Climatology, sources, and transport characteristics of observed water vapor extrema in the lower stratosphere

Abstract. Stratospheric water vapor (H₂O) is a substantial component of the global radiation budget, and therefore important to variability of the climate system. Efforts to understand the distribution, transport, and sources of stratospheric water vapor have increased in recent years, with many studies utilizing long-term satellite observations. Previous work to examine stratospheric H₂O extrema has typically focused on the stratospheric overworld (pressures ≤ 100 hPa) to ensure the observations used are truly stratospheric. However, this leads to the broad exclusion of the lowermost stratosphere, which can extend over depths more than 5 km below the 100 hPa level in the midlatitudes and polar regions and has been shown to be the largest contributing layer to the stratospheric H₂O feedback. Moreover, focusing on the overworld only can lead to a large underestimation of stratospheric H₂O extrema occurrence. Therefore, we expand on previous work by examining 16 years of Microwave Limb Sounder (MLS) observations of water vapor extrema (≥ 8 ppmv) in both the stratospheric overworld and the lowermost stratosphere to create a new lower stratosphere climatology. The resulting frequency of H₂O extrema increases by more than 300 % globally compared to extrema frequencies within stratospheric overworld observations only, though the percentage increase varies substantially by region and season. Additional context is provided to this climatology through a backward isentropic trajectory analysis to identify potential sources of the extrema. We show that, in general, tropopause-overshooting convection presents as a likely source of H₂O extrema in much of the world, while meridional isentropic transport of air from the tropical upper troposphere to the extratropical lower stratosphere is also possible.