HYDRODYNAMICS OF AN OPEN SEA ALGERIAN EDDY

During the Mediprod-5 cruise, conducted in May-June 1986 in the Algeri an Basin, an anticyclonic open sea eddy, similar to those depicted pre viously with infrared images, was shown deflecting the Algerian Curren t seawards for months. CTD profiles collected similar to 10 km apart d own to similar to 800 decibars throughout the eddy allow a close exami nation of the density, velocity, vorticity and energy fields. All the major water masses were recognized in the eddy with rather heterogeneo us characteristics. Intrusions of surface water, directly issued from the Algerian Current in the form of streamers, were found in places ac ross the eddy. A lens of Modified Atlantic Water, locally cooled durin g the preceding winter and trapped within the central region of the ed dy, was isolated from outside and was in near solid-body rotation. Rel atively unmixed Winter Intermediate Water and Levantine Intermediate W ater also encountered in the eddy could be more or less exchanged with the surroundings. In a similar to 50 m mixed layer, the water in the central region of the eddy was denser than in the outer region. As the reverse occurs below, this leads to relatively large velocities (up t o 25-30 cm/s) at 50-100 m and similar to 25 km from the eddy axis. Ben eath, all the isopleths were depressed by 100-250 m. Although the eddy was mainly a cylinder of 120-140 km diameter and much more than simil ar to 800 m deep, the distributions of the period of rotation and of t he potential vorticity reveal an overall conical structure of the whol e eddy. The inner and upper part (diameter of 80-100 km at the surface and depth of similar to 300 m near the axis) were relatively isolated from, and rotated more rapidly than, the remainder of the eddy. Effec tive Rossby numbers for the eddy are significantly <1, and the centrif ugal force cannot be neglected. Strong interactions occurred between t he eddy and the Algerian Current. The whole current first deviated fro m the coast seawards, but then streamers crossed the eddy and carried water back towards the coast. These interactions account for the marke dly asymmetrical shape of the eddy. They might also account for its re latively long persistence, through transfers of energy from the Algeri an Current itself.