A shallow meddy (a smeddy) from the secondary Mediterranean salinity maximum

Abstract

A smeddy, or shallow meddy with temperature and salinity core characteristics of the Secondary Mediterranean Salinity Maximum (SMSM), was found near 36.75°N, 12.5°W in March 1992, west of Cape St. Vincent and some 600 km from the Strait of Gibraltar. A detailed survey defined the temperature, salinity, nitrate, silicate, oxygen, and light transmission structure of the smeddy. The smeddy core had a maximum salinity of 36.483 psu at a depth of 775 m, and a maximum temperature of 13.55°C at a depth of 725 m. The salinity anomaly was 9 standard deviations from the mean at a depth of 600 m. The property distributions suggest that about 0.5 km of a central water column (∼500–1000 m) was traveling with the smeddy, though the dynamic influence extended from the surface to a depth of about ∼1.5 km. The velocity field was derived from dynamic height differences, the gradient equation, a drogued Argos buoy, and acoustic Doppler current profiler (ADCP) measurements. At the core depth of ∼700 m a number of well‐defined structures were found that characterized the horizontal influence of the smeddy. Surrounding an inner core (temperature > ∼ 13.25°C, salinity > ∼36.4 psu) of radius about 7 km, in near solid body rotation (with center period ∼3.7 days), there was a region of maximum azimuthal currents (∼20 cm/s) at a radius of ∼12 km. At a radius of ∼17 km, horizontal gradients of properties reached maximum values. This water mass boundary was thought to define the horizontal extent of the water actually traveling with the smeddy, giving it an aspect ratio of 1.5%. The smeddy was observed to move about 150 km southward over a period of ∼30 days (∼6 cm/s), passing from the Tagus Abyssal Plain to the Horseshoe Abyssal Plain. Remote sensing and other hydrographic data are used to suggest one route whereby water found in the core of the isolated eddy reached the Horseshoe Abyssal Plain.