Interannual Variability of Zonal Mean Temperature, Water Vapor, and Clouds in the Tropical Tropopause Layer.

Water vapor and cirrus clouds in the tropical tropopause layer (TTL) are important for the climate and are largely controlled by temperature in the TTL. On interannual timescales, both stratospheric and tropospheric modes of the large‐scale variability could affect temperatures in the TTL. Here multiple linear regression (MLR) is used to investigate explained variance in the cold point tropopause temperature (CPT), cold point tropopause height (CPZ), 83 hPa water vapor (WV83), 83 hPa ozone (O383), and total cirrus cloud fraction with cloud base (TTLCCF) and top (ALLCF) above 14.5 km, all averaged over 15°S‐15°N. Predictors of the MLR are a set of stratospheric and tropospheric large‐scale modes of variability. The MLR explains significant variance in CPT (76%), CPZ (78%), WV83 (65%), O383 (62%), TTLCCF (52%), and ALLCF (36%). The interannual variability of CPT and WV83 is dominated by stratospheric processes associated with the Quasi‐Biennial Oscillation (QBO) and Brewer‐Dobson Circulation (BDC), whereas the variability of CPZ, O383, TTLCCF and ALLCF is also controlled by 500 hPa temperature (T500). Residual variability in CPT and CPZ not captured by the MLR are further significantly correlated to stratospheric temperature. It is shown that the portion of the BDC's shallow branch missed by the eddy heat flux based BDC index contributes significant amounts of the explained variances. Plain Language Summary: Between the tropical upper troposphere and lower stratosphere, water can exist as either vapor or ice. The amount of water that enters the stratosphere depends on the portion of vapor that is frozen out by the coldest temperature that air experiences in this region, which on interannual timescales could be modulated by both large‐scale stratospheric and tropospheric modes of variability. Here we show that 76%, 65%, and 52% of the interannual variance in cold point temperature, water vapor at 83 hPa, and ice cloud fraction in this region can be explained using a multiple linear regression (MLR), where the predictors are the modes of the large‐scale variability. Stratospheric processes are much more important in controlling the interannual variance of cold point temperature and water vapor at 83 hPa, but notably, the height of the cold point is controlled by both stratospheric and tropospheric processes. Residual variability of the cold point temperature not captured by the MLR is still connected to temperature variability in the stratosphere. Key Points: The interannual variability in the cold point tropopause temperature averaged over 15°S‐15°N is driven by stratospheric processesThis cold point tropopause temperature residual after regressing out large‐scale modes is still correlated with stratospheric temperatureThe portion of the BDC's shallow branch, which is independent of our eddy heat flux BDC index, is an important source of TTL variability [ABSTRACT FROM AUTHOR]