Results from a recent hydrographic survey show that an influx of Aegean Sea water has replaced 20 percent of the deep and bottom waters of the eastern Mediterranean. Previously, the only source of such waters was the Adriatic Sea, and the waters of the eastern Mediterranean were in near-steady state. The flux changed the water characteristics and displaced older waters upward. Its cause was increasing Aegean Sea salinity, resulting from changes in either the circulation pattern or the large-scale freshwater balance. Current deepwater studies may be affected by the intrusion, but effects might be found also at shallower depths and over a larger region.

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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.

Abstract. Adriatic and Ionian seas are Mediterranean subbasins linked through the Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basinwide circulation. Altimetric maps showed that the last cyclonic mode started in 2011 but unexpectedly in 2012 reversed to anticyclonic. We related this "premature" inversion to the extremely strong winter in 2012, which caused the formation of very dense Adriatic waters, flooding Ionian flanks in May and inverting the bottom pressure gradient. Using Lagrangian float measurements, the linear regression between the sea surface height and three isopycnal depths suggests that the southward deep-layer flow coincided with the surface northward geostrophic current and the anticyclonic circulation regime. Density variations at depth in the northwestern Ionian revealed the arrival of Adriatic dense waters in May and maximum density in September. Comparison between the sea level height in the northwestern Ionian and in the basin centre showed that in coincidence with the arrival of the newly formed Adriatic dense waters the sea level was lowered in the northwestern flank, inverting the surface pressure gradient. Toward the end of 2012, the density gradient between the basin flanks and its centre went to zero, coinciding with the weakening of the anticyclonic circulation and eventually with its return to the cyclonic pattern. Thus, the premature and transient reversal of Ionian surface circulation originated from the extremely harsh winter in the Adriatic, resulting in the formation and spreading of highly dense bottom waters. The present study highlights the remarkable sensitiveness of the Adriatic-Ionian BiOS to climatic forcing.