Dynamics and Evolution of a Northern Meddy

Abstract

A meddy was discovered in April 1997 off the northwestern corner of Spain, near 45°N, 11°30?W. It was tracked during 18 months with Lagrangian floats and deep drogued buoys, and several cruises were set to collect further hydrological and Lowered-ADCP measurements on it. The meddy, named Ulla, was a one-core lens with maximum values of temperature and salinity of 11.5°C and 36.17 psu near 1000-m depth, yielding anomalies above 2.5°C and 0.5 psu compared to its environment. Its rotation frequency was close to 1 loop every 5 days. The maximum azimuthal velocities of 15?20 cm s?1 were reached near a 15-km radius. The meddy had a wide remote influence, notably up to the surface, and was associated with a total azimuthal volume transport of around 10 Sv, (Sv = 106 m3 s?1), of which around 2 Sv was trapped in the core. A widening of the radial structure with decreasing depth was notable in August 1997. Meddy Ulla was significantly elliptic for most of the time and, depending on the periods, the main ellipse axis either slowly rotated clockwise, or kept a constant orientation. The hydrological properties and vorticity of the core remained relatively constant during the observation period. Beyond the radius of maximum azimuthal velocity, the velocity structure was extremely variable and cannot be modeled simply. Meddy Ulla had two other characteristics that make it a particular member of the meddy family: first, it is the northernmost meddy, by around 5 latitude, that has ever been thoroughly studied. It is believed that Ulla was generated in the Cape Finisterre?Cape Ortegal area, far to the north of previously known formation sites. Second, Meddy Ulla stayed in the same area for 11 months while meddies usually drift quite rapidly, in a generally southwestward direction. Among the reasons advocated for its near stagnation are the frequent interaction with other ?northern meddies? that drifted past Ulla to its south and the likely interaction with deep seamounts. After 11 months of stagnation, the meddy suddenly accelerated southwestward and lost some volume at the same time. Finally, after 18 months of observations, it was lost: at that time its net displacement was only 190 km southwestward, at a mean velocity of 4 mm s?1. Meddy Ulla is the first of a series of ?northern meddies? identified during the Action de Recherche sur la Circulation en Atlantique Nord Est (ARCANE) program.