Intraseasonal variations in the tropical tropopause temperature revealed by cluster analysis of convective activity

Space‐time variations of the tropical convective activities and temperatures around the tropical tropopause associated with the intraseasonal oscillation (ISO) are investigated using outgoing longwave radiation (OLR) data from the National Oceanic and Atmospheric Administration and atmospheric fields from the European Centre for Medium‐range Weather Forecasts Interim reanalysis data. By performing cluster analysis using the locus of convective activities observed in the unfiltered OLR field, the 72 ISO events selected during the southern summers from 1979–2011 are mainly grouped into four clusters. Two of the clusters exhibit the slow (<2 m/s) propagation speed in the unfiltered OLR field and the others fast (∼4 m/s). One cluster characterized as the El Niño phase has the fast speed while passing over the date line and another as the La Niña phase has the slow speed while propagating to ∼120°E. Compared with the other two clusters characterized as the weak El Niño‐Southern Oscillation phase, the speed is slow while propagating to ∼135°E when the SSTs over the Western Pacific are relatively low. Low temperatures around the tropical tropopause appear to the east of the eastward‐propagating convection in the tropics and to the west in the subtropics, forming a horseshoe‐shaped structure. The strength of the horseshoe‐shaped temperature structure is determined by that of the convective activities. Furthermore, the strength and location of the 100 hPa temperature minima differ among the clusters. This study implies that the different ISOs would cause different impacts on the dehydration process depending on their types.