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

Abstract

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.

Climatic, Decadal, and Interannual Variability in the Upper Layer of the Mediterranean Sea Using Remotely Sensed and In-Situ Data

“…Interannual and decadal variabilities are related to changes of the thermohaline properties and circulation regimes such as Eastern Mediterranean Transient (EMT), Western Mediterranean Transition (WMT), and Adriatic-Ionian Bimodal Oscillating System (BiOS) [37][38][39][40][41]. The WMT can be defined as a climate shift, which changed the basic structure and properties of the intermediate and deep layers of the western Mediterranean, with an abrupt increase in temperature, salinities, and densities [39,42]; it affects the characteristics of the water exiting the Gibraltar Strait.…”

“…This event influences not only the deep layer but also the entire water column and the circulation structures of the Eastern Mediterranean [19,44]. The BiOS is a feedback mechanism, driven by the difference in salinity between the salty and warmer waters originating in the eastern Mediterranean and the less saline AW entering from the Sicily Channel [39,41,45,46], that is accredited to having led to the quasi-decadal reversal of the Northern Ionian Gyre (NIG) from anticyclonic to cyclonic and vice-versa. Evidence of the importance of this mechanism was obtained by dimensional analysis and laboratory experiments [39][40][41]45,46].…”

“…The BiOS is a feedback mechanism, driven by the difference in salinity between the salty and warmer waters originating in the eastern Mediterranean and the less saline AW entering from the Sicily Channel [39,41,45,46], that is accredited to having led to the quasi-decadal reversal of the Northern Ionian Gyre (NIG) from anticyclonic to cyclonic and vice-versa. Evidence of the importance of this mechanism was obtained by dimensional analysis and laboratory experiments [39][40][41]45,46]. Another possible mechanism proposed for explaining the NIG reversal is wind forcing [47][48][49][50][51].…”

A reconciling vision of the Adriatic-Ionian Bimodal Oscillating System

“…The first theory relates NI current reversals to changes in the wind stress curl [9][10][11] ; the second relates current reversals of the Ionian near-surface circulation to baroclinic (internal) vorticity production, induced by changes in the horizontal pressure gradient due to injections of Adriatic Deep Water (AdDW) 12 . Rubino et al 13 and Gačić et al 14 , relying on tank experiments and numerical modeling, demonstrated that the polarity switch of the near-surface circulation in the NIG can be indeed induced by the injection of dense water on a sloping bottom.The two theories offer an explanation about the physics underlying the BiOS paradigm, but both have flaws. Specifically, the first theory, although based on the most natural approach to explain the inversion of the…”

Copernicus Ocean State Report, issue 6

“…The complex thermohaline circulation of the SAP (Figure 4.7.1) is driven by the interaction between different processes: the wintertime dense water formation events that occur through open-ocean convection (Gačić et al 2002); the inflow of fresher water from the northern Adriatic rivers (Vilibić et al 2013) and of dense water produced in the northern Adriatic (Bensi et al 2013); and the inflow/outflow of surfaceintermediate/dense water masses from/to the Otranto Channel (Yari et al 2012). Moreover, thermohaline and biogeochemical properties in the SAP are closely related to the periodic reversal (from anticyclonic to cyclonic and vice-versa) of the Northern Ionian Gyre (NIG; Gačić et al , 2021Menna et al 2019;Rubino et al 2020) on a quasi-decadal temporal scale Mihanović et al 2021). NIG reversals affect the water mass distribution among the Eastern Mediterranean sub-basins (Gačić et al 2011;Bessieres et al 2013;Reale et al 2017;Notarstefano et al 2019), in turn influencing the thermohaline properties of the whole Mediterranean on decadal and multi-decadal scales (Gačić et al 2013;Schroeder et al 2017;Menna et al 2022).…”