The tropopause: definition, trends, and tropical broadeningThe lifetime of aviation water vapour emissions in the stratosphere, and their climate impact, is dependent on their ini tial height above the tropopause. As the height of the tropopause can vary substantially with changes in tropopause defi nition, estimates of the climate impact of water vapour emissions are also expected to be sensitive to the tropopause de finition. There is no universally correct definition of the tropopause, and several different definitions are in use.
In addition to being important to the evaluation of the climate impact of water vapour emissions, variables on the tropo pause can provide a unique insight into atmospheric dynamics. In recent decades the height of the global tropopause has been observed to be increasing, primarily due to increases in well-mixed greenhouse gases. Tropopause trends provide a sensitive indicator of climate change due the smaller natural variability at the tropopause compared to the surface m eans that changes there can be detected sooner.
I developed a new definition of the tropopause for use in the evaluation of the climate impact of aviation water vapour emissions. I also investigated the differences in this surface between ERA-Interim and ERA-40, and analysed global and r egional trends, with a focus on the expansion of the tropics. The tropopause data used in this work are available for download.