Transient synoptic systems as mechanisms for meridional transport: an observational study in the southern hemisphere

Abstract

A classification of satellite cloud vortices compiled for the southern hemisphere by Streten and Troup is used to provide an inventory of the incidence of various stages in the life-cycle of depressions as a function of latitude. These figures are then allied with estimated meridional transient eddy fluxes of both heat and momentum to assess the varying effectiveness of the systems during their life-cycles as mechanisms of meridional transport, in comparison with the net contribution from other transient systems (travelling long waves and temporal variations in stationary waves). The developing stage of the cloud vortex is shown to be associated with a strong poleward heat flux, decreasing with the age of the system, with only a small (poleward) contribution from other transient mechanisms. For momentum, the developing stage is associated with a moderate poleward flux and the mature stage with relatively little net flux, while strong equatorward transport occurs during the decaying stage. A major poleward momentum flux is attributed to other transient mechanisms, particularly in the upper troposphere. The results are consistent with the hypothesis that the occurrence at preferred latitudes of different stages in the life-cycle of systems is a major factor controlling the latitude profile of meridional transient eddy flux. A postulated meridional eddy flux distribution within a frame of influence for a mid-latitude disturbance during its life-cycle, when related to the broad-scale zonal current, can account for salient features in the observed global distribution of meridional eddy fluxes of both heat and momentum. An indirect toroidal circulation is inferred when such a system interacts with the mean zonal current, and other synoptic implications are discussed.