Indications of deep water formation in Gulf of Lion

By Hayley Evers-King and Ben Loveday (PML)

The circulation of oxygen-rich surface waters into the deeper layers of the ocean is known as deep water formation, a process essential for life.

In the western Mediterranean Sea, the process of deep water formation is confined to the Gulf of Lion, located between Barcelona and Marseille.

Here, thanks to a regional cyclonic circulation, and the occurrence of cold and dry winters, the thermal stratification of the region is greatly reduced at this time.

Subsequently, during winter periods of extensive surface cooling, driven by the cold mistral winds, a deepening of the mixed layer causes the formation of small, intense convective ‘plumes’ and an overturning of the water column. As surface waters sink down into the depths, as newly-formed deep water, they spread into the wider region, providing nutrients and oxygen to the entire Mediterranean basin.

Data from the SLSTR and SRAL sensors aboard the Sentinel-3A platform (Figure 1) show the presence of an extensive patch of cold water, centred over the Gulf of Lion (4 °E, 42 °N), corresponding to a high wind speed signal, indicative of a strong mistral.

While these satellite data are unable to fully confirm the occurrence of deep water formation episodes in this case, the conditions presented here are highly indicative of its occurrence. Also, the surface temperatures observed in the ‘cold patch’ are consistent with in situ temperature measurements of the onset of convection.

Data from buoys with sensors moored at different depths can be used to provide a third dimension when combined with the satellite data.

The Gulf of Lion is a fascinating region that was the recent focus for a mini project during a EUMETSAT Copernicus training course and also features in the EUMETSAT Ocean from Space MOOC as one of the filming sites (Figure 2).