Tropical troposphere-to-stratosphere transport inferred from trajectory calculations

We present an analysis of trajectory calculations in the tropical tropopause layer (TTL) based on European Centre for Medium-Range Weather Forecasts (ECMWF) analysis wind and temperature fields. Over 500,000 forward and backward trajectories were calculated for January/February and July/August 2001. We analyze the pathways between 340 K and 400 K potential temperature (θ) of those trajectories involved in troposphere-to-stratosphere transport (TST). Even though trajectory calculations in this region may suffer from deficiencies in the underlying vertical wind field, they incorporate not only slow radiative ascent but also effects of deep convection, zonal and meridional transport, and their regional variability. From the trajectory calculations we derive a mean residence time of air parcels in the TTL, which shows a maximum at θ ≈ 360 K of ∼ 13 days for a change in potential temperature of ±10 K. The analysis of trajectory pathways reveals that approximately 80% of the trajectories ascending into the stratosphere enter the TTL over the western Pacific. Upon further ascent, they typically travel ∼ 5000-10,000 km before they arrive at the location where they assume minimum water mixing ratios. These pathways show regional and seasonal patterns and are largely controlled by the upper level circulation of the Asian-Australian monsoon, the northern hemispherical subtropical jet and the equatorial easterly jet from South Asia to Africa. As a consequence of the interplay of these meteorological systems, about 70% of TST trajectories assume their minimum water mixing ratio over the western Pacific, which shows also a global minimum in tropopause temperatures. Average water mixing ratios of air after TST are _χH2O = 1.6 ppmv for January/February and _χH2O = 3.6 ppmv for July/August 2001. Mixing of stratospherically young air, which just underwent TST, with older air masses entering the lower tropical stratosphere sideways yields an estimate of _χH2O = 2.3 ppmv for January/February and _χH2O = 3.7 ppmv for July/August for air at θ = 400 K, which compares favorably with satellite observations. Our analysis emphasizes the importance of particular pathways for tropical TST, with the western Pacific being the dominant source of stratospheric air in general and being the place, in particular, where ∼ 70% of tropical TST assumes its final water mixing ratio.