An underwater glider of PLOCAN finds a Mediterranean water eddy “Meddie” between the eastern Canary Islands

An underwater glider Slocum G2 has found a Mediterranean water eddy (“Meddie”) of an approximate dimension of the island of Gran Canaria near the western coasts of the islands of Lanzarote and Fuerteventura during the last seasonal mission of the year developed between the European Station for Time-series in the Ocean (ESTOC), located 60 miles north of Gran Canaria, and the islands of Lanzarote and Fuerteventura.

The difficulties that have been generated by the pandemic, have significantly highlighted the added value in efficiency and sustainability, among others, that involves the use of state-of-the-art technological solutions in marine robotics for ocean monitoring. PLOCAN’s Integrated Observatory and the European Station for Time-series in the Ocean (ESTOC), which have concluded their activities satisfactorily with the seasonal glider mission ESTOC2020_3, have been a clear example of this fact during 2020.

This last mission has been developed uninterruptedly for 15 days and has sailed just over two hundred nautical miles with a Slocum G2 underwater glider, making dives every three hours at 1000 meters depth, which has made it possible to collect nearly a million data from each one of the five observed seawater biogeochemical parameters (temperature, conductivity, dissolved oxygen, chlorophyll and turbidity), in accordance with current international standard programs and methodologies regarding the ocean observation.

During this last mission, the unusual find of a Mediterranean water eddy has to be highlighted.

A Mediterranean water eddy is a lenticular-shaped underwater structure that flows by rotating on itself, clockwise and at depths between 800m and 1500m in the waters of the North Atlantic Ocean. These structures are known oceanographically by the name of “Meddies” for its English terminology (“Med” – Mediterranean and “Eddy” – eddy). The name was given by two oceanographers, McDowell and Rossby, in 1978.

These scientists discovered a subsurface eddy near to the island of Hispaniola in the western North Atlantic Ocean, about 6000 km from its area of origin in the Gulf of Cadiz, located southwest of the Iberian Peninsula. Since then, these structures have been the focus of numerous investigations due to being one of the most outstanding features of the subsurface hydrography of the North Atlantic Ocean. These eddies are made up of Mediterranean water that flows into the Atlantic through the Strait of Gibraltar.

The temperature and salinity of the “Meddies” are higher than those of the surrounding waters, its shape is lenticular with diameters usually 40-150 km and typical translation speeds of 2 cm/s. In the vertical profiles of the water column, they appear associated with maximum temperatures and salinity. They present diverse trajectories, heading mainly west and southwest from Cape San Vicente, although there are also trajectories to the northwest and south.

It is estimated that around 17 Meddies are generated per year and some studies have calculated the life of these structures to reach 5 years. However, due to the obstacles that they find in their trajectories, mainly the seamounts against which they collide and cause their breakage, an average life of 1.7 years is estimated.

The Meddy observed during this mission has probably followed the less usual trajectory, the south direction, reaching the archipelago of the Canary Islands next to the coast of Lanzarote, in the channel formed by this island together with Fuerteventura and Gran Canaria.

The arrival of the Meddy to the east of the Canary Islands is a singular event compared to the censuses of the Meddies recorded in the scientific bibliography. This small piece of Mare Nostrum has reached the Canarian Atlantic avoiding probable collisions with the great seamounts existing at the north of the island of Lanzarote. It only remains to know if it will be able to surpass the archipelago or if the contact with the islands will erode its structure until it is fully integrated into the Atlantic Ocean waters.

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