Internal tides off the Amazon shelf during two contrasted seasons: Interactions with background circulation and SSH imprints

Abstract

Abstract. The Amazon shelf break is a key region for internal tides (IT) generation. The region also shows a large seasonal variation of circulation and associated stratification. The objective of this study is to document how these variations will impact IT generation and propagation properties. A high-resolution regional model (1/36° horizontal resolution), explicitly resolving IT is analyzed to investigate their interactions with the background circulation and stratification, over two seasons: first MAMJJ (March to July), with weaker mesoscale currents, shallower and stronger pycnocline, and second ASOND (August to December) with stronger mesoscale currents, deeper and weaker pycnocline. IT are generated on the shelf break between the 100 and 1800 m isobaths, with a maximum on average at about 10 km offshore. South of 2° N, the conversion from barotropic to baroclinic tide is more efficient in MAMJJ than in ASOND. At the eight main IT generations sites, the local dissipation is higher in MAMJJ (30 %) than in ASOND (22 %). The remaining fraction propagates away from the generation sites and mainly dissipates locally every 90–120 km. The remote dissipation increases slightly during ASOND and the coherent M2 fluxes seem blocked between 4°–6° N west of 47° W. Further analysis of 25 hours mean snapshots of the baroclinic flux shows deviation and branching of the IT when interacting with strong mesoscale and stratification. We evaluated sea surface height (SSH) frequency and wavenumber spectra for subtidal (f < 1/28h−1), tidal (1/28h−1 < f < 1/11h−1) and super tidal (f > 1/11h−1) frequencies. Tidal frequencies explain most of the SSH variability for wavelengths between 300 km and 70 km. Below 70 km, the SSH is mainly incoherent and supertidal. The length scale at which the SSH becomes dominated by unbalanced IT was estimated to be around 250 km. Our results highlight the complexity of correctly predicting IT SSH in order to better observe mesoscale and submesoscale from existing and upcoming altmetrics missions, notably the Surface Water Ocean Topography (SWOT) mission.

Nonlinear Internal Tides in a Realistically Forced Global Ocean Simulation

“…The faster mode 1 wave overtakes the slower mode 2 wave, causing alternating patterns of reduced and elevated surface kinetic energy. Similar banding patterns have been observed in satellite altimetry and model simulations of the Amazon Shelf region by Tchilibou et al (2022). These banding patterns are not present in KE after depth integration.…”

“…The main internal tide beam is generated in at least three hot spots along the steep Amazon Shelf slope (Figure 5a), in agreement with previous studies (Magalhaes et al, 2016;Tchilibou et al, 2022). The tidal energy flux is high at the shelf slope and along the internal tide propagation path with values >14 kW/m.…”

“…The regularly spaced banding patterns, for example, in semidiurnal surface KE, are not unique to our simulation. Similar patterns in semidiurnal SSSH variance at the same locations and with the same separation distances have been detected in satellite altimetry and regional model simulations of the Amazon Shelf region as shown in Figures 2c and 2d of Tchilibou et al (2022). As in our simulations, the patches shown in Tchilibou et al (2022) occur near 2.5° and 4°N along the main beam.…”

“…In this beam, nonlinear internal solitary waves appear about 500 km from the coastline, coinciding with the rapid decay of the primary tide energy fluxes (Magalhaes et al, 2016). The direction of the main wave beam is modulated by the boundary currents along the shelf, namely the North Brazil Current and North Equatorial Counter Current (Magalhaes et al, 2016;Tchilibou et al, 2022). In this section we analyze the energy budget terms along the main internal tide beam generated at the Amazon Shelf to indicate potential sources of supertidal energy.…”

Global submesoscale diagnosis using alongtrack satellite altimetry

“…An important cascade of energy is apparent in the SSH spectrum around the tropical latitudes (Tchilibou et al, 2018), with an increased high-frequency variability of tidal origin (mainly non-phase locked) for wavelengths shorter than 70 km (Tchilibou et al 2022). Using a highresolution global OGCM, Qiu et al (2018) explored the influence of the internal tides on their 505 estimates of Lt, concluding that the removal of the phase-locked internal tide contribution significantly reduces the values of Lt by 50 to 100 km in many regions.…”