Composition of air and its seasonality within the tropical tropopause layer TTL: Impact of the Asian monsoon

The tropical tropopause layer (TTL) has been described as a transition layer coupling the convectively dominated free troposphere with the Brewer-Dobson circulation in the stratosphere. Multi-annual simulations (2001-2006) with the Chemical Model of the Stratosphere (CLaMS) are used to study transport of air and its seasonality within the TTL. In agreement with recently published satellite and in-situ observations at the tropical tropopause, CLaMS simulations show a pronounced annual cycle in O3, CO and, in addition, in the mean age. Within the model, this seasonality is driven by the seasonality of both upwelling and in-mixing. The latter process describes enhanced meridional transport from the extratropics into the TTL. The strongest in-mixing occurs from the northern hemisphere during the boreal summer in the potential temperature range between 380 and 420 K. Contrary, an increase of upwelling with highest values in winter reduces O3 up to the lowest values in early spring. Both, CLaMS simulations and Aura MLS O3 observations show that this enhanced equatorward transport in summer is mainly driven by the Asian monsoon anticyclone. ClaMS simulations reproduce the transport barrier between interior and exterior of the anticyclone fairly well compared to the recent observations from the satellites. Both, enhanced vertical velocities within the upper part of the anticyclone and the enhanced mixing mainly along the inner flanks of the jets drive in CLaMS an effective upward transport up to 70 hPa level.