The history and decay of a Mediterranean salt lens

Abstract

Subsurface coherent vortices called Meddies¹ are formed by the outflow of salty water from the Mediterranean Sea^1,2 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.