Mesoscale variability of the Brazil Current in the Santos Bight: is it locally or remotely forced?

• Ano de defesa: 2017
• Autor(a) principal: Almeida, Helio Miguel dos Reis
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Bacia de Santos; Brazil Current; Corrente do Brasil; Linha não dispersiva; Nondispersive Line; Ondas de Vorticidades; Santos Bight; Subinertial Variability; Variabilidade Subinercial; Vorticity Waves
• Link de acesso: http://www.teses.usp.br/teses/disponiveis/21/21135/tde-26032018-143335/

The Brazil Current (BC) is possibly one of the least studied western boundary currents (WBCs) in the world ocean. Its unique vertical structure makes this a different WBC in terms of velocity and transport. Although significant progress has been made over the last decade, the BC system variability within the Santos Bight (SB, 23°S-28°S) is very poorly understood despite its strategic role for Brazil in terms of energy. The BC thickens and changes its vertical structure and dynamical modal composition as it crosses the SB. As it enters the bight by trying to contour Cape Frio (Cf, 23°S), the BC is about 500 m deep. As the current leaves the bight south of Cape Santa Marta (CSM, 28°S) it is 1300 m. These differences in thickness and accompanying velocity vertical shear are due to the impinging branch of the intermediate South Equatorial Current (SEC) generating the so called Santos Bifurcation (SBi). We here investigate the subinertial variability of the BC system within the SB primarily using satellite altimetry data and some traditional spectral analysis techniques. We also analyzed quasi-synoptic observations of an oceanographic cruise, which was part of the IOUSP-PETROBRAS CERES Experiment. We identified that there are different phenomena occurring to the north and to the south of the bifurcation and this might be caused by the SBi feature itself and/or the consequential change in the BC vertical structure downstream. North of the SBi, we detected oscillations with period of 92 days and a wavelength of 490 km and which represent about a fifth of the BC variability at 25°S. The 92-day oscillations are nonlinear vorticity waves, which are originated to the east of 35°W and propagate zonally towards the BC axis. These waves then perturb the current exciting oscillatory motions of the same period and which propagate downstream. South of the SBi axis (~28°S), we identified two different maxima in the ω - κ spectrum. They correspond to oscillations with periods (wavelengths) of 68 and 148 days (397 km and 790 km). The longer wave is also a baroclinic first-mode nonlinear vorticity waves propagating with westward phase speed of the nondispersive planetary Rossby waves of same period. They are originated in the ocean interior and we were able to track its signal as far as 5°W. The 68-day waves has its existence limited to vicinities of the BC. Phase speeds have approximately the BC axis orientation and they can only be clearly detected between ~27°S and ~32°S. This propagating signal accounts with a variance of ~21% on the BC axis. Quasi-synoptic observations allowed the mapping of a anticyclone and a cyclone of such waves. They seem to have modal composition very similar to the BC itself, which might provide additional evidence of local origin.