Meso and submesoscale dynamics of the western boundary currents in the Vitória Trindade Ridge region

• Autor(a) principal: Napolitano, Dante Campagnoli
• Data de Publicação: 2020
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/21/21135/tde-11052022-143455/
• topic: Brazil Current; Cadeia Vitória Trindade; Corrente de Contorno Intermediária; Corrente do Brazil; flow-topography interactions; interações escoamento-topografia; Intermediate Western Boundary Current; mesoescala; mesoscale; submesoescala; submesoscale; Vitória-Trindade Ridge

This work adresses the dynamics of western boundary currents in the region of the Vitória Trindade Ridge and adjacent Tubarão Bight. First, we deepen into our understanding of the dynamics of the Intermediate Western Boundary Current (IWBC) recirculation at Tubarão Bight using new direct observations, an Argo float climatology, and a ROMS numerical simulation. With a quasi-geostrophic (QG) intermediate-layer model, we show that the ROMS time-mean flow is a good proxy for the IWBC steady state. Constrained by the Tubarão Bight topography, the IWBC recirculation is part of its steady state. Further analysis of the ROMS simulation reveals that incoming westward-propagating nonlinear eddies enter Tubarão Bight, breaking the steadiness of the flow. The perturbations are advected downstream and strongly interact with the IWBC, experiencing explosive growth through horizontal shear production as they negotiate the local topography. In the upper layers, the Brazil Current (BC) meanders cyclonically within Tubarão Bight, occasionally forming the Vitória Eddy above the topographically-constrained IWBC recirculation. We present an analysis of AVISO observations which suggest that the Vitória Eddy formation is conditioned to the BC strength upstream of Tubarão Bight. We build a simple two-layer QG model (in which the IWBC recirculation effectively acts as a topographic bump) and conduct experiments varying the upper-layer jet speed. A weak upper-layer jet slowly meanders and develops a cyclone above the lower-layer eddy. But a too-strong jet sweeps away the potential vorticity anomalies, triggering downstream meander growth. The dynamics leading to eddy formation in the upper-layer are both linear (the initial trigger by the lower-layer eddy) and nonlinear (downstream advection and meander growth), being intimately related to the ratio between the velocity in the upper and lower layers. In a third study, we downscale from the meso to the submesoscale. Using original high-resolution sampling, we capture the BC and IWBC flowing across the Vitória Trindade Ridge banks and channels. A potential vorticity (PV) analysis reveals large patches of low and negative PV where the flows interact with topography. From this interaction, symmetric instabilites develop below the mixed-layer depth. We then use a 2-km ROMS numerical simulation to characterize the seasonality of the flow: while transects without direct interaction with topography show a clear seasonal cycle in the submesoscale, flow-topography interactions---generating unstable conditions below the mixed layer throughout the year---mask this seasonality. As captured by the observations, symmetric instabilities account for nearly all the deeper patches of unstable flow.