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Accueil > Actualités > Doctorats > Doctorats 2015

Mercredi 4 novembre 2015, soutenance de thèse d’Aimie MOULIN - 14h30, Amphithéâtre K118, site Bergès

Interaction air-mer à l’échelle synoptique et méso-échelle

- Achim Wirth, DR CNRS


This thesis considers air-sea interaction, due to momentum exchange, in an idealized but consistent model. Two superposed one-layer fine-resolution shallow-water models are numerically integrated. The upper layer represents the atmosphere and the lower layer the ocean. The interaction is only due to the shear between the two layers. The shear applied to the ocean is calculated using the velocity difference between the ocean and the atmosphere. The frictional force between the two-layers is implemented using the quadratic drag law. Three idealized configurations are explored.

- First, a new mechanism that induces barotropic instability in the ocean is discussed. It is due to air-sea interaction with a quadratic drag law and horizontal viscous dissipation in the atmosphere. I show that the instability spreads to the atmosphere. The preferred spatial scale of the instability is that of the oceanic baroclinic Rossby radius of deformation. It can only be represented in numerical models, when the dynamics at this scale is resolved in the atmosphere and the ocean. In one-way interaction the shear applied to the atmosphere neglects the ocean dynamics, it is calculated using the atmospheric wind, only. In two-way interaction it is opposite to the shear applied to the ocean. In the one-way interaction the atmospheric shear leads to a barotropic instability in the ocean. The instability in the ocean is amplified, in amplitude and scale, in two-way interaction and also triggers an instability in the atmosphere.

- Second, the air-sea interaction at the atmospheric synoptic and mesoscale due to momentum transfer, only, is considered. Experiments with different values of the surface friction drag coefficient are performed, with a different atmospheric forcing from the first configuration, that leads to a turbulent dynamics in the atmosphere and the ocean. The actual energy loss of the atmosphere and the energy gain by the ocean, due to the interfacial shear, is determined and compared to the estimates based on average speeds. The correlation between the vorticity in the atmosphere and the ocean is determined. Results differ from previous investigations where the exchange of momentum was considered at basin scale. It is shown that the ocean has a passive role, absorbing kinetic energy at nearly all times and locations. Due to the feeble velocities in the ocean, the energy transfer depends only weakly on the ocean velocity. The ocean dynamics leaves nevertheless its imprint in the atmospheric dynamics leading to a quenched disordered state of the atmosphere-ocean system, for the highest value of the friction coefficient considered. This finding questions the ergodic hypothesis, which is at the basis of a large number of experimental, observational and numerical results in ocean, atmosphere and climate dynamics.

- The last configuration considers the air-sea interaction, due to momentum exchange, around a circular island. In todays simulations of the ocean dynamics, the atmospheric forcing fields are usually too coarse to include the presence of smaller islands (typically < 100km). In the calculations presented here, the island is represented in the atmospheric layer by a hundred fold increased drag coefficient above the island as compared to the ocean. It leads to an increased atmospheric vorticity in the vicinity and in the wake of the island. The influence of the atmospheric vorticity on the ocean vorticity, upwelling,turbulence and energy transfer is considered by performing fully coupled simulations of the atmosphere-ocean dynamics. The results are compared to simulations with a constant, in space and time, atmospheric forcing (no wake) and simulations with one-way coupling only (where the ocean velocity has no influence on the atmosphere). Results of our simulations agree with previous published work and observations, and confirm that the wind-wake is the main process leading to mesoscale oceanic eddies in the lee of an island. It is shown that vorticity is injected directly by the curl of the wind stress, but also by wind stress orthogonal to the gradient of the oceanic surface-layer thickness. Furthermore, the importance of the horizontal boundary layer friction (at the island), in higher Reynolds number simulations, leading to intense submesoscale vortices, is evaluated.