Transport in the low-latitude tropopause zone diagnosed using particle trajectories

Recent literature has described a "transition zone'' between the average to p of deep convection in the Tropics and the stratosphere. Here transport ac ross this zone is investigated using an offline trajectory model. Particles were advected by the resolved winds from the European Centre for Medium-Ra nge Weather Forecasts reanalyses. For each boreal winter clusters of partic les were released in the upper troposphere over the four main regions of tr opical deep convection (Indonesia, central Pacific, South America, and Afri ca). Most particles remain in the troposphere, descending on average for ev ery cluster. The horizontal components of 5-day trajectories are strongly i nfluenced by the El Nino-Southern Oscillation (ENSO), but the Lagrangian av erage descent does not have a clear ENSO signature. Tropopause crossing locations are first identified by recording events when trajectories from the same release regions cross the World Meteorological Organization lapse rate tropopause. Most crossing events occur 5-15 days af ter release, and 30-day trajectories are sufficiently long to estimate cros sing number densities. In a further two experiments slight excursions acros s the lapse rate tropopause are differentiated from the drift deeper into t he stratosphere by defining the "tropopause zone'' as a layer bounded by th e average potential temperature of the lapse rate tropopause and the profil e temperature minimum. Transport upward across this zone is studied using f orward trajectories released from the lower bound and back trajectories arr iving at the upper bound. Histograms of particle potential temperature (the ta) show marked differences between the transition zone, where there is a s low spread in theta values about a peak that shifts slowly upward, and the troposphere below 350 K. There forward trajectories experience slow radiati ve cooling interspersed with bursts of convective heating resulting in a we ll-mixed distribution. In contrast theta histograms for back trajectories a rriving in the stratosphere have two distinct peaks just above 300 and 350 K, indicating the sharp change from rapid convective heating in the well-mi xed troposphere to slow ascent in the transition zone. Although trajectorie s slowly cross the tropopause zone throughout the Tropics, all three experi ments show that most trajectories reaching the stratosphere from the lower troposphere within 30 days do so over the west Pacific warm pool. This pref erred location moves about 30 degrees -50 degrees farther east in an El Nin o year (1982/83) and about 30 degrees farther west in a La Nina year (1988/ 89). These results could have important implications for upper-troposphere- lower-stratosphere pollution and chemistry studies.