Interannual Variability of Temperature, Water Vapor, and Clouds in the
Tropical Tropopause Layer
Abstract
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 variability affect temperatures in the TTL. In this study, we
use satellite observations to investigate the explained variance in cold
point temperature (CPT), 83 hPa water vapor (WV83), and TTL cirrus cloud
fraction (TTLCCF) over the equatorial region (15°N - 15°S) using a
multiple linear regression (MLR) model where predictors are
stratospheric and tropospheric modes of variability. The MLR model can
explain 68%, 60%, and 52% of the variance in CPT, WV83, and TTLCCF.
The model suggests that these variables are dominated by stratospheric
‘top-down’ processes associated with the Quasi-Biennial Oscillation
(QBO) and Brewer Dobson Circulation (BDC) as opposed to tropospheric
‘bottom-up’ processes associated with the El Nino Southern Oscillation
(ENSO) and the Madden-Julian Oscillation (MJO). Although cold point
temperature is controlled by ‘top-down’ mechanisms, the cold point
tropopause height is related to both ‘top-down’ stratospheric and
‘bottom-up’ tropospheric processes. Our MLR model explains more variance
during boreal winter. We also investigate how these modes of variability
correlate with zonal mean temperature, water vapor, and cloud fraction
globally in the upper troposphere and lower stratosphere (UTLS) and find
significant relationships between clouds and the modes of variability.