Interannual variability of tropical tropopause layer clouds

The quasi-biennial oscillation (QBO), El Niño Southern Oscillation (ENSO), and upwelling in the tropical branch of the Brewer-Dobson circulation (BDC) impact tropical tropopause layer (TTL, 14–19 km) 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 S–10 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 S–30 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 Niño is weakly associated with enhanced zonal-mean cloudiness in the uppermost TTL over the short satellite record.