Observations and Laboratory Modeling of Meddy Generation at Cape St. Vincent

The AMUSE, CANIGO, and AMPOR observations provided a large amount of subsurface float data showing evidence of several distinctly different flow behaviors within the Mediterranean Water layers in the vicinity of Cape St. Vincent (southwest Portugal). Some of the floats followed the coastline and moved northward, and some were trapped either in anticyclonic lenses of Mediterranean Water (meddies) generated at Cape St. Vincent or in cyclonic structures. These observations clearly indicated that the anticyclones were accompanied by cyclones, at least during their early stages. As the dipolar structure moves away from the cape, the cyclone tends to shift slowly away from the anticyclone while decaying so that after some time only the meddy persists. These in situ observations are compared with large-scale laboratory experiments and the agreement between the two sets of observations is remarkably good. Near the cape and shortly after the generation, the nondimensional radius, relative vorticity, and rotation period of the anticyclonic lenses observed in situ and in the model are in good agreement. Thus, the experiments and the 3D velocity measurements, which are possible in the test tank, provide useful details of the generation mechanisms and of the lens dynamics. They prove that the Rossby and Froude numbers of the (Mediterranean) current upstream of the cape are determining factors in predicting the behavior at the cape. For sufficiently low Rossby number (0.1 ≤ Ro ≤ 0.6) the laboratory measurements show that there is formation of a dipolar structure at the cape either by instability of the current at the cape or by coupling of the anticyclone created at the cape by the current and of a cyclone created upstream and advected by the current. This dipolar structure then detaches from the cape and, as is observed in nature, the cyclone tends to weaken in strength while separating from the cyclone, whereas the anticyclone moves away from the cape and has a much longer lifetime. The laboratory measurements also show that there is a quick upward diffusion of the negative relative vorticity.