On the Steadiness and Instability of the Intermediate Western Boundary Current between 24° and 18°S

Abstract

The Intermediate Western Boundary Current (IWBC) transports Antarctic Intermediate Water across the Vitória–Trindade Ridge (VTR), a seamount chain at ~20°S off Brazil. Recent studies suggest that the IWBC develops a strong cyclonic recirculation in Tubarão Bight, upstream of the VTR, with weak time dependency. We herein use new quasi-synoptic observations, data from the Argo array, and a regional numerical model to describe the structure and variability of the IWBC and to investigate its dynamics. Both shipboard acoustic Doppler current profiler (ADCP) data and trajectories of Argo floats confirm the existence of the IWBC recirculation, which is also captured by our Regional Oceanic Modeling System (ROMS) simulation. An “intermediate-layer” quasigeostrophic (QG) model indicates that the ROMS time-mean flow is a good proxy for the IWBC steady state, as revealed by largely parallel isolines of streamfunction [Formula: see text] and potential vorticity [Formula: see text]; a [Formula: see text] scatter diagram also shows that the IWBC is potentially unstable. Further analysis of the ROMS simulation reveals that remotely generated, westward-propagating nonlinear eddies are the main source of variability in the region. These eddies enter the domain through the Tubarão Bight eastern edge and strongly interact with the IWBC. As they are advected downstream and negotiate the local topography, the eddies grow explosively through horizontal shear production.

On the Deep Western Boundary Current Separation and Anticyclone Genesis off Northeast Brazil

“…We evaluate conversions within the kinetic energy budget by analyzing barotropic conversions as a mechanism for the DWBC anticyclones growth. We follow Vallis (2017) and Napolitano et al (2019) to calculate the horizontal shear production (HSP),…”

“…In the DWBC, barotropic instabilities have been shown to play a role in regional dynamics (Biló et al, 2021) used an eddy-rich ocean model and revealed that barotropic instabilities dominate over baroclinic instabilities within the DWBC, with a gain in E KE resulting from strong horizontal shear. Moreover, the presence of the DWBC rubbing against the PP further advocates for the importance of horizontal shear instabilities, as they can draw energy from western boundary currents into eddies (e.g., Mata et al, 2006;Napolitano et al, 2019).…”

“…In a detailed assessment of the energetics in the Gulf Stream and Kuroshio extensions, Chen et al (2014) showed that both barotropic and baroclinic conversions act on the mean flow, driving kinetic energy production. Using a regional numerical simulation off east Brazil, Napolitano et al (2019) showed that eddies grew from perturbations fed by the mean flow of the Intermediate Western Boundary Current, mainly through lateral shear.…”

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

“…Also, effects of topography, and eddy decay processes could be included. Recently, Napolitano et al (2019) showed that the variability in the region is dominated by westward-propagating eddies, which are likely to complicate the picture. To further address the mechanisms that drive the Vitória Eddy, we call for a hierarchy of models, from solutions as simple as ours to idealized primitive-equation simulations to complex regional numerical models.…”

The role played by density in meso and large scale ocean dynamics

“…Therefore, we are introducing the same bias in both the reconstructed and the "new" altimeter sea surface height. Moreover, we ensured that our reconstructed η fields (hereafter η rec ) using HYCOM's velocities preserved the variance of the original altimetric sea level anomaly field by multiplying each field by a constant factor defined as the RMS of the latter divided by the RMS η a (NAPOLITANO et al, 2019). Figures 3.16, 3.15 and 3.17 show η a , derived from altimeter's geostrophic velocities, and η rec , calculated using HYCOM's velocities and ISAS T and S, for all latitudes at each ocean basin.…”