Research axis 1 : the ocean engine at very high resolution

Coordinators : Patrice KLEIN (Ifremer / LPO) and Bertrand CHAPRON (Ifremer / LOS)

Participating laboratories

LPO (Laboratoire de physiques de océan), CNRS, Ifremer, IRD, UBO

LOS (Laboratoire d'Océanographie Spatiale), Ifremer

Département Géosciences Marines, Ifremer

Research thematics

Ocean dynamics is driven by motions involving a large range of scales from 10,000 km to a few meters and even a few centimeters. However geostrophic eddies (with diameters ranging from a few tens to 200 km), that are the building blocks of the Ocean Weather, are now known to contain almost 90% of the total kinetic energy (KE) of the flow. As such they are the major driver of ocean currents, the heat transport and of the ocean biogeochemical system. Dynamical impact of smaller scales has been for a long time mostly ignored or, for some of them (such as surface waves), studied within a small spectral range. But recent results of the last 5-10 years strongly question the interaction of these small scales with the larger ones (for example in terms of eddy dissipation) and point out the need to further decipher the scale interactions over a large spectral range.

These recent results originated from the new satellite and in-situ observations with much higher resolution than before as well as from the power increase of supercomputers. Among the major scientific advances, we can mention three breakthroughs:

• In the whole ocean interior, seismic reflection methods have been successfully used to map the internal structure of the entire water column with an outstanding resolution of 10 meters, over sections of several hundreds of kilometers length. This unveiled ubiquitous thin layers surrounding geostrophic vortices. Such layers have been reproduced through non-hydrostatic simulations of comparable resolution, revealing their coincidence with “dissipation hot spots” of kinetic energy, thus indicating a possible route to dissipation in the ocean interior.
• In the upper 500m of the ocean, submesoscales (such as 10km-wide filaments ubiquitous on high resolution satellite images) explain more than 50% the vertical velocity field, so important for the ocean biological system. More generally, numerical simulations, performed with unprecedented high resolution (500m horizontally) at basin scale, have revealed that submesoscales have strong impacts on 3D dynamics; a particular impact was shown on both the intensification and dissipation of geostrophic eddies.
• At the ocean surface, the dynamics of breaking waves is better understood to a point where their statistical properties may be predicted, and applied to upper ocean mixing, air-sea gas exchange and coastal inundation problems. While still poorly understood, models and observation indicate that this superficial mixing is partly entrained to the base of the mixed layer by Langmuir circulations induced by wave-turbulence interactions.

One common finding of these scientific advances is that all these small scales have strong impacts on the larger ones and vice-versa. Such results are important for the ocean KE pathways and therefore for the ocean engine. Efforts are timely to further delve into and reinforce these results before taking them into account in the new generation of climate models (through the development of new parameterisations) and for the improvement of ocean forecast models.

The next step to significantly deepen and strengthen these scientific advances within the next ten years requires to use an integrated approach.

The combined and detailed analysis of the most recent high resolution numerical simulations with high resolution experimental data and the resulting interpretation is a major challenge for the next years.

Scientists from Brest Laboratories are major actors in several of the scientific advances of the last 5 years and have been very active within the international community. The existence of strong numerical, theoretical and experimental expertises in these new fields on the Brest campus means that all ingredients exist to meet this new challenge by following the integrated approach mentioned before, which is the originality of the project.

Expected results

Anticipated scientific results concern the identification and quantification of the different energetic pathways that connect small and large ocean scales. This should produce a new vision of the ocean engine. The deliverables should include new parameterizations of the small-scales processes to be used in the new generation of climate models as well as the improvement of ocean forecast models.

More information : see research project for the 2012-2014 period