Influence of convection on the water isotopic composition of the tropical tropopause layer and tropical stratosphere

Abstract

We present the first in situ measurements of HDO across the tropical tropopause, obtained by the integrated cavity output spectroscopy (ICOS) and Hoxotope water isotope instruments during the Costa Rica Aura Validation Experiment (CR‐AVE) and Tropical Composition, Cloud and Climate Coupling (TC4) aircraft campaigns out of Costa Rica in winter and summer, respectively. We use these data to explore the role convection plays in delivering water to the tropical tropopause layer (TTL) and stratosphere. We find that isotopic ratios within the TTL are inconsistent with gradual ascent and dehydration by in‐situ cirrus formation and suggest that convective ice lofting and evaporation play a strong role throughout the TTL. We use a convective influence model and a simple parameterized model of dehydration along back trajectories to demonstrate that the convective injection of isotopically heavy water can account for the predominant isotopic profile in the TTL. Air parcels with significantly enhanced water vapor and isotopic composition can be linked via trajectory analysis to specific convective events in the Western Tropical Pacific, Southern Pacific Ocean, and South America. Using a simple model of dehydration and hydration along trajectories we show that convection during the summertime TC4 campaign moistened the upper part of the TTL by as much as 2.0 ppmv water vapor. The results suggest that deep convection is significant for the moisture budget of the tropical near‐tropopause region and must be included to fully model the dynamics and chemistry of the TTL and lower stratosphere.