Background stratification impacts on internal tide generation and abyssal propagation in the western equatorial Atlantic and the Bay of Biscay

Abstract

Abstract. The forthcoming SWOT altimetric missions aim to resolve the mesoscale with an unprecedented spatial resolution and swath. However, high-frequency processes, such as tides, are undersampled in time and aliased onto lower frequencies, so they need to be corrected properly. Unlike barotropic tides, internal tides (ITs) are not completely stationary and have significant temporal variability due to their interactions with the ocean circulation and the stratification variability. Stratification changes impact both the generation and the propagation of ITs. The present study proposes a methodology to quantify the impacts of background stratification using a clustering method for the classification of a broad range of stratification and idealized modeling of ITs in the frequency domain. The methodology is successfully tested in the western equatorial Atlantic and in the Bay of Biscay. For the western equatorial Atlantic, a single pycnocline is observed and only the two first vertical modes of ITs have significant amplitudes. With no variation in the stratification intensity, the variation in the depth of this single pycnocline linearly impacts the elevation amplitude, energy fluxes and surface wavelength of the two modes. In the Bay of Biscay, there is a permanent deep pycnocline and secondary seasonal pycnoclines near the surface. No proxy have been found to describe the changes in ITs, so a seasonal climatology is explored. The seasonality of the stratification strongly affects the elevation amplitudes as well as the energy fluxes of modes 1, 2 and 3. The distribution of the modes vary with the background stratification, changing the horizontal scales of the ITs.

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

“…Larger conversion rate might be due to the stratification modulation from MAMJJ (shallower and slightly stronger stratification) compared to ASOND. This is in good agreement with idealized simulations of Barbot et al (2021), that show that shallower stratification enhanced the conversion rate in this area.…”

“…For all sites except F, P 2 ratio is stronger in MAMJJ than ASOND, meaning that MAMJJ is more favorable to local dissipation. In the 8 boxes, the generation of mode 2 and 3 is larger in MAMJJ compared to ASOND (see CVR for mode 2 and 3 columns of Table 3), as expected for a season with shallower pycnocline Barbot et al (2021). Once higher modes are generated, instabilities are more probable, and thus local dissipation is higher.…”

“…In ASOND, the pycnocline deepens by more than 50 m and reaches 170 to 190 m in the area delimited by the NBC and its retroflection (Figure 5d). The internal tides generated in ASOND are expected to have less higher modes and a larger wavelength of the mode 1 (Barbot et al, 2021) . Also, the steep pycnocline slope established along the NBCR / NECC (front) in ASOND, might act as a kind of barrier to the free propagation of the internal tide.…”

“…The distance (90 km) smaller than a mode 1 baroclinic wavelength (120 km) suggest that the dissipation would occurs in the model in the water column between 100 and 500m. It is also possible that the 90 km distance is a consequence of a change in stratification and particularly in the depth of the pycnocline as discussed by Barbot et al (2021).…”

“…The seasonality of the stratification induces a seasonality of the internal tides. Seasons with a shallow pycnocline coincide with the generation of high vertical modes, while a deeper pycnocline leads mostly to mode 1 internal tide generation (Tchilibou et al, 2020;Barbot et al, 2021). Idealized and, less often, realistic studies have looked at internal tides and current interactions.…”