A last‐saturation diagnosis of subtropical water vapor response to global warming

We used boreal winter output from an IPCC AR4 GCM simulation to drive an atmospheric tracer transport model with water vapor last saturation tracers over the northern hemisphere in order to identify processes that contribute to the projected increase in specific humidity of the subtropical relative humidity minimum. Most of the increase in specific humidity at the relative humidity minimum can be attributed to last saturation within what will be a warmer free troposphere. The water vapor content of the relative humidity minimum is projected to increase in the 21st century primarily as a function of increased last saturation temperatures in the tropical lower troposphere, effectively yielding more water vapor via poleward transport. The increase in specific humidity is to a degree kept in check by a poleward and upward shift of baroclinic instability and saturation patterns that effectively yield drier air to the RH minimum via isentropic transport.