Effect of Tropical Waves on the Tropical Tropopause-Transition-Layer Upwelling

An initial-value problem is employed with a GCM to investigate the role of the convectively driven Rossby and Kelvin waves for tropopause-transition-layer (TTL) upwelling in the tropics. The convective heating is mimicked with a prescribed heating field and the Lagrangian upwelling is identified by examining the evolution of passive tracer fields whose initial distribution is identical to the initial heating field. This study shows that an overturning circulation, induced by the tropical Rossby waves, is capable of generating the TTL upwelling. Even when the heating is placed in the eastern Pacific, the TTL upwelling occurs only over the western tropical Pacific, indicating that the background flow plays a crucial role. The results from a Rossby-wave source analysis suggest that a key feature of the background flow is the strong absolute vorticity gradient associated with the Asian subtropical jet. In addition, static stability is relatively weak over the western Pacific, providing additional favorable conditions for TTL upwelling. The background flow also modulates the internal Kelvin waves in such a manner that the coldest region in the TTL, resembling the observed ‘cold trap’, occurs over the western tropical Pacific. As a consequence, the upwelling air, induced by the Rossby wave flux, passes through the cold trap generated by the Kelvin wave. Since, in reality, the background flow is shaped by the convective heating, we conclude that the climatological western tropical Pacific heating is ultimately responsible for both the TTL upwelling and the ‘cold trap’, but that both processes are realized indirectly through its impact on the background flow and the generation of the tropical waves.