We analyze output from a domain-filling forward trajectory model in order to better understand the annual cycle of water vapor entering the stratosphere. To do this, we determine the minimum water vapor saturation mixing ratio along each trajectory (the final dehydration point or FDP) and assume that the parcel carries that much water vapor into the stratosphere. In the annual average, the tropical Western Pacific, equatorial Africa and South America, and Southeast Asia are found to be the locations of the most frequent FDPs. Looking at individual seasons, we find that FDPs in the tropical western Pacific tend to occur in the summer hemisphere, with FDPs over South America and Africa occurring predominantly during the boreal winter. During boreal summer, a dehydration maximum occurs in the Asian monsoon region. In the annual average, FDP maxima occur at 99 and 84 hPa. Looking at individual seasons, we find that FDPs occur at higher altitudes (centered at 84 hPa) during boreal winter and at lower altitudes (99 hPa) during boreal summer. The annual cycle in FDP altitude combines with the annual cycle in tropical tropopause layer temperatures to generate the observed annual variations in water vapor entering the stratosphere.
