Effects of Convective Ice Evaporation on Water Vapor Interannual Variability in the Tropical Tropopause Layer (TTL)

Abstract

The objective of this dissertation is to investigate the physical processes controlling the spatial distribution of water vapor in the tropical tropopause layer (TTL) and the impact of convective ice evaporation on TTL water vapor using a forward domain-filling trajectory model. With large-scale circulation and temperature fields, the trajectory model is able to accurately reproduce the TTL water vapor patterns of the satellite observations and climate model simulations. Comparison of TTL water vapor simulations from the trajectory model to Aura Microwave Limb Sounder (MLS) observations and climate model simulations indicates that, over decadal periods, TTL water vapor variations are primarily controlled by variability of TTL temperature. On longer century timescales, the evaporation of convective ice plays an important role and contributes to more than half of the total trend of TTL water vapor in the 21st century. We investigate the impact of the Brewer-Dobson circulation (BDC), the quasibiennial oscillation (QBO), and the tropospheric temperature (ΔT) on the spatial distribution of TTL water vapor using a multivariate linear regression model to decompose TTL water vapor variability. We find that the BDC and QBO affect TTL water vapor mainly by changing TTL temperatures. But for ΔT, we find that the impact on TTL water vapor cannot be explained through the TTL temperatures alone. We hypothesize a moistening role for the evaporation of convective ice from increased deep convection as the troposphere warms. Parallel analysis using a climate-chemistry model, the GEOSCCM, supports that variability in the evaporation of convective ice plays a role in water vapor variability in the TTL.