Control of the Tropical Tropopause and Vertical Transport across It

Abstract

A 3D primitive equation model is used to investigate how the tropical tropopause is influenced by cumulus convection in the troposphere and mean upwelling in the stratosphere. The model simulates the residual mean circulation explicitly, whereas it represents the influence of convection on large-scale structure through statistical properties of cloud. Within this global framework, a change of extratropical planetary waves induces a large change of downwelling over the winter hemisphere, compensated at lower latitudes by a change of upwelling. This exerts a major influence on thermal structure in the extratropics, but only a minor one in the Tropics. On seasonal time scales, however, the influence in the Tropics is significant. During northern winter, extratropical planetary waves are sharply amplified. The accompanying intensification of tropical upwelling, while small compared to the intensification of extratropical downwelling, accounts for about half of the observed seasonal change of the tropical tropopause. Remarkably, anomalous thermal structure extends even into the summer hemisphere where the tropopause is anomalously high and cold. Just the reverse is found in the winter hemisphere. Contrasting with this is the dependence on convection, which is large in the Tropics. An intensification or deepening of convection elevates and cools the tropical tropopause. Accompanying those changes overhead is anomalous downwelling in the lowermost stratosphere. It is forced by convective cooling, at and above the level of neutral buoyancy (LNB), where overshooting cumulus are colder than their environment. Anomalous temperature is out of phase above and below the LNB, consistent with cumulus detrainment and observed changes that accompany the outbreak of cold cloud. Conversely, an elevation of the LNB, as would accompany an increase of moist static energy (e.g., SST), elevates but warms the tropical tropopause. This dependence may explain geographical variations of the tropopause. A change of tropical convection also influences the extratropical circulation, secondarily through the absorption of planetary waves, which then modulates downwelling and temperature over the winter hemisphere. Vertical transport into the stratosphere depends on both mechanisms, which interact. Above the LNB, convective cooling drives environmental downwelling that transports stratospheric air into the troposphere at sites of deep convection. There, air of high ? mixes with air of low ? that has been convected above the LNB inside overshooting cumulus. The mixture, having been cooled mechanically, then experiences enhanced radiative warming that carries it upward at sites removed from convection.