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. The North Ionian Gyre (NIG) displays prominent inversions ondecadal scales. We investigate the role of internal forcing induced bychanges in the horizontal pressure gradient due to the varying density ofAdriatic Deep Water (AdDW), which spreads into the deep layers of thenorthern Ionian Sea. In turn, the AdDW density fluctuates according to thecirculation of the NIG through a feedback mechanism known as the bimodaloscillating system. We set up laboratory experiments with a two-layerambient fluid in a circular rotating tank, where densities of 1000 and 1015 kg m−3 characterize the upper and lower layers, respectively. From thepotential vorticity evolution during the dense-water outflow from a marginalsea, we analyze the response of the open-sea circulation to the along-slopedense-water flow. In addition, we show some features of thecyclonic and anticyclonic eddies that form in the upper layer over the slopearea. We illustrate the outcome of the experiments of varying density andvarying discharge rates associated with dense-water injection. When thedensity is high (1020 kg m−3) and the discharge is large, the kineticenergy of the mean flow is stronger than the eddy kinetic energy. Conversely, when the density is lower (1010 kg m−3) and the dischargeis reduced, vortices are more energetic than the mean flow – that is, theeddy kinetic energy is larger than the kinetic energy of the mean flow. Ingeneral, over the slope, following the onset of dense-water injection, thecyclonic vorticity associated with current shear develops in the upperlayer. The vorticity behaves in a two-layer fashion, thereby becominganticyclonic in the lower layer of the slope area. Concurrently, over thedeep flat-bottom portion of the basin, a large-scale anticyclonic gyre formsin the upper layer extending partly toward a sloping rim. The density recordshows the rise of the pycnocline due to the dense-water sinking toward theflat-bottom portion of the tank. We show that the rate of increase in theanticyclonic potential vorticity is proportional to the rate of the rise ofthe interface, namely to the rate of decrease in the upper-layer thickness(i.e., the upper-layer squeezing). The comparison of laboratory experimentswith the Ionian Sea is made for a situation when the sudden switch fromcyclonic to anticyclonic basin-wide circulation took place followingextremely dense Adriatic water overflow after the harsh winter in 2012. Weshow how similar the temporal evolution and the vertical structure are inboth laboratory and oceanic conditions. The demonstrated similarity furthersupports the assertion that the wind-stress curl over the Ionian Sea is notof paramount importance in generating basin-wide circulation inversionscompared with the internal forcing.

Impact of dense-water flow over a sloping bottom on open-sea circulation: laboratory experiments and an Ionian Sea (Mediterranean) example