Linking fine-scale ocean physics and marine conservation efforts: The importance of internal tides around New Caledonia

New Caledonia, an archipelago in the Coral Sea in the southwestern tropical Pacific, is internationally recognized for its marine biodiversity including its coral reef systems and lagoons (UNESCO World Heritage Site). Yet, only 10 % of the Coral Sea Marine Park is currently under full protection by restricting human and economic activities. Further conservation efforts have failed inter alia linked to political conflicts of interest and scientific knowledge gaps of the local ecosystem. Continuous efforts for marine reserves in ecologically important and vulnerable places such as New Caledonia are highly needed, which may eventually help maintain ocean resilience in the face of climate change. To close the current knowledge gaps, a clear understanding of the governing physical mechanisms at work and their implications for biogeochemical processes are crucial for transdisciplinary ocean governance and management.

Fine-scale ocean physics around New Caledonia are to a large part dominated by internal tides. Internal tides are freely propagating waves at tidal frequency in the ocean interior. They are ubiquitous in the global ocean and are argued to play an essential role in our understanding of open-ocean mixing and the global oceanic energy budget. They are generated by the interaction of tidal currents with the seafloor topography and feature oscillations of surfaces of constant density as large as 100 m. Therefore, internal tides impact local hydrodynamical and biogeochemical properties and have important implications for marine ecosystems and biodiversity through nutrient inputs while thriving biological productivity - potentially up to high trophic levels and marine fauna.

Internal tides around New Caledonia have very recently been studied and quantified using state-of-the-art numerical modeling giving insight into temporal and spatial variability of their generation, propagation, and dissipation. They are closely linked to the major bathymetric features such as continental slopes, shelf breaks, ridges, and seamounts, which represent potential hot spots of marine biodiversity. Yet, in-situ and remote sensing observations are missing to assess the relevance of our findings based on numerical simulations. Promising insight into internal tides is given by the Surface Water Ocean Topography (SWOT) satellite altimetry mission. Launched in December 2022, SWOT represents a breakthrough in sea surface height (SSH) observations along two swaths of 60-km width at unprecedented spatial scales of up to 10x higher resolution than conventional altimetry. The internal-tide generation hot spot around New Caledonia is located just beneath those two swaths during SWOT’s fast-sampling phase, during which SWOT orbited in a 1-day cycle for a duration of 3 months representing a unique study site to infer high-frequency internal-tide variability from SSH. Available in-situ mooring observations at fixed locations beneath the swaths allow for a dynamical interpretation of SWOT SSH while capturing the temporal evolution of the internal tide’s vertical structure.

Several efforts have been initiated in this area of exceptional biodiversity to characterize internal tides (SWOTALIS, SWOTOBS-NC, ScInObs, etc.). We are encouraged that observing and understanding the local impact of internal tides on the ecosystem will play an essential role in marine conservation efforts around New Caledonia.