The role of equatorial waves in stratosphere-troposphere exchange

An observational evidence that equatorial planetary-scale gravity waves transport the tropical stratospheric ozone into the upper troposphere was recently obtained by Fujiwara et al. (JGR, 1998). They concluded that downward displacement due to an equatorial Kelvin wave confined near the tropopause and the passage of its breaking phase caused an irreversible exchange. By using an atmospheric general circulation model (AGCM), this study investigates the seasonal-longitudinal dependence of the transport associated with the equatorial waves. The study also trys to estimate its contribution to the ozone budget in the tropical troposphere in comparison with other processes such as tropical deep convections, tropopause foldings at the subtropical westerly jets, and isentropic cross-tropopause exchanges within the "middle world".

The model used here is the first version AGCM developed at the Center for Climate System Research (CCSR) and the National Institute for Environmental Studies (NIES). It has a horizontal resolution of T42 (about 2.8 degree x 2.8 degree) and 60 vertical levels with its top boundary at about 50 km altitude, having a vertical resolution of 550 m in the upper troposphere and lower stratosphere. This very fine vertical resolution is essential to simulate vertically thin waves near the tropical tropopause. Arakawa-Schubert scheme is used for the cumulus cloud parameterization. For simulating the stratospheric ozone, modified Chapman reactions are included, and the ozone is fully interactive with the dynamical and radiative processes.

In the model, many events of ozone transport across the tropical tropopause in association with the eastward-moving large-scale gravity wave packets are observed over Indian Ocean, Indonesia, and the western Pacific. These waves seem to be excited at the top of cumulonimbus clouds and over the high mountains in Africa. In the tropical western hemisphere where the average zonal wind direction in the upper troposphere is eastward, westward-moving gravity wave packets are observed, but the vertical displacements are much smaller. This model also simulates tropopause foldings and ozone filaments on isentropes within the "middle world" from the higher latitudes.