Rossby Wave Radiation in the Cape Verde Frontal Zone

Abstract

Radiating baroclinic Rossby waves excited through instability of the Cape Verde frontal zone are proposed as a mechanism for the generation of mesoscale variability at middepth (1000 m) in the southeastern North Atlantic basin. Linear quasigeostrophic theory is applied to an idealized front representative of the Cape Verde frontal zone to demonstrate that the front is unstable to modes that may radiate away from the frontal region as baroclinic Rossby waves. Evidence for the existence of these waves is obtained from an eddy-resolving, basin-scale general circulation primitive equation model. In addition, the model fields are used to identify characteristic signature of the waves in terms of quantities that may be directly observed in the ocean. Lagrangian trajectories, Reynolds stress, eddy kinetic energy, and frequency spectra taken from SOFAR float and current-meter records are all in good agreement with the amplitude and distribution implied by the wave radiation in both the linear theory and the full primitive equation model. It is concluded that the Cape Verde frontal zone is a source of radiating baroclinic Rossby waves and that these waves are an important component of the low-frequency eddy energy at the middepth ocean in the southeastern basin.