The vertical structure of large-scale unsteady currents

Friday, 19 December 2014
Antoine Hochet, Robert Scott and Alain Colin De Verdiere, Laboratoire de Physique des Océans, Brest, France
Oceanic Rossby waves are the main way by which information is transmitted in the zonal direction. As a result, they set up the response time of the ocean to changes and are essential in understanding the climate system. Although surface characteristics of Rossby waves have been widely studied thanks to advances in satellite observations, their vertical structure is poorly known. Therefore, the purpose of the present study is to give some insight on their vertical structure and, more broadly, on the vertical structure of large scale sea surface height anomalies.
A linear model based on the quasi-geostrophic equations is thus constructed. The subsurface field is reconstructed from sea surface height and climatological stratification. The solution is calculated in periodic rectangular regions with a 3D discrete Fourier transform. The effect of the mean flow on Rossby waves is neglected, which we believe is a reasonable approximation for low latitudes. The method used has been tested with an idealized double gyre simulation (performed with the MICOM ocean model). The linear model is able to give reasonable predictions of subsurface currents at low latitudes (below approximately 30°) and for relatively weak mean flow. However, the predictions degrade with stronger mean flows and higher latitudes. The subsurface velocities calculated with our model using AVISO altimetric data and velocities from current meters have also been compared. Results show that the model gives reasonably accurate results away from the top and bottom boundaries, side boundaries and far from western boundary currents. We found for the regions where the model is valid, an energy partition of the traditional modes of approximately 67% in the barotropic mode and 25% in the first baroclinic mode. Only 20% of the observed kinetic energy can be attributed to free Rossby waves of long periods that propagate energy to the west. A large part of this energy lies between the curves of the free waves dispersion relationships. Therefore raising the question of the mechanism responsible of this energy distribution.