Tropical–Extratropical Interactions Associated with an Atlantic Tropical Plume and Subtropical Jet Streak

Abstract

Tropical plumes (TPs) are elongated bands of upper- and midlevel clouds stretching from the Tropics poleward and eastward into the subtropics, typically accompanied by a subtropical jet (STJ) streak and a trough on their poleward side. This study uses ECMWF analyses and high-resolution University of Wisconsin?Nonhydrostatic Modeling System trajectories to analyze the multiscale complex tropical?extratropical interactions involved in the genesis of a pronounced TP and STJ over the NH Atlantic Ocean in late March 2002 that was associated with extreme precipitation in arid northwest Africa. Previous concepts for TP genesis from the literature are discussed in the light of this case study. Analysis of the upper-level flow prior to the TP formation shows a northeastward propagation and a continuous acceleration of the STJ over the Atlantic Ocean equatorward of a positively tilted upper-level trough to the west of northwest Africa. Both dynamic and advective processes contribute to the generation of the accompanying cloud band. The northern portion of the TP consists of parcels that exit a strong STJ streak over North America, enter the deep Tropics over South America, and then accelerate into the Atlantic STJ, accompanied by strong cross-jet ageostrophic motions, rising, and cloud formation. The southern portion is formed by parcels originating in the divergent outflow from strong near-equatorial convection accompanying the TP genesis. A local increase in the Hadley overturning is found over the tropical Atlantic and east Pacific/South America and appears to be related to low inertial stability at the outflow level and to low-level trade surges associated with the cold advection, sinking, and lower-level divergence underneath two strong upper-level convergence centers in the eastern portions of both a subtropical ridge over North America and an extratropical ridge over the North Atlantic Ocean. Evidence is presented that the convective response lags the trade surge by several days.