I am focusing on ocean dynamics, and especially the understanding of submesoscale processes (1-10km) in the ocean which are a major concern for oceanic and climate modelling. Such processes are currently absent in oceanic general circulation models and global climate models and have an impact on the large scale mean circulation. By combining state-of-the-art high resolution numerical modelling with theory and idealized simulations, I am investigating the dynamics of these small scale processes and how they contribute to regional climate equilibria and intrinsic variability.

SST of the Gulf Stream

SST of the Gulf Stream from a nested sequence of simulations with horizontal resolutions 6 km, 2.5 km, and 750 m.

Series of nested regional models of the Gulf Stream system along the east coast of the United States have been developped. The range of numerical resolution of these models is between 7 km and 150 m, which allows to investigate the dynamics of the flow at a wide range of scales, from gyre-scale balance to submesoscale generation.

Gulf Stream surface relative vorticity

Snaphot of surface relative vorticity in a dx=500m ROMS nest over the Gulf Stream. Animation [.mov ]

Results from high resolution Gulf Stream simulations clearly highlight the emergence of smaller submesoscale currents full of sharp fronts, filaments, and coherent vortices. They are created by mesoscale strain fields that explosively sharpens lateral buoyancy gradients, which in turn become highly unstable. These structures catalyze energy dissipation for the large-scale circulation and are responsible for a large part of the vertical fluxes of mass, buoyancy, and materials in the upper ocean layers.

Submesocale processes

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Submesocale cold filaments

Intense submesocale cold filaments are abundant in the Gulf Stream region. The mechanism of filamentogenesis is triggered by the deformation flow which acts on the favorably-aligned, heavy density filament, causing rapid narrowing and a two-celled secondary circulation with even stronger surface convergence and downwelling at its center than in frontogenesis. The life cycle of a filament is typically a few days in duration, from intensification to quasi-stationarity to instability to dissipation.

Instability and mixing at the North Wall

In support of field experiments by the Office of Naval Research ("LATeral MIXing"), high resolutions simulations allow to further investigate manifestations of surface layer fronts and filaments. The North Wall of the Gulf Stream exhibits regions with intense, small scale vorticity structures that have been successfully observed using a combination of satellite and in-situ measurements and identified as submesoscale mixed-layer instabilities using high resolution simulations.

Topographic control of the Gulf Stream

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Gulf Stream along the U.S. Seaboard

The Gulf Stream strongly interacts with the topography along the U.S. Eastern Seaboard, between the Straits of Florida and Cape Hatteras. Very high resolution simulations are used to to understand and quantify the role played by the interactions of the flow with topographic features, and the subsequent impact of nonlinear eddy-mean flow interactions, to set the characteristics of the Stream.

North Atlantic Gyre Balance

Western boundary currents do not imply a viscous balance against the seaboard (as in the traditional view based on models with flat bottoms and vertical sidewalls), but a nearly inviscid one. The bottom pressure torque is the term locally enabling the return flow of the wind-driven transport in western boundary currents and providing most of the overall positive input of vorticity balancing the negative input by anticyclonic wind curl on the scale of the gyre.