Can the Intermediate Western Boundary Current recirculation trigger the Vitória Eddy formation?

Abstract

Abstract South of the Vitória-Trindade Ridge, a seamount chain off East Brazil, the Brazil Current (BC) meanders cyclonically within Tubarão Bight, occasionally forming the Vitória Eddy. It was recently found that the Intermediate Western Boundary Current (IWBC), which flows equatorward below the BC, cyclonically recirculate within Tubarão Bight. We present an analysis of AVISO observations that suggest that the Vitória Eddy formation is conditioned by the strength of the BC upstream of Tubarão Bight. A weak BC is prone to local meandering and eddy formation in the bight, while a strong BC suppresses eddy formation in the bight but triggers downstream meander growth. To study the effects of the IWBC recirculation on the BC meandering and the Vitória Eddy formation, we formulate a simple two-layer quasi-geostrophic model. In the model, the BC is represented by a meridional jet in the upper layer and the IWBC recirculation is a steady eddy in the lower layer. The lower-layer eddy effectively acts as a topographic bump, affecting the upper-layer jet via the stretching term

ψ

2

/

R

d

2

$\psi _{2}/{R_{d}^{2}}$ , where ψ2 is the lower-layer streamfunction and Rd is the baroclinic deformation radius. Based on the AVISO sea-surface height data and previous observational studies, we define a stationary eddy and reference jet. We conduct a number of initial-value problem experiments varying the upper-layer jet speed. A weak upper-layer jet slowly meanders and develops a cyclone above the lower-layer eddy. As we increase the jet velocity, the meandering is faster and the cyclone is larger. But a too-strong jet has an opposite effect: the potential vorticity anomalies induced by the lower-layer eddy are quickly swept away, leading to explosive downstream meander growth; no cyclone is formed above the lower-layer eddy. In all cases, the initial meandering trigger is a linear process (the steering of the upper-layer jet by the lower-layer eddy). But even when the upper-layer jet is weak, nonlinearity quickly becomes important, dominating the dynamics after 10 days of simulation. The downstream meander growth is fully nonlinear. Our idealized QG model confirms that the IWBC recirculation can trigger the Vitória Eddy formation and elucidates the mechanisms involved in this process.