Impact of dense-water flow over a sloping bottom on open-sea circulation: laboratory experiments and an Ionian Sea (Mediterranean) example
Authors: Gačić Miroslav , Ursella Laura , Kovačević Vedrana , Menna Milena , Malačič Vlado , Bensi Manuel , Negretti Maria-Eletta , Cardin Vanessa , Orlić Mirko , Sommeria Joël , Viana Barreto Ricardo , Viboud Samuel , Valran Thomas , Petelin Boris , Siena Giuseppe , Rubino Angelo 
Published: July 2021

Abstract. The North Ionian Gyre (NIG) displays prominent inversions on
decadal scales. We investigate the role of internal forcing induced by
changes in the horizontal pressure gradient due to the varying density of
Adriatic Deep Water (AdDW), which spreads into the deep layers of the
northern Ionian Sea. In turn, the AdDW density fluctuates according to the
circulation of the NIG through a feedback mechanism known as the bimodal
oscillating system. We set up laboratory experiments with a two-layer
ambient fluid in a circular rotating tank, where densities of 1000 and 1015 kg m−3 characterize the upper and lower layers, respectively. From the
potential vorticity evolution during the dense-water outflow from a marginal
sea, we analyze the response of the open-sea circulation to the along-slope
dense-water flow. In addition, we show some features of the
cyclonic and anticyclonic eddies that form in the upper layer over the slope
area. We illustrate the outcome of the experiments of varying density and
varying discharge rates associated with dense-water injection. When the
density is high (1020 kg m−3) and the discharge is large, the kinetic
energy of the mean flow is stronger than the eddy kinetic energy. Conversely, when the density is lower (1010 kg m−3) and the discharge
is reduced, vortices are more energetic than the mean flow – that is, the
eddy kinetic energy is larger than the kinetic energy of the mean flow. In
general, over the slope, following the onset of dense-water injection, the
cyclonic vorticity associated with current shear develops in the upper
layer. The vorticity behaves in a two-layer fashion, thereby becoming
anticyclonic in the lower layer of the slope area. Concurrently, over the
deep flat-bottom portion of the basin, a large-scale anticyclonic gyre forms
in the upper layer extending partly toward a sloping rim. The density record
shows the rise of the pycnocline due to the dense-water sinking toward the
flat-bottom portion of the tank. We show that the rate of increase in the
anticyclonic potential vorticity is proportional to the rate of the rise of
the interface, namely to the rate of decrease in the upper-layer thickness
(i.e., the upper-layer squeezing). The comparison of laboratory experiments
with the Ionian Sea is made for a situation when the sudden switch from
cyclonic to anticyclonic basin-wide circulation took place following
extremely dense Adriatic water overflow after the harsh winter in 2012. We
show how similar the temporal evolution and the vertical structure are in
both laboratory and oceanic conditions. The demonstrated similarity further
supports the assertion that the wind-stress curl over the Ionian Sea is not
of paramount importance in generating basin-wide circulation inversions
compared with the internal forcing.