Calil, P.H.R., Suzuki, N., Baschek, B., & da Silveira, I.C.A. (2021): Filaments, Fronts and Eddies in the Cabo Frio Coastal Upwelling System, Brazil. Fluids 2021, 6, 54, doi:10.3390/fluids6020054
We investigate the dynamics of meso- and submesoscale features of the northern South Brazil Bight shelf region with a 500-m horizontal resolution regional model. We focus on the Cabo Frio upwelling center, where nutrient-rich, coastal waters are transported into the mid- and outer shelf, because of its importance for local and remote productivity. The Cabo Frio upwelling center undergoes an upwelling phase, from late September to March, and a relaxation phase, from April to early September. During the upwelling phase, an intense front around 200 km long and 20 km wide with horizontal temperature gradients as large as 8 ∘C over less than 10 km develops. A surface-intensified frontal jet of 0.7 ms−1 in the upper 20 m and velocities of around 0.3 ms−1 reaching down to 65 m depth makes this front a preferential cross-shelf transport pathway. Large vertical mixing and vertical velocities are observed within the frontal region. The front is associated with strong cyclonic vorticity and strong variance in relative vorticity, frequently with O(1) Rossby numbers. The dynamical balance within the front is between the pressure gradient, Coriolis and vertical mixing terms, which are induced both by the winds, during the upwelling season, and by the geostrophic frontal jet. Therefore, the frontal dynamics may be largely described as sum of Ekman and turbulent thermal wind balances. During the upwelling phase, a mix of barotropic and baroclinic instabilities dominates in the upwelling center. However, these instabilities do not lead to the local formation of coherent eddies when the front is strong. In the relaxation phase, the front vanishes, and the water column becomes less stratified. The interaction between eastward coastal currents generated by sea level variability, coastal intrusions of the Brazil Current, and sporadic wind-driven, coastal upwelling events induce the formation of cyclonic eddies with diameters of, approximately, 20 km. They are in gradient-wind balance and propagate along the 100-m isobath on the shelf. During this phase baroclinic instability dominates. Cold filaments with widths of 2 km are formed due to straining and stretching of cold, coastal temperature anomalies. They last for a few days and are characterized by downwelling as large as 1 cms−1. The turbulent thermal wind balance provides a good first order estimate of the dynamical balance within the filament, but vertical and horizontal advection are shown to be important. To our knowledge, this is the first account of these smaller scale features in the region. Because these meso- and submesoscale features on the shelf heavily affect the water properties crucial to productivity of the South Brazil Bight, it is important to take these features into account for a better understanding of the functioning of this ecosystem and its resilience to both direct human activities as well as to climate change.
Napolitano, D.C., da Silveira, I.C.A., Tandon, A., & Calil, P.H.R. (2021): Submesoscale phenomena due to the Brazil Current crossing of the Vitória‐Trindade Ridge. Journal of Geophysical Research: Oceans, 126, e2020JC016731, doi:10.1029/2020JC016731
At 20.5°S, the Brazil Current and the Intermediate Western Boundary Current interact with a quasi‐zonal seamount chain, the Vitória‐Trindade Ridge (VTR). While the mesoscale variability generated due to these western boundary currents crossing the VTR has been recently studied, the submesoscale dynamics associated with such features have never been addressed. Here, we use new observations and a 2‐ km‐resolution model to analyze the role of the VTR seamounts in the regional submesoscale dynamics, their seasonality, and instabilities. We present new high‐resolution velocity and density observations that capture submesoscale features associated with the flow. Within these regions, potential vorticity (PV) reveals patches of symmetrically unstable flow close to seamounts. The horizontal resolution (Δx ≃ 1.5 km) of our quasi‐synoptic observations (10 h) partially resolves submesoscale instabilities. Our Regional Oceanic Modeling System simulation identifies two regimes of submesoscale activity in the region, one typically associated with the seasonal cycle of the mixed layer, and a second associated with flow–topography interactions. A spatiotemporal analysis of the vertical buoyancy fluxes points to these flow–topography interactions as the main source of recurrent, deeper instabilities. As the VTR emerges as a submesoscale hotspot in the oligotrophic South Atlantic, the lack of observations still remains the main obstacle to better understand submesoscale processes in the region.
Plain Language Summary:
At 20.5°S, strong currents interact with a submarine chain, the Vitória‐Trindade Ridge (VTR). In this study, we use new observations and a 2‐ km‐resolution regional numerical model to analyze how the interaction between the Brazil Current (BC) and the VTR seamounts give rise to submesoscale instabilities. We present new high‐resolution velocity and density observations that capture submesoscale features associated with the flow, with patches of unstable flow associated with the BC interacting with the seamounts. In the same transects of the cruise, our simulation shows that submesoscale activity follows a typical seasonal cycle. But this seasonality is masked in regions where the flow intercepts topography. A spatiotemporal analysis of the vertical fluxes points to flow‐topography interactions as the main source for these recurrent, deeper instabilities. As the VTR emerges as a submesoscale hotspot in the oligotrophic South Atlantic, the lack of observations still remains the main obstacle to better understand the submesoscale processes in the region.