Zonally asymmetric temperature structure around the tropical tropopause and its relationship to deep convection

Tropical tropopause temperature is one of the most important factors which control the dehydration mechanism of air entering the lower stratosphere from the upper troposphere. Around the tropical tropopause during Northern Hemisphere (NH) summer and Southern Hemisphere (SH) summer, there exist zonally asymmetric temperature structures with a 'horseshoe' shape; cold anomalies on the equator extend to north-west and south-west, surrounding warm anomalies in the west of it. The structure is theoretically interpreted as the Matsuno-Gill pattern, which is probably induced by near equatorial diabatic heating such as due to convective activity and has Rossby gyres to the west of the heating and Kelvin waves to the east of it (e.g., Highwood and Hoskins [1998]). During NH summer, such a structure would be coupled with deep convection within the Asian summer monsoon region, and is also associated with the distribution of minor constituents in the upper troposphere and the lower stratosphere (Park et al. [2007]). During SH summer, deep convection in the Australia summer monsoon region may also result in such a characteristic atmospheric structure. In spite of possible influence of the horseshoe-shaped temperature structure on stratosphere-troposphere exchange, its spatial and temporal variation and relationship to deep convection have not clearly documented yet.

In this study, we investigate the temperature structure around the tropical tropopause, and its variability associated with near equatorial deep convection. We use the ERA-40 temperature data at 100hPa and the outgoing longwave radiation (OLR) data for the proxy of deep convection for about 23 years from January 1979 to August 2002. The southern oscillation index data are also examined because of the possible influence of the El Nino/Southern Oscillation (ENSO) on the interannual variation of deep convection. Paying attention to the structure, we define two indices; the latitudinal component of HorseShoe Index (HSIy) as the meridional temperature difference between the tropics and the equator, and the longitudinal component of HorseShoe Index (HSIx) as the zonal temperature difference on the equator. HSIy and HSIx represent for the strength of the Rossby and the Kelvin wave response, respectively. The negative values suggest that the horizontal distribution of cold anomalies represents a horseshoe shape.

Longitude-time sections for HSIy and HSIx show two prominent minima in NH summer around the Indian Ocean and in SH summer around the western pacific. The NH summer HSIy minima are extended more widely at longitude and have a larger absolute value than the SH summer ones. The seasonal variations of HSIx and OLR are closely related. Although the indices are defined at all longitude, we use the representative values of them as follows: the minimum HSIy value along the longitude coordinate, the minimum HSIx value in the region near the minimum HSIy value, and the minimum mean tropical (15S-15N) OLR value near the minimum HSIy value. We calculate correlation coefficients between HSIx and HSIy, HSIy and OLR, and HSIx and OLR for each of the NH and SH summers. In particular HSIx is associated with HSIy during NH summer and SH summer. In NH summer, the intraseasonal variation of HSIx, HSIy and OLR is dominant. Although the intraseasonal components of HSIx and HSIy are correlated, OLR is not related with them. In SH summer, the interannual variation of HSIx, HSIy and OLR is also dominant, and it is clearly related to ENSO. We can conclude that in NH and SH summers, the horseshoe-shaped temperature structures are dominant, and it would be produced by deep convection particularly in SH summer.