Observation of a western Alborلn gyre migration event

In the Western Alborلn 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 Alborلn gyres. However, the circulation in the Alborلn Sea is strongly varying in time, with episodes where neither the western Alborلn gyre nor the eastern Alborلn gyre are present. In October 1996 four consecutive high-resolution surveys of the western Alborلn gyre (WAG) were carried out and Lagrangian floats were released. During the first survey the western Alborلn 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 Alborلn Sea during a WAG migration event, relating the migration to changes in the inflow through the Strait of Gibraltar.