Regional modeling of internal tide dynamics around New Caledonia: energetics and sea surface height signature

Abstract

Abstract. The Southwestern Tropical Pacific exhibits a complex bathymetry and represents a hot spot of internal tide generation. Based on a tailored high-resolution regional model, we investigate for the first time the internal tide field around the New Caledonia islands through energy budgets that quantify the internal tide generation, propagation, and dissipation. A total of 15.97 GW is converted from the barotropic to the baroclinic tide with the main conversion sites associated with the most prominent bathymetric structures such as continental slopes and narrow passages in the north (2.17 GW) and ridges and seamounts south of New Caledonia (3.92 GW). The bulk of baroclinic energy is generated in shallow waters around 500 m depth and on critical to supercritical slopes highlighting the limitations of linear semi-analytical models in those areas. Despite the strongly dominant mode-1 generation, more than 50 % of the locally generated energy dissipates in the near-field close to the generation sites. The remaining energy propagates within well-defined tidal beams with baroclinic energy fluxes of up to 30 kW m−1 toward the open ocean, strongly dominated by mode-1. The energetic mesoscale eddy activity in the region appears to be the main source of tidal incoherence. Locally, mesoscale eddy-driven stratification changes induce variations of the conversion term. In the far-field, incoherence of the energy flux arises through the interaction of the tidal beam with the eddying background flow. The New Caledonia site represents a challenge for SWOT (Surface Water Ocean Topography) observability of meso- and submesoscale dynamics in the presence of internal tides with sea surface height signatures > 6 cm. We show that a correction of the coherent baroclinic tide may improve the observability range by shifting the transition scale between balanced and unbalanced flow in winter from 180 km to 50–80 km. In contrast, in summer observability increases only marginally due to the seasonally amplified signature of the incoherent tide at scales below 100 km.

Mode-1 N2 internal tides observed by satellite altimetry

“…In this study, it is trimmed to 30 • S-0 • , 145 • E-125 • W. Colors show flux magnitudes, and black arrows show flux vectors. This region is chosen because (1) it features various topographic obstacles such as mid-ocean ridges and island chains and (2) the New Caledonia region is one site for SWOT calibration/validation field experiments (Bendinger et al, 2023). There are numerous N 2 and M 2 internal tidal beams in this region.…”

Ocean 2D eddy energy fluxes from small mesoscale processes with SWOT

“…Most of these studies are based on eddy-resolving simulations (1/12 • or 1/10 • spatial resolution) or global altimetry maps that resolve the larger mesoscales > 100 km in diameter but not the smaller mesoscales or submesoscales, yet the ocean fine-scale dynamics (15 to 150 km), with their strong gradients in ocean properties and temporal scales ranging from days to weeks, are now understood to affect the ocean physics and biomass up to climate scales (Ferrari and Wunsch, 2008). Over the last decade, high-resolution ocean models have made great advances in resolving finer-spatial-and finer-temporal-scale dynamics, down to a few kilometres regionally (Renault et al, 2018;Verron et al, 2020;Contreras et al, 2023;Bendinger et al, 2023) and globally (Arbic et al, 2022). These models provide a more detailed description of the dynamics of energetic Western Boundary Current systems and in particular the role of meso-and submesoscale structures and their rectification on the mean circulation (McWilliams, 2008;Gula et al, 2014;Renault et al, 2016;Chassignet and Xu, 2017;Renault et al, 2017;Contreras et al, 2023).…”