Interannual variability of tropical tropopause layer clouds

The quasi-biennial oscillation (QBO), El Nino Southern Oscillation (ENSO), and upwelling in the tropical branch of the Brewer-Dobson circulation (BDC) impact tropical tropopause layer (TTL, 14-19km) temperature (T) and relative humidity (RH), and thus it is likely that they also affect the TTL cloud distribution. Satellite data reveal extreme interannual variability in the zonal-mean TTL cloud occurrence frequency (CF) in the deep tropics (10 degrees S-10 degrees N). This zonal-mean interannual variability is related to the QBO and BDC, with a relatively minor role for ENSO. However, over the whole tropics (30 degrees S-30 degrees N), the dominant mode of variability in the longitudinally resolved CF field is an ENSO-related dipole pattern of positive and negative anomalies centered over the Pacific that mimics the RH/T fields. The ENSO effects largely cancel in the zonal mean, although El Nino is weakly associated with enhanced zonal-mean cloudiness in the uppermost TTL over the short satellite record.