheric Models Intercomparison Project 2 (AMIP 2) [Gates et al., 1999] simulation, which is based on NCAR/CCM3 with SUNYA ozone archive [Wang et al., 1995; Liang et al., 1997], is used to study the characteristics of the zonal and monthly mean tropopause. For comparison purposes, the NCEP/NCAR reanalysis data are also analyzed. Both data sets show strong interhemispheric asymmetry in the seasonal variation of tropopause. In addition, the physical and dynamical mechanisms driving the tropopause variation are studied by diagnosing the seasonal heating/cooling components affecting the temperature tendencies around the tropopause. Interhemispheric asymmetry in the seasonal variation of tropopause is related to the asymmetry in temperature, wave activity, and deep convective activity between the two hemispheres. In the Northern Hemispheric (NH) polar region during spring, the warming in the lower stratosphere ends earlier and thus yields a rising tropopause in the coming summer; however, the polar tropopause in the Southern Hemisphere (SH) sinks during SH summer. This asymmetry is attributed to the stronger thermal infrared cooling associated with warmer temperatures in the NH lower stratosphere. In the NH subpolar region, temperature tendencies are greater below the tropopause during spring and autumn; however, they are greater above the subpolar tropopause in the SFI during SH spring and autumn. As a result, the tropopause in the NH subpolar region varies in the direction opposite to that in the SH subpolar region in these seasons. This asymmetry is associated with a stronger dynamical warming in the NH during spring and a stronger thermal infrared cooling in the NH during autumn. In middle latitudes the variation of tropopause is symmetric between the two hemispheres during the respective seasons, but the NH tropopause varies with larger amplitudes. This asymmetry in amplitudes is related to the greater NH wave activity, which induces a greater warming above the tropopause in winter through the descending branch of the Brewer‐Dobson circulation, and the greater NH deep convective activity, which induces a greater warming below the tropopause in summer through latent heat release. The greater NH stratospheric wave activity also results in larger amplitudes of the tropopause variation in the SH subtropics, because of the greater cooling above the tropopause induced by the ascending branch of the Brewer‐Dobson circulation in NH winter. Compared to the situation in middle latitudes, latent heat release in the tropics plays a less direct role in driving the seasonal variation of the tropical zonal mean tropopause.