Hydrodynamics of an open sea Algerian eddy

During the Médiprod-5 cruise, conducted in May–June 1986 in the Algerian Basin, an anticyclonic open sea eddy, similar to those depicted previously with infrared images, was shown deflecting the Algerian Current seawards for months. CTD profiles collected ∼10 km apart down to ∼ 800 decibars throughout the eddy allow a close examination of the density, velocity, vorticity and energy fields. All the major water masses were recognized in the eddy with rather heterogeneous characteristics. Intrusions of surface water, directly issued from the Algerian Current in the form of streamers, were found in places across the eddy. A lens of Modified Atlantic Water, locally cooled during the preceding winter and trapped within the central region of the eddy, was isolated from outside and was in near solid-body rotation. Relatively unmixed Winter Intermediate Water and Levantine Intermediate Water also encountered in the eddy could be more or less exchanged with the surroundings. In a ∼ 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 to 25–30 cm/s) at 50–100 m and ∼ 25 km from the eddy axis. Beneath, 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 ∼ 800 m deep, the distributions of the period of rotation and of the potential vorticity reveal an overall conical structure of the whole eddy. The inner and upper part (diameter of 80–100 km at the surface and depth of ∼300 m near the axis) were relatively isolated from, and rotated more rapidly than, the remainder of the eddy. Effective Rossby numbers for the eddy arc significantly <1, and the centrifugal force cannot be neglected. Strong interactions occurred between the eddy and the Algerian Current. The whole current first deviated from the coast seawards, but then streamers crossed the eddy and carried water back towards the coast. These interactions account for the markedly asymmetrical shape of the eddy. They might also account for its relatively long persistence, through transfers of energy from the Algerian Current itself.