Tropical plumes (TPs) are several thousand kilometers long bands of mainly upper- and mid-level clouds stretching from the Tropics poleward and eastward into the subtropics, typically accompanied by a subtropical trough to the west penetrating into the Tropics and high upper-level wind speeds. This paper uses ECMWF analyses and data from a case study with the UW-NMS model of a TP over the Atlantic Ocean in March 2002 to test different hypothesis and model results with respect to TP genesis. The high spatial (75 km) and temporal (1 h) resolution of the available data allows for the calculation of very accurate trajectories to trace back the origin and characteristics of ‘plume parcels’. The model’s water condensate density is used as a proxy for clouds. An updated schematic/dynamic model of TPs and the accompanying subtropical jet (STJ) will be presented at the conference.

The trajectory analysis corroborates the shallow-water model derived theory that tropical plumes are forced by a positive vorticity tendency in the ridge to the west of the subtropical trough produced by the convergent part of the wind field. The resulting equatorward amplification and zonal contraction of the trough cause the enhanced equatorward flow from the subtropics in the western portion of the trough to lose its rotational balance when entering the deep Tropics. The ensuing deceleration, sharp cyclonic turning and then acceleration of the flow away from the Tropics are clearly depicted by the trajectories. This adjustment process of the mass and momentum fields produces strong convergence and divergence in the entrance region of the emerging STJ, with the latter located over South America in the considered case. Here, trajectories rise in convective updrafts, cross the equator at upper-levels and finally form the southern portion of the jet/plume over the Atlantic Ocean. The advection of negative potential vorticity in this region lowers the inertial stability for the convection to spread its outflow northward. The strong poleward ageostrophic motions associated with the acceleration of the STJ forces plume parcels to rise on isentropic surfaces (latent heat release is very small at upper-levels) on their way from the Tropics to the subtropics. It is shown that a large portion of the cloud plume is actually formed by this rising and cooling rather than by a pure advection of convective outflow. The described mechanism explains the puzzling fact that plumes tend to spread faster than the wind at outflow levels. In addition, kinetic energy considerations are employed to show that the acceleration of the jet is mainly achieved by the rotational component of the wind flowing down the geopotential gradient and not primarily by the divergent outflow of convection.