Tropical–extratropical interactions related to upper-level troughs at low latitudes

Momentum and kinetic energy fluxes associated with low-latitude transient disturbances at upper-levels play an important role in the general circulation of the atmosphere. They are related to eastward and equatorward propagating, positively tilted wave trains from the extratropics. Theoretical, modelling and observational studies show that this particular kind of tropical–extratropical interaction is most common in regions of mean upper-level westerlies at low latitudes, i.e. over the central and eastern Pacific and Atlantic Oceans during boreal winter and spring. The penetration of an upper-level trough into the Tropics is often associated with enhanced convection and the formation of an east- and poleward stretching elongated band of upper- and midlevel clouds, usually referred to as a ‘tropical plume’ (TP).The present study provides an overview of various aspects related to the penetration of upper-level disturbances to low latitudes, including a description of the involved meteorological phenomena, climatological aspects, interannual variability, linear Rossby-wave and critical line theory, results from barotropic and higher-complexity modelling studies, the vertical structure of the disturbances as well as sources for the wave energy. In addition, the dynamical relation of the upper-troughs to convection, moisture transports and precipitation in the subtropics will be discussed. The paper concludes with a number of research perspectives for future investigations.