The history and decay of a Mediterranean salt lens

Document Type


Date of Original Version



Subsurface coherent vortices called Meddies1 are formed by the outflow of salty water from the Mediterranean Sea1,2 into the North Atlantic. In October 1984 we began a study to observe the life history and large-scale changes of a Meddy by identifying a specific Meddy, and carefully mapping it and seeding it with Sofar floats3. (These neutrally buoyant floats are tracked acoustically and can be located aboard ship.) As this Meddy moved southward across the Madeira Abyssal plain, it was resurveyed three more times during a span of two years. Being able to find this same lens (100km in diameter) on successive surveys was itself a unique achievement that allowed us to observe the Meddy evolution and to gain new insight into the importance of different mixing mechanisms that cause Meddy decay. We find evidence of mixing by at least three processes: (1) lateral mixing by the exchange of layers of water ('thermohaline intrusions')4,5, (2) vertical mixing at the underside of the lens by salt fingers6 and (3) mixing by turbulence. Together these cause the net heat and salt anomalies to decay with an e-folding time of about one year. Despite the mixing, the relative vorticity at the core remained constant for the first year and the Meddy retained its coherent shape over a two-year period. © 1988 Nature Publishing Group.

Publication Title, e.g., Journal