The International Team activity "Tropical Width Impacts on the Stratosphere (TWIST)" was selected for funding by the International Space Science Institute (ISSI) in 2019.

The topic of the activity is the width and position of the tropics in the UTLS (upper troposphere - lower stratosphere) region. It is based on the outcome of the predecessor ISSI International team activity "Tropical width diagnostics Intercomparison project (TWDIP)" in which a surprising inconsistency between so-called "lower" and "upper" atmospheric metrics for the width of the tropospheric tropics has been found. While the "lower" atmospheric metrics co-vary with the Hadley cell edge, the "upper" metrics based on the subtropical jet and tropopause do not, although a close link between the relevant parameters is to be expected. TWIST focusses on the identification of robust satellite-observed metrics of the tropical width in the UTLS, the characterization of relationships between UTLS tropical width and circulation, and the identification of the relation between tropical width variations and variability and trends in trace gas concentrations in the UTLS.

Contribution of IMK-ASF: Provide observational data of numerous trace gases suited for study of TWIST-related topics, AoA assessments and assessment of the subtropical mixing barriers from MIPAS/Envisat, in continuation of work published in Stiller et al., 2012, Haenel et al. 2015, and Stiller et al., 2017.