Rossby waves in the Levantine Basin driven by the North Ionian Gyre vorticity changes: formation mechanism and implications

The eastern Mediterranean Levantine Basin (LB) is characterized by a complicated multi scales system of meandering jets, strong currents and eddies. The most prominent and persistent feature is a warm anticyclonic eddy, the Cyprus Gyre (CG), located south of the island of Cyprus and above the Eratosthenes Seamount (ESM). A small cyclonic and anticyclonic eddy, the South Shikmona Eddy (SSE) and North Shikmona Eddy (NSE), occasionally form downstream of the CG. The center of the CG migrates seasonally between 31.75° – 33°E, and the reason for this east-west drift, as well as the generation mechanisms of the CG today are largely controversial. In the present work, we hypothesize that the CG is an anticyclonic Taylor column generated by the eastward Mid Mediterranean Jet (MMJ) impinging over the ESM, and that the downstream meandering wake is a standing Rossby wave whose embedded cyclonic and anticyclonic eddies correspond to the SSE and NSE, respectively. It is also shown that the position of the CG center is regulated by the intensity of the incoming MMJ and therefore, by the vorticity changes of the North Ionian Gyre. This hypothesis is proven using a series of scaled (based on in-situ ocean data) laboratory experiments conducted in the Coriolis Rotating Platform (LEGI, Grenoble), using a submerged obstacle on a ß plane in both homogeneous and stratified ambient flow conditions (McCartney (1975)). Reanalysis products, Argo floats and glider data are used to corroborate our analysis. These Rossby waves have been shown to lead to local upwelling and transport of nutrient-rich coastal water offshore, with important consequences not only on the physical, but also on the biological dynamics of the eastern Mediterranean.