In 1997 an inversion in the Ionian upper‐layer circulation was documented and ascribed to a massive inflow of Aegean dense waters associated with the Eastern Mediterranean Transient (EMT) and not to the wind‐stress (Borzelli et al., 2009). Here we generalize the concept hypothesizing that such inversions are possible even in the absence of the Aegean influence. Indeed, salinity and density data collected in the Southern Adriatic, the main source of the Eastern Mediterranean deep water, show decadal variations coherent with changes in the sea level height in the northern Ionian. Scaling considerations suggest that the redistribution of Ionian water masses, resulting from changes in the thermohaline properties of waters entering the basin, can sustain inversions of the upper‐layer circulation. Therefore, we propose a feedback mechanism (named the Adriatic‐Ionian Bimodal Oscillating System – BiOS) between variations in the thermohaline properties of waters formed in the Southern Adriatic and the Ionian circulation.

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Abstract. Analysis of 20-year time-series of the vertically averaged salinity and nutrient data in the Southern Adriatic shows that the two parameters are subject to strong decadal variability. In addition, it is documented that nutrient and salinity variations are out of phase. Nutrients in the Ionian and in the Adriatic vary in parallel except that generally the nutrient content in the Adriatic is lower than in the Ionian, a fact that has been attributed to primary producer consumption following the winter convective mixing. As shown earlier, North Ionian Gyre (NIG) changes its circulation sense on a decadal scale due to the Bimodal Oscillating System, i.e. the feedback mechanism between the Adriatic and Ionian. Cyclonic circulation causes a downwelling of the nitracline along the borders of the NIG and a decrease in the nutrient content of the water flowing into the Adriatic across the Otranto Strait, and vice versa. In addition, the highly oligotrophic central area of the Ionian shows annual blooms only during cyclonic NIG circulation. Inversion of the sense of the NIG results in the advection of Modified Atlantic Water or of the Levantine/Eastern Mediterranean waters in the Adriatic. Here, we show that the presence of allochtonous organisms from Atlantic/Western Mediterranean and Eastern Mediterranean/temperate zone in the Adriatic are concurrent with the anticyclonic and cyclonic circulations of the NIG, respectively. On the basis of the results presented, a revision of the theory of Adriatic ingressions formulated in the early 1950s is proposed.

Abstract. The Mediterranean Sea is a semi-enclosed sea characterized by high salinities, temperatures and densities. The net evaporation exceeds the precipitation, driving an anti-estuarine circulation through the Strait of Gibraltar, contributing to very low nutrient concentrations. The Mediterranean Sea has an active overturning circulation, one shallow cell that communicates directly with the Atlantic Ocean, and two deep overturning cells, one in each of the two main basins. It is surrounded by populated areas and is thus sensitive to anthropogenic forcing. Several dramatic changes in the oceanographic and biogeochemical conditions have been observed during the past several decades, emphasizing the need to better monitor and understand the changing conditions and their drivers. During 2011 three oceanographic cruises were conducted in a coordinated fashion in order to produce baseline data of important physical and biogeochemical parameters that can be compared to historic data and be used as reference for future observational campaigns. In this article we provide information on the Mediterranean Sea oceanographic situation, and present a short review that will serve as background information for the special issue in Ocean Science on "Physical, chemical and biological oceanography of the Mediterranean Sea". An important contribution of this article is the set of figures showing the large-scale distributions of physical and chemical properties along the full length of the Mediterranean Sea.

We study the impact of decadal inversions of the Ionian upper layer circulation (denominated as Adriatic‐Ionian Bimodal Oscillation System) on thermohaline properties of the Levantine and Cretan Seas. Lagrangian drifter data and surface geostrophic currents show that the Atlantic Water (AW) flow is well organized and most intense when the Ionian circulation is cyclonic. During the Ionian anticyclonic phase, the AW spreading pathway is the longest, contributing to its prolonged mixing and higher salinity once it reaches the Levantine. Thus, the Levantine basin is subject to less dilution by AW during the anticyclonic surface circulation phase. Empirical orthogonal function analysis of the sea level shows a large‐amplitude circular feature in the northern Ionian which matches the cyclonic/anticyclonic gyre obtained from Lagrangian measurements. Furthermore, it reveals the out‐of‐phase variability of the North Ionian Gyre and the Aegean and Levantine sea levels. We further show that the surface salinity of the Levantine basin variation is out of phase with that of the Ionian surface layers. Salinity variations of the deepwater column in the Aegean are out of phase with the Ionian surface salinity values, owing probably to a fast transfer of the surface salinity changes via winter deep convection. The changing of the Levantine and Cretan Seas' salinity parallel to the Ionian circulation inversions suggests that the preconditioning for the eastern Mediterranean transient (EMT) is driven by internal processes. As the Ionian inversions are cyclical events, we conclude that the EMT is not an isolated episode but potentially a recurrent phenomenon.