Simple models of tropical plumes

Abstract

Tropical plumes are upper and mid-level cloud bands at least 2000 km long that cross 15' latitude. The simplest conditions that lead to tropical plume development are sought in a barotropic model simulating winter 200 mb flow. The features sought are: (1) a subtropical trough along the western edge of the plume, (2) an anticyclonically curving jet emanating from the plume source region, (3) a convergence/divergence dipole equatorward of 15'N straddling the trough axis, with the divergence center at the jet entrance to the east. It is found that these features can result from just a zonally symmetric subtropical jet near imposed mass convergence in the tropics. However, the evolution is highly sensitive to the locations of these two features relative to each other and to the planetary vorticity field. The development of the tropical plume is largely a consequence of quasigeostrophic adjustment of forced synoptic scale waves as the subtropical jet moves out of the tropical forcing region. The plume development takes place in two stages. First, mass removal by the convergent forcing creates a large scale trough, bending the subtropical jet into the deep tropics. As the jet leaves the tropics on the east side of the trough, a smaller synoptic scale Rossby wave train forms as the subtropical jet attempts to come into balance with the large scale height field. This smaller scale wave, which is highly transient, is the tropical plume.