What does cause the collapse of the Western Alboran Gyre? Results of an operational ocean model

• We numerically investigate the collapse of the Western Alboran Gyre.
• The collapse is triggered by the development of a submesoscale cyclonic vortex.
• The origin of the cyclonic vortex is explained in terms of vorticity advection.
• Vorticity is created in the strait lateral boundary layers.
• Tidal and subinertial flows have the potential to generate high relative vorticity.

The stability of the mean state of the surface circulation of the Western Alboran Sea, characterized by a jet of Atlantic water (AJ) surrounding the Western Alboran Gyre (WAG), is investigated with a submesoscale-resolving operational ocean model. It is shown that this circulation state collapses through the interaction between the AJ-WAG system and a growing cyclonic eddy that arises close to the Spanish coast. This eddy develops as the WAG partially blocks the positive potential vorticity (PV) flux coming from the Strait of Gibraltar. It is found that tides increase the positive PV flux with respect to a non-tidal simulation, thus making the destabilization of the mean circulation state more likely. Atmospheric pressure driven flows are also shown to have the potential to destabilize the circulation by rising dramatically the PV flux within a time scale of some days. This provides an explanation as to why the circulation of the Western Alboran Sea exhibits more variability toward winter time, when meteorological fluctuations are enhanced.