Upper ocean physics: observations and applications

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Mesoscale (~100 km) and submesoscale (~1-10 km) processes in the upper ocean play a fundamental role in the vertical transport of tracers, exchanging heat and gases with the atmosphere, and supplying nutrients from the ocean's interior into the sun-lit euphotic zone. However, estimating the vertical velocities responsible for this transport from in-situ observations would require to sample the ocean vertically at a prohibitive horizontal resolution. A new method to diagnose vertical velocities using only surface observations, while taking into account the key physical processes of the mixed layer, has been developed and tested against idealized and realistic numerical simulations during the past 5 years.***The long-term objective of the proposed research program is to combine high-resolution satellite***observations of sea surface height, temperature and salinity, and in-situ observations of stratification provided by drifting floats, to estimate: i) the horizontal and vertical turbulent fluxes of tracers in the upper ocean, and ii) kinetic energy transfers between large-, meso- and submeso-scales. To attain this objective, the following short-term objectives will be pursued in the next 5 years:***1) Further develop and test the new method to diagnose vertical velocities from surface observations;***2) Develop methods to filter signals associated with internal waves from satellite altimeter observations;***3) Estimate kinetic energy transfers between meso- and submeso-scales using high-frequency radar***observations to investigate the relative importance of low-frequency versus high-frequency oceanic processes.***The proposed research activities will open new perspectives to fully exploit the high-resolution data from the Surface Water Ocean Topography (SWOT) satellite altimeter that will become available in 2021. Expected outcomes are global observational estimates of upper ocean turbulent tracers fluxes and kinetic energy transfers in the range of 10-100 km. These observational estimates will in turn provide valuable benchmarks to tune and refine parameterizations of unresolved processes in global ocean circulation models used to predict climate change.