A Study of the Observed Increases in Tropical Stratospheric Water Vapor in the Early 1990s

Previous analysis of stratospheric water vapor data from the Halogen Occultation Experiment (HALOE) has shown a large upward trend in the mixing ratio between 40 and 60 km over the period from 1992 to 1997 [Nedoluha et. al., 1998; Evans et. al., 1998]. This study combines data from HALOE and the Microwave Limb Sounder (MLS), which observed the tropical stratosphere from September 1991 to May 1993, in order to investigate in detail the first 18 months of increasing water vapor. The use of MLS data allows the analysis to be extended into the lower stratosphere in the immediate post-Pinatubo period, and allows us to determine the time period and altitude region over which the HALOE water vapor data is contaminated by the Pinatubo aerosols. This period is of particular interest since it has been speculated that the eruption of Mt.Pinatubo resulted in a large increase in water vapor entering the stratosphere [Evans et. al., 1998; Considine et. al., 2001; Nedoluha et. al., 2003]. Changes in water vapor over the 18 month time period are calculated for HALOE and MLS at individual pressure levels throughout the stratosphere and averaged over the latitude range from 10 S to 10 N by removing annual, semi-annual, and QBO related variations in the data. Details of the technique that are used to the model the water vapor variability and remove it from the HALOE and MLS data are presented. Estimating the QBO signal is particularly important to an understanding of the short MLS data record. A comparison of the observed increases between 1 and 8 mb in HALOE and MLS water vapor shows good agreement, with the increases peaking at 3 mb with a value of 0.4 ppmv between September 1991 and May 1993. Between 10 and 100 mb, MLS shows near zero changes at most pressure levels and no clear increases throughout the lower and middle stratosphere which could be related to the observed water vapor increases above 8 mb.