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Center for Nonlinear Studies |
The strength of the water vapor feedback response to global warming is particularly sensitive to changes in subtropical humidity. Specific humidity of the free troposphere has been monitored, and is projected, to increase in such a way as to maintain a near-constant relative humidity. In order to better understand how humidity of the subtropics does change, we explored observed ENSO and projected global warming variability of subtropical humidity using a Eulerian last saturation framework. Water vapor tracers of last saturation were used in an atmospheric tracer transport model to evaluate ENSO variability in the generation of the dry air that defines the subtropical middle troposphere over the north Pacific. We evaluated the probability distributions of last saturation position for the dry air in the middle troposphere over the subtropical north Pacific and found there to be further poleward, higher altitude, and more westerly components during the warm phase compared to the cold phase. Humidity reconstructions from the water vapor tracers capture observed ENSO humidity variability and demonstrate that it can be explained in terms of changes in the location of last saturation, and not by changes in the temperature field. This study shows how teleconnections between the tropical ocean and the extratropical upper troposphere can impact the humidity of the middle troposphere of the subtropical dry regions. In a similar study, 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.
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