Estudo teórico da dinâmica da confluência Brasil-Malvinas

Abstract

ABSTRACT. The study of the mesoscale dynamics of the confluence of two western boundary currents was conducted in this work via a quasi-geostrophic inviscid f -plane two-layer ocean theoretical approach. The simplified vertical structure was calculated through the employment of a dynamic calibration scheme based on the dynamical mode structure of the Brazil-Malvinas Confluence Region. The Brazil-Malvinas mode structure was computed from climatological hydrography of the area delimited by 35 • -40 • S and 50 • -60 • W. Two contour dynamics models were constructed: a linear and a nonlinear version. The basic flow configuration consisted of two converging western boundary currents that form a zonal eastward current in the upper layer. The lower layer flow was essentially divergent as a result of a westward zonal jet impinging on the western border. This vertical shear choice assured that the system was baroclinically unstable. The results of the model experiments showed that the presence of the confluence, the western meridional boundary and the barotropic mode in the dynamical structure of the basic flow favored long wave patterns. The three experiments conducted with the nonlinear model exhibited the development of both a reflection pattern and vortical dipoles. The dipoles pinched off from either the retroflection lobe (i.e., the primary crest of the wave train) or the primary trough when the baroclinically unstable current system was perturbed at the boundary vicinities.It was verified that the nonlinear model simulations followed the instability properties predicted by the linear model in terms of meander growth rates, phase speeds and most unstable wavelengths. This suggested that while the baroclinic instability mechanism was responsible for the temporal growth of the meanders, the nonlinear effects caused the dipole isolation and pinch off of the finite amplitude meanders. These dipoles could leave and propagate away from the current axis.Keywords: Contour dynamics, calibrated layer models, baroclinic instability, Western Boundary Currents, Brazil-Malvinas Confluence. RESUMO.O estudo da dinâmica de meso-escala da confluência de duas correntes de contorno oeste num oceano de duas camadas quase-geostrófico, não viscoso e inercial no plano f foi conduzido neste trabalho. A estrutura vertical simplificada do modelo foi obtida através de calibração dinâmica em relaçãoà estrutura modal da região da Confluência Brasil-Malvinas. Essa estrutura modal foi estimada a partir da climatologia daárea compreendida entre 35 • -40 • S e 50 • -60 • W. Dois modelos de dinâmica de contornos foram então desenvolvidos. Estes apresentaram sistema de correntes de contorno oeste com transporte simétrico e duas frentes de vorticidade, uma por camada. O escoamento na camada superior foi sempre convergente, formando um jato zonal que flui para leste. Já o escoamento na camada inferior foi divergente, com jato zonal de velocidades fluindo dominantemente para oeste. Tal configuração garantiu que o sistema fosse baroclinicamente instável....