Eddy-internal tide interactions around New Caledonia: Insight from regional numerical modeling, in-situ observations, and SWOT

Internal tides, internal waves at tidal frequency, represent a considerable reservoir of kinetic energy within the broad spectrum of motions at frequencies higher than the inertial frequency f. Further, they are argued to play an important role in the modulation of energy pathways, such as the redistribution of energy toward smaller scales, promoting an energy transfer via the forward energy cascade.

New Caledonia, an archipelago in the southwestern tropical Pacific, has recently been identified as a hot spot for semidiurnal internal tides based on regional numerical modeling, with comparable energetics to those at Luzon Strait and the Hawaiian Ridge. Alongside strong internal tides, New Caledonia features elevated mesoscale-eddy activity, and is subject to eddy-internal tide interactions. A twin simulation without tides suggests that tidal forcing has a considerable impact on cross-scale energy exchanges by amplifying the forward energy cascade while modulating the transition scale between forward and inverse cascade toward larger wavelengths. Though, these modulations underlie strong seasonal variations. In turn, the eddy-internal tide interactions impact the internal-tide life cycle, from the barotropic-to-baroclinic tidal energy conversion to the energy propagation pathways. Specifically, mesoscale-eddy-induced stratification changes at the generation sites can considerably enhance/reduce the barotropic-to-barotropic conversion rate by more than 20% on monthly to intraseasonal time scales. In propagation direction, the tidal beams are primarily refracted by mesoscale currents and are characterized by overall increased phase variability and dispersion.

Important insights are gained by moorings deployed in the internal-tide generation hot spot and in the propagation direction south of New Caledonia, enhancing our understanding of eddy–internal tide interactions from an in-situ perspective. Mooring observations at fixed locations are further complemented by daily sea surface height (SSH) measurements provided by the Surface Water Ocean Topography (SWOT) satellite altimetry mission during its fast-sampling phase (1-day repeat orbit). Together, these datasets will help understand the governing dynamics of internal tides and their broader impact on oceanic energy pathways.