Drift forecast with Mercator Ocean velocity fields and addition of external wind/wave contribution

Law Chune, S. ; Drillet, Y. ; Mey, P. De ; Daniel, Pierre

Année de publication
2012

Predicting the fate of sea pollutions or drifting objects is a crucial need during disasters and also a very challenging task for ocean models. In case of incident over the French marine territory, Météo France has the responsibility to provide reliable ocean drift forecasts for authorities and decision makers. For that purpose, the oil spill model MOTHY (Daniel et al, 1996) was developed and is operated on duty 24/7/365. This system quickly computes the top layer surface ocean's response to rapid changes in the atmospheric forcing to estimate surface trajectories, but provide limited forecasts in waters dominated by large scale currents or meso-scale features and eddies. Oceanographic operational systems are an efficient tool to have access to realistic surface currents, and since 2007, MOTHY is fed with currents forecasted by Mercator Océan's assimilated systems. This cooperation already provided helpful assistance in the past, like during the Prestige incident where forecasts in the Bay of Biscay were improved thanks to large scale currents supplied by operational oceanography, in particular by Mercator-Océan.
The first part of this paper focusses on the forecast error ranges obtained with several oceanic simulations used in the reproduction of surface buoys trajectories collected in two specific areas: the Western Mediterranean sea and the southern part of the Guinea Gulf along the Angola coast. Drift forecasts have been computed with the 1/12° oceanographic system operated by Mercator Océan (PSY2V3R1) (Dombrowsky et al, 2009) and with some regional nested configurations especially developed for this study to evaluate benefits of some modeling improvements. In a second part, we present a simple way to take into account the windage and the Stokes drift, the latter leading to a strong improvement of forecast in case of significant wind. The forecast period that we are interested in goes from a few hours up to three days, a typical timescale for a quick action when an incident occurs.

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