Air mass export from the Asian monsoon into theextratropical stratosphere: impact on chemistry and radiation (AirExam)

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The Asian summer monsoon is associated with deep convection over south Asia linked to the Asian monsoon anticyclone (AMA) that extends from the upper troposphere into the lower stratosphere (UTLS) region, which is the most pronounced circulation pattern in these altitudes during boreal summer. It is known that the export of monsoon air quasi-isentropically out of the AMA both to the east and to the west has a large impact on the composition of the extratropical lower stratosphere, however their relative strengths is unknown hitherto. The transport of air masses out of the AMA into the northernextratropical UTLS crucially affects stratospheric chemistry and its radiative budget (e.g., through transport of H2O, aerosol or ozone-depleting substances). Within this project AirExam the quasi-isentropical air mass export from the AMA by different pathways into ExUTLS and its impact on chemistry and radiation will be quantified using i.a. HALO aircraft measurements (in particular from the PHILEAS campaign scheduled for summer 2023), simulations with the Chemical Transport Model CLaMS and radiative calculations.Our project AirExam will address the following key open questions:1) What are the relative contributions of the two quasi-horizontal transport pathways (to the west and to the east) out of the Asian monsoon anticyclone to the composition of the extratropical lower stratosphere?2) How large is the intra-annual variability of the transport out of the Asian monsoon anticyclone to the the extratropical lower stratosphere and what are the main source regions on the Earth’s surface?3) What is the impact of water vapor transport out of the Asian monsoon anticycloneto the H2O budget of the extratropical UTLS and its radiative impact?In our project, we will combine HALO measurements (in particular H2O) with global 3-dimensional CLaMS simulations including artificial tracers of air mass origin as well as 'pathway finder tracers'. CLaMS simulations will be driven by the new ECMWF ERA-5 reanalysis providing a much higher spatial and temporal resolution than the older ERA-Interim product. CLaMS simulations driven by ERA-5 are a unique tool to describe transport processes reliably in the region of the Asian monsoon and its global impact. The radiative impact of the H2O increase during summer and autumn caused by the Asian monsoon will be calculated using Edwards and Slingo radiative transfer code. H2O is the most important greenhouse gas and moistening of the stratosphere is an important driver of climate change. Our project AirExam will quantify the impact of enhanced H2O transport into the lower stratosphere and therefore can help to assess the potential risks of air mass transport from the Asian monsoon region on the global stratosphere.