Circulation in the Alboran Sea as Determined by Quasi-Synoptic Hydrographic Observations. Part I: Three-Dimensional Structure of the Two Anticyclonic Gyres

Abstract

The circulation and dynamics of the Modified Atlantic Water have been studied using data from an intensive field experiment carried out between 22 September and 7 October 1992. Data included 134 CTD casts, ADCP, and satellite imagery. A well-defined wavelike front was observed with two significant anticyclonic gyres in the western and eastern Alboran Sea. Smaller-scale cyclonic eddies were also observed. The front separates the more saline, older modified Atlantic water (S>38) in the northern region from the fresher, more recent modified Atlantic water (S<36.8) in the south. The associated baroclinic jet had a mean transport of 1 Sv and maximum geostrophic velocities of 1.0 ms−1. The three-dimensional structure and spatial scales of both gyres were similar, that is, 90 km long and 220 m deep. In the eastern Alboran, northeast of Oran, the origin of the Algerian Current was also detected with an eastward transport of 1.8 Sv. The general picture can be presented as a structure formed by a wavelike front coupled with two large anticyclonic gyre-small cyclonic eddy systems. The relative importance of stratification, relative vorticity, and Froude number in the distribution of Extel’s potential vorticity has been examined, and potential vorticity conservation is used to infer vertical motion. The vertical velocity forcing has been computed using the quasigeostrophic Q vector formulation of the omega equation. It is found that the differential vorticity advection due to mesoscale phenomena in the western Alboran plays a main role in this forcing. The vertical velocities associated with these mesoscale structures reach maximum absolute values of 15 m day−1.