Baroclinic instabilities and forced oscillations in the Brazil/Malvinas confluence front

Abstract

Observations collected at the confluence between the Brazil and Malvinas currents are analysed to study the high-frequency oscillations in the range of periods from 55 to 2 days. The variability in time and space of the thermohaline front originates at the confluence. The meandering of the Brazil Current after the encounter and the presence of an abrupt shelf break, create the conditions for the generation of baroclinic and internal waves. Analysis of time-series of dynamic height, wind magnitude, and position of the front indicates the following: in the band from 20 to 55 day periods, two waves propagate with similar characteristics but in the opposite direction; neither of these waves appears to be forced by the wind. The westward-propagating wave ( T = 37.4 days) is related to the position of the front and is assumed to be forced by its north-south displacement. The eastward-propagating wave ( T = 29.1 days) has the characteristics of a topographic Rossby wave. At shorter periods, in the band from 2 to 10 days, the spectrum of the atomosphere is very energetic. The passage of cyclones can be detected by the seasonal variability of the energy spectra. The ocean is forced by the wind at periods of 5 and 2.6 days. In addition to these wind-forced oscillations, the presence of the front originates baroclinic instabilities in the same band.