Tropical dehydration processes constrained by the seasonality of stratospheric deuterated water

Stratospheric water vapour affects Earth’s radiation budget. In addition, it has a key role in stratospheric chemistry and in processes that permit ozone depletion. Air largely enters the stratosphere in the tropics, but the processes that bring water through the cold tropopause into the stratosphere are not well understood. Here we present a 19-month record of non-deuterated (H2O) and deuterated (HDO) water in the tropical stratosphere, collected through remote-sensing measurements with the Michelson Interferometer for Passive Atmospheric Sounding. Our data show a clear seasonal cycle in the isotopic composition that propagates upward in the tropical stratosphere, and is most likely created in the tropical tropopause layer. In addition, we find that the slope of the HDO–H2O correlation of water entering the stratosphere in the tropics is close to, but slightly steeper than the slope expected from Rayleigh fractionation. We propose that gradual dehydration of air by cirrus clouds that are formed in situ, together with a seasonally varying contribution from the evaporation of convectively lofted ice, provides the most plausible explanation for our measurements. We conclude that potential changes in the water budget of the tropical tropopause layer and the stratosphere should be detectable in isotopic measurements.