Analyzing the effects of Brewer-Dobson circulation upwelling and horizontal transport on the TTL composition

We analyze the effects of the Brewer-Dobson circulation tropical upwelling and of quasi-horizontal isentropic transport from the mid-latitudes into the tropics (in-mixing) on the seasonality of the trace gas composition of the Tropical Tropopause Layer (TTL). For that purpose, we use three-dimensional back trajectories, driven by ECMWF ERA-Interim winds, and a simple diagnostic one-dimensional tropical trace gas model. Our analysis shows that carbon monoxide and ozone are complementary trace gas species in the TTL, with carbon monoxide mixing ratios affected mainly by vertical upwelling and ozone affected by both upwelling and in-mixing. Reconstructed carbon monoxide and ozone profiles as well as an independent radiative heating rate calculation further indicate, that the upwelling velocity throughout the TTL in ERA-Interim reanalyses is about 40% too fast. The impact of horizontal in-mixing on TTL mixing ratios largely depends on the meridional extratropics-tropics gradient of the particular species. For ozone, the meridional gradient is large and the contribution of in-mixing to the ozone maximum during NH summer is about 50%, strongly related to the Asian monsoon anticyclone and not sensitive to the tropical upwelling velocity. The combination of both seasonally varying upwelling and in-mixing rates, with minimum upwelling and maximum in-mixing rate during summer, causes the annual variation of ozone in the upper TTL. These findings are consistent with the result that seasonally varying vertical advection dominates the ozone balance at a given level closely above the tropical tropopause.