What controls stratospheric water vapor in the NH summer monsoon regions?

Water vapor in the lower stratosphere exhibits localized maximum values during Northern Hemisphere summer in the Asian and North American (NA) monsoon regions. The processes maintaining these maxima are not well understood. We analyze the variability of water vapor in the monsoon regions based on Aura Microwave Limb Sounder satellite observations during 2005–2013 and quantify links to deep convection, large-scale temperatures, and monsoonal circulations. Strong subseasonal variations in stratospheric water vapor are closely linked with deep convection in the monsoon regions, with the surprising result that stronger convection leads to a relatively dry stratosphere and weaker convection to a wetter stratosphere. This result is observed for both the Asian and NA monsoon regions. This behavior is explained by temperature changes in the stratosphere tied to deep convection: stronger convection leads to relatively cold temperatures in the subtropical lower stratosphere, which is the key region controlling large-scale dehydration within the anticyclonic monsoonal circulations. Likewise, weaker convection leads to warmer subtropical stratospheric temperatures, relatively less dehydration, and enhanced water vapor. The observed water vapor changes are in approximate agreement with those expected from the subtropical temperature variations, taking into account dilution of the dehydrated air into the larger monsoon region. These results demonstrate that stratospheric water vapor in the Northern Hemisphere monsoon regions is mainly controlled by large-scale circulation and temperatures, and overshooting deep convection plays a relatively minor role.