Reconsideration of the cause of dry air in the southern middle latitude stratosphere

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Abstract
A previous comparison of data from the Halogen Occultation Experiment (HALOE) and of output from the NCAR Community Climate Model (CCM2) in austral spring overstated the effect of dehydration in the southern polar vortex on lower stratospheric water vapor in part due to shortcomings in the HALOE retrieval algorithm and in part to remnants of the initially dry stratosphere in the CCM2 run. A new HALOE retrieval and a longer CCM2 run both show more water vapor than previously, but HALOE still shows less in the southern hemisphere than in the northern in September‐October 1992. A “dried‐air” tracer in the CCM2 suggests that polar dehydration influences middle latitudes only below about 100 hPa, and seasonal variations of HALOE H2O imply no influence north of 50°S.