STE in the vicinity of North Atlantic cyclones

This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract It is well known that the storm tracks are a preferred region of stratosphere–troposphere exchange (STE), but a systematic and climatological investigation of the connection between cyclones and STE has not yet been performed. We use two established ERA-Interim climatologies of STE and cyclones for the years 1979–2011 to quantify the 5 amount of STE that occurs during the life cycle of North Atlantic cyclones. A Lagrangian method serves to identify individual STE events, and a sophisticated cyclone identification tool detects cyclones, their shape and size from the sea-level pressure field. Combining the two data sets reveals that roughly half of the total STE in the North Atlantic occurs in the vicinity of cyclones and that both downward and upward fluxes of 10 mass across the tropopause (STT and TST, respectively) are more intense in deeper cyclones (lower minimum pressure) compared to less intense cyclones. In summer, STT and TST in the vicinity of cyclones are almost equal; in the other seasons, STT is larger by 15–45 %. Cross-tropopause mass fluxes are enhanced by a factor of about two compared to climatology when a cyclone is present. On average, STE is strongest 15 during the mature phase of cyclones, i.e., in a 24 h time window around the time of maximum intensity. Systematic patterns of exchange locations relative to the cyclone centre are identified via composite analysis and shed light on the different characteristics of STT and TST. During cyclone intensification and in the mature stage, TST is mainly confined to the cyclone centre, whereas STT occurs mainly in a region further 20 southwest. During the decay of the cyclones, both STT and TST are most frequent close the cyclone centre, in a region with a fairly low tropopause.