Marine Heat Wave sensibility to tidal and atmospheric forcing, case study in French Polynesia

Marine Heat Waves (MHW) are heat extrema in the ocean, with temperature significantly exceeding climatological standards. They can greatly impact ecosystems, coral reefs, and fisheries, in particular in insular economies. Polynesia is wide archipelago in the South Pacific ocean, highly dependent on marine resources for its economy, to which MHW are an increasing threat.

MHW are monitored since 2 decades using remote-sensing and in situ data, and in some cases assimilated simulations. There is already an observed trend of increased MHW frequency in various part of the world ocean. Additionnally, MHW dissipation was observed in several studied cases to be linked with strong winds and high-frequency atmospheric events (e.g. tropical cyclones). Polynesian seamounts are also active internal tides generation hotspots. Internal tides increase vertical mixing and can decrease sea surface temperature, hence tides could also locally impact MHW dynamics. However so far none of these drivers has been  systematically investigated.

Here we explore the sensibility of MHW to these physical drivers, in a case study in the Polynesian archipelago and using free (non-assimilated) 3D realistic simulations with the CROCO model. Air-sea fluxes are parametrized from ERA5 atmospheric reanalysis. Tides have a significant impact on sea surface temperature, but not on the heat wave. On the other hand high-frequency atmospheric forcing is shown to greatly affect MHW regional extent and dissipation. Last, Sensitivity to model grid resolution is also investigated to assess reliability of MHW forecasting skills.