Relevant CMEMS products to predict oil slick drift in the Grande America accident

The Copernicus Marine Environmental Monitoring Service (CMEMS) is a unique capability to provide daily state-of-the-art ocean analyses and forecasts. In the event of accidental marine pollution incidents, those products can help predict where slicks of pollutant and other substances spilled at sea will drift. For a given area, several products are generally available and it is sometimes difficult to know which is the most suitable for this type of use. The use of several CMEMS products during a major accident in the Bay of Biscay is presented here.

On March 12, 2019, the merchant ship Grande America sank at a depth of 4600 m, 350 km off the French coast, in the Bay of Biscay. It caused a spill of bunker oil and loss of containers. The MOTHY drift model was used daily during the aerial surveillance and recovery at sea period. It provided drift forecasts for oil slicks and containers up to 3 days in deterministic mode and up to 10 days in probabilistic mode. Long-term modelling of residual diffused pollution was also carried out, in particular to manage continuous leakage from the wreck. A technical committee of experts met daily to evaluate drift observations and forecasts. It focused on the best choices of available ocean models.

The operational ocean analysis and forecasting systems IBI (Iberian Biscay Irish) at 1/36 degree and Global Mercator at 1/12 degree were used. They led to significant differences in drift predictions, and only one of the two systems was retained after a few days of use. These differences are analysed in the light of available observations.

Drift forecasts did not indicate any oil arrival to the coast. This allowed the authorities to organize the response at sea without mobilizing resources ashore. No pollution was indeed observed on the coasts.