Diagnosis of relative humidity changes in a warmer climate using tracers of last saturation

The zonal mean signature of the relative humidity response to a doubling of CO₂ is qualitatively robust among most climate models. This signature is characterized by a decrease in relative humidity in the tropical upper troposphere and subtropics and an increase in the tropical mid-troposphere and extratropical tropopause layer. Two climate model simulations are performed, one control and one with doubled CO₂. The output of these simulations is used to drive a three-dimensional tracer transport model that contains both an independent hydrologic cycle and a zonally symmetric last saturation tracer scheme. The tracers are capable of recreating both the modeled relative humidity field and the relative humidity response to warming. The zonal mean pattern of relative humidity response is shown to depend more strongly on changes in atmospheric circulation than inhomogeneous changes in temperature, particularly in the tropics and subtropics. The most influential circulation changes are identified and illustrated using the last saturation tracers.