The temperature response to stratospheric water vapor changes

This study uses an analytical model, based on the cooling-to-space approximation, and a fixed dynamical heating model to investigate the structure of the temperature response to a uniform increase in stratospheric water vapor (SWV). The temperature response consists of lobes of maximum cooling in the extra-tropical lower stratosphere and a decrease in cooling with increasing altitude. This is markedly different to the response to an increase in CO₂, which shows stratospheric cooling that increases with altitude, and a net warming in the tropical lower stratosphere.

The differences in the temperature responses can be explained by the smaller opacity of water vapor in the stratosphere compared to that of CO₂; this means that, for an increase in SWV, the fractional difference in the instantaneous heating rate perturbation between the upper and lower stratosphere is significantly smaller than for an increase in CO₂. Hence, for a SWV perturbation, the largest change in temperature required to balance the instantaneous heating rate perturbation is in the lower stratosphere, where gases are less able to effectively radiate to space. The background concentration of SWV is therefore essential for determining the structure of the temperature response to a SWV perturbation: indeed for background SWV concentrations ≥30ppmv, the instantaneous heating rate perturbation and temperature change resemble those from an increase in CO₂.

In the extra-tropical lower stratosphere, the lower height of the tropopause is found to cause the enhancement in cooling due to a reduction in the net exchange of radiation between stratospheric layers. Meridional gradients in upwelling infra-red radiation from the troposphere and spatial differences in the background stratospheric temperature are found to have a minor effect on the cooling.