Understanding seasonality and trends of transport pathways and composition in the upper troposphere lower stratosphere (UTLS) region

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Seasonality and trends in transport pathways affect the trace gas composition of the upper troposphere lower stratosphere (UTLS) which, in turn, impacts radiative forcing and climate change. Currently, transport in the UTLS and its link to a changing climate is a rapidly expanding field in atmospheric science. This proposal for a research fellowship at the National Center for Atmospheric Research (NCAR/ACD) at Boulder/USA aims at developing novel diagnostic tools for an improved understanding of transport pathways in the UTLS, with particular focus on seasonality and trends and the impact on trace gas composition. Using the chemistry transport model CLaMS and the climate model WACCM, a new diagnostic tool for calculating the age spectrum of stratospheric air, the probability distribution of transport time scales of air parcels originating at the tropical tropopause, will be developed, based on various decaying tracers. Considering the age spectrum is a major step forward going beyond current research where mainly the mean age is considered. The method will be validated against age spectra from pulse experiments and against trajectory transit time distributions. The new age spectrum diagnostic will be applied to the CLaMS and WACCM models as a tool for model transport evaluation. The combination of the different age spectrum diagnostics and trajectory statistics will provide new deep insight into the relative strength and the seasonality of different pathways between the tropical and the extratropical lower stratosphere and long-term circulation trends. Hence, the proposed work will be an important step towards an improved understanding of our changing climate system.