Internal Tide Generation at the Vitória‐Trindade Ridge, South Atlantic Ocean

Abstract

Sea surface height data from multiple satellite altimetric missions are analyzed in order to investigate the surface manifestation of internal tides generated at the Vitória‐Trindade Ridge (VTR), in the western South Atlantic Ocean. The VTR is formed by a sequence of submerged seamounts disposed in a west‐east alignment around 20.5°S, extending for over 1,000 km from the Brazilian continental margin to the Trindade and Martim Vaz islands. Point‐wise harmonic analysis of the along‐track sea surface height data, combined with a two‐dimensional plane wave fitting at selected subregions, depicts well‐organized northbound and southbound wave beams radiating from the VTR, and propagating as far as ~1,000 km from the ridge. Energy convergence is observed at a focal point located circa 300 km south of the VTR, associated with a lens effect due to the arc‐shaped ridge. Wavenumber spectra show highest energy associated with wavelengths of 140 and 70 km, close to the theoretical values expected for the first and second baroclinic modes, respectively. Energy fluxes for the mode 1 principal lunar (M2) constituent reach values of 2 to 3 kW/m for the southward beams and 1 kW/m north of the VTR. Judging from its surface signature, the VTR appears to be the most significant internal tide generation site in the western South Atlantic.

Observations of Internal Tides at the Vitória‐Trindade Ridge, South Atlantic Ocean

“…This is roughly in agreement with the results from Paiva et al. (2018) based on multimission satellite data, and with the analysis presented in Figure 1 based on Cryosat data, which indicate southward fluxes. Note that the satellite‐based analysis integrates the information in 120 × 120 km wide boxes, while the mooring reflects the data at a single location, what might explain the relatively small difference in direction.…”

“…Figure 1 depicts the same energy beams derived from the multimission altimetry data and shown in Paiva et al. (2018) corroborating their study, although with somewhat smaller values (up to 3 kW/m near the seamounts, as derived from CryoSat‐2 data).…”

“…Such analysis was possible since the larger horizontal resolution of the Cryosat-2 data, when compared to the multimission altimetry, compensates for the shorter time period of observations, as demonstrated by Zhao (2016b). Figure 1 depicts the same energy beams derived from the multimission altimetry data and shown in Paiva et al (2018) corroborating their study, although with somewhat smaller values (up to 3 kW/m near the seamounts, as derived from CryoSat-2 data).…”

“…Note that the satellite‐based analysis integrates the information in 120 × 120 km wide boxes, while the mooring reflects the data at a single location, what might explain the relatively small difference in direction. The magnitude of the computed fluxes is significantly lower, however, than those based on the satellite data (∼3 kW/m from Cryosat, and up to 5 kW/m according to Paiva et al., 2018). While differences between in situ and satellite‐based estimates are not unexpected, and may reflect imperfect data or other factors (Chiswell, 2006), it is possibly related in the present case to the relatively shallow depth at the mooring site, which does not allow the in situ observations to capture the most energetic fluxes in the nearby region.…”

Submesoscale Coherent Vortices in the South Atlantic Ocean: A Pathway for Energy Dissipation

“…However, the combination of weak stratification, high vorticity, low PV, and low Ri corroborates the hypothesis of intense mixing and energy dissipation within such features. On the one hand, part of the measured energy dissipation could arise from the manifestation of internal waves generated in the VTR (Paiva et al., 2018) by the acceleration of the flow or by the collapse of fronts (Shakespeare & Taylor, 2014) or due to the interaction with the local topography. Inertial oscillations are excited in boundary layers (Barkan et al., 2017; Nikurashin & Ferrari, 2011; Thomas, 2017), radiated, and reabsorbed by larger‐scale flows, while only a small fraction of the energy of these oscillations is dissipated (Nagai et al., 2015).…”