Near real-time analyses of the mesoscale circulation during the POMME experiment

This paper presents an analysis scheme applied to the POMME (Programme d'Océanographie Multi-disciplinaire Méso-Échelle) multi-parameter data set in order to investigate the near-surface circulation. It is based on an objective mapping scheme incorporating velocity, displacement and density data, assuming geostrophic dynamics. Emphasis is on the near surface layers for which most of the velocity and displacement data were obtained. The objective mapping is applied successively from the deepest level to the shallowest level, carrying upward the correction done on the guess field by the mapping routine. The mapping was done on a weekly basis with a guess field from a quasi-geostrophic model incorporating altimetric sea level data. The data set investigated covered a little over a year from early September 2000 to late September 2001 with the largest quantity of data collected within a region 500 km in longitude (15-21W) and 800 km in latitude (38-45N). In a few instances, separate analyses with independent data could be done that are used to assess the errors in the analysed fields. These are found to be much smaller than the mesoscale signals. The analysis is clearly better than the guess in many respects. The time evolution of kinetic energy (KE) is more regular, and its spatial structure more consistent than in the guess fields. Structures are better identified and for one particular eddy independently tracked, the accuracy of the positioning of the structure centres is assessed with an rms error less than 15 km compared to a rms error of about 25 km for the guess field. However, the scales in the analysed coherent structures are somewhat larger than the ones observed. This method is also found not to be adequate for mapping the vertical shear. Diagnostics based on following Lagrangian trajectories in these analysed velocity fields indicate that the current structure near 41N corresponds to a front that acts as a barrier to the diffusion of tracers, except in March-April 2001. This interpretation is supported by the hydrographic and biological surveys done during POMME. Data distribution is, however, not always sufficient to constrain the velocity field, and the analysis provides estimates on where this happened.