Upward transport of boundary layer air to altitudes of the Asian summer monsoon anticyclone in Eulerian and Lagrangian model simulations

The Asian summer monsoon anticyclone is a dominant circulation system in the upper troposphere and lower stratosphere (UTLS) in boreal summer (about June–September). An appropriate simulation of the monsoon anticyclone is an important challenge for chemistry climate and chemistry transport models. Here we compare simulations of the ECHAM5/MESSy Chemistry Climate model (EMAC) and the Chemical Lagrangian Model of the Stratosphere (CLaMS) based on the European Centre for Medium-Range Weather Forecasts Reanalysis-Interim (ERA-Interim); EMAC simulations are nudged towards ERA-Interim, whereas transport in CLaMS is driven by ERA-Interim. We employ surface origin tracers for continental South Asia. These surface origin tracers are lifted upward into the Asian summer monsoon anticyclone, both in EMAC and CLaMS. We investigate monsoon conditions for boreal summer 2015. In summer 2015, the entire monsoon, and in particular upward transport in the monsoon anticyclone, was strongly influenced by El Niño. In both models, in 2015, the simulated impact of surface origin tracers on the composition of air in the Asian summer monsoon anticyclone is very weak at 420 K. Further, in both models, a very strong decline with altitude (between ≈ 370–400 K) of surface origin tracers is obvious. The pattern of the Asian monsoon anticyclone in August and early September is represented very similarly in EMAC and CLaMS, with a lower fraction of the surface origin tracer for continental South Asia in CLaMS. The simulated pattern of surface origin tracers in the Asian summer monsoon anticyclone in CLaMS is much less smooth than in EMAC. Finally, we find a strong day-to-day variability in the Asian summer monsoon anticyclone and a confinement of monsoon air at UTLS altitudes (≈370 K to 400 K) similarly in both, EMAC and CLaMS.