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 the 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 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. Regardless of the location of the heating, this TTL upwelling occurs only over the western tropical Pacific, indicating that the background flow plays a crucial role. A key feature of the background flow in that role is found to be the strong absolute vorticity gradient associated with the Asian subtropial jet. 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 the cold trap generated by the Kelvin wave. Since, in reality, the background flow is shaped by the convective heating, we conclude that the 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.