Upwind convective influences on the isotopic composition of atmospheric water vapor over the tropical Andes

Abstract

We take advantage of the spatial coverage provided by the Tropospheric Emission Spectrometer on-board the Aura satellite to evaluate processes that control seasonal variations in atmospheric water vapor HDO/H2O values (δDvapor) over the tropical Andes. δDvapor is lower in austral summer (December, January, and February, DJF) than austral winter (June, July, and August, JJA), which is broadly consistent with precipitation studies and with δ18Osnow preserved in tropical Andean glaciers. In DJF, 64% of δDvapor measurements over the tropical Andes are lower than predicted by Rayleigh distillation while 40% of JJA δDvapor measurements are lower than predicted by Rayleigh distillation. Air that has lower δDvapor than predicted by Rayleigh distillation at a given water vapor concentration (q) encounters low minimum outgoing longwave radiation (<240 W m−2) en route to the tropical Andes, suggesting convective intensity controls the isotopic ratios of these measurements. The broad regional coverage of the satellite data allows us to map the spatial extent of the region where isotopic ratios reflect convective processes in different seasons. In DJF, convection strongly influences δDvapor in the central tropical Andes. In JJA, convection influences δDvapor north of the tropical Andes. This pattern suggests that monsoon convection controls δDvapor in austral summer while large-scale advective mixing controls Andean δDvapor in austral winter.