This session will focus on the structure of the UTLS (upper troposphere / lower stratosphere) region in the extratropics and in the tropics and interactions between both regions. It will be formally divided into an extratropical and a tropical part. However, emphasis will be given to studies which investigate the coupling and mutual interaction between the tropical and extratropical tropopause region.

We invite observational as well as model based contributions that investigate dynamics and chemistry in each respective region. This includes studies of tracer transport and cross tropopause exchange, the structure of the TTL and the ExTL (tropical and extratropical transition layer, respectively) and time scales of relevant transport
processes.

Particularly welcome are papers that combine in-situ or remote sensing observational data sets with model based investigations or output from chemistry climate models addressing one of the following aspects:
1. Chemical composition and structure of the UTLS in both the tropics and the extratropics
2. Dynamical and chemical interactions between both regions
3. Seasonal and long-term changes of the chemical composition in both regions as well as the of the underlying transport processes