A mechanism for the TTL upwelling

Upwelling processes in the tropopause transition layer (TTL) are studied by performing simulations with a global primitive equation model. Convection is not modeled explicitly. Instead, convective tropical heating is modeled with a localized equatorial heat source in the middle troposphere. Air and liquid/solid water in the convective cloud is modeled with a pack of passive tracers placed at the same location as the localized heating. While the idealized heating produces a Gill-type equatorial wave response regardless of the background flow (at rest, zonally symmetric, or the zonally varying 3-D climatological flow), vertical transport of the TTL occurs only for the 3-D climatological background flow over the tropical western Pacific. By day 13 of the integration, the tracer field starts to form a thin layer confined mostly to the TTL, indicating that the upwelling occurs within a relatively short time period even without the aid of convective overshooting.

Diagnostic analyses of these runs show that this vertical transport is induced by meridional convergence of zonal momentum by Rossby waves, consistent with the Boehm-Lee-Norton mechanism. This Rossby wave momentum flux convergence occurs only for the 3-D basic state over the tropical western Pacific, indicating that the upward transport, via the Boehm-Lee-Norton mechanism, hinges on the background flow structure over the western Pacific. For the atmosphere, this result implies that it may be the convective heating-induced large-scale background flow, rather than the convection itself, that is important for the vertical transport.

The same idealized heating generates a cold anomaly in the TTL over the western tropical Pacific, whose location and structure closely resembles that of the observed ``cold trap". This cold anomaly is associated with vertically propagating Kelvin waves in the UT/LS. Because these Kelvin waves were shown to be modulated by the background flow so as to attain their maximum amplitude over the western Pacific, our results suggest that the background flow over the western tropical Pacific, through its impact on both the Rossby and the vertically propagatin Kelvin waves, plays a pivotal role for generating the TTL upwelling as well as the cold trap. Based on these grounds, we contend that the encounter of the upwelling air with the cold trap is not coincidental, but is orchestrated by the western tropical Pacific heating and the attendant background flow.