Seasonal and interannual variability in the temperature structure around the tropical tropopause and its relationship with convective activities

Abstract

Seasonal and interannual variability in the tropical tropopause temperatures and its relationship with convective activities are examined by using the ECMWF 40 year reanalysis data and NOAA/OLR data. Low temperatures generally occur over the equator in the eastern hemisphere and extend northwestward and southwestward in the subtropics to form a horseshoe‐shaped structure. Because this structure resembles a stationary wave response known as the Matsuno‐Gill pattern, which is a superposition of the Rossby and Kelvin responses, the two preliminary indices are defined to represent the two responses. The horseshoe‐shaped structure index is then calculated from the two indices. The seasonal cycle in the horseshoe‐shaped structure index is significantly related to that observed in convective activities adjacent to three monsoon regions: the South Asian monsoon (SoAM) and the North Pacific monsoon (NPM) areas during the northern summer and the Australian monsoon (AUM) area during the southern summer. The convective activities in the SoAM and NPM areas individually influence the horseshoe‐shaped structure. During the northern summer, interannual variation in the horseshoe‐shaped structure index in the NPM area is related to that observed in convective activities associated with the El Niño–Southern Oscillation (ENSO) cycle with about a half‐year time lag. In the SoAM area, the variation is mainly controlled by isolated high temperatures, which are surrounded by the horseshoe‐shaped temperature structures and are not related to convective activities. During the southern summer, the horseshoe‐shaped structure index is related to convective anomalies associated with the ENSO cycle, shifting eastward in El Niño years.