Observation of a western Alborán gyre migration event

Abstract

In the Western Alborán Sea light Atlantic water and dense Mediterranean waters meet. This gives rise to intense density fronts and energetic mesoscale features such as two semi-permanent anticyclonic vortices: the western and eastern Alborán gyres. However, the circulation in the Alborán Sea is strongly varying in time, with episodes where neither the western Alborán gyre nor the eastern Alborán gyre are present. In October 1996 four consecutive high-resolution surveys of the western Alborán gyre (WAG) were carried out and Lagrangian floats were released. During the first survey the western Alborán Sea was occupied by the WAG, surrounded by the Atlantic jet. The following surveys showed the WAG drifting eastward, beginning a WAG migration event, as confirmed 15 days later by the Lagrangian float tracks and the SST images. To replace the old gyre, one month later a new gyre was formed. Analysis of ADCP velocities, SST images and TS properties show a change, between the first and the second survey, in the Atlantic inflow through the Strait of Gibraltar. The relationships between this change in the Atlantic inflow, a drop in the atmospheric pressure at Ceuta, the tidal regime and the WAG migration are discussed. The existence of a high resolution sampling in space together with the possibility of addressing the temporal variability has allowed us to describe, in detail and for the first time, the time evolution of the Western Alborán Sea during a WAG migration event, relating the migration to changes in the inflow through the Strait of Gibraltar.