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.

[1] More than 120 satellite-tracked drifters were deployed in the northern and middle Adriatic (NMA) Sea between September 2002 and November 2003, with the purpose of studying the surface circulation at mesoscale to seasonal scale in relation to wind forcing, river runoff, and bottom topography. Pseudo-Eulerian and Lagrangian statistics were calculated from the low-pass-filtered drifter velocity data between September 2002 and December 2003. The structure of the mean circulation is determined with unprecedented high horizontal resolution by the new data. In particular, mean currents, velocity variance, and kinetic energy levels are shown to be maximal in the Western Adriatic Current (WAC). Separating data into seasons, we found that the mean kinetic energy is maximal in fall, with high values also in winter, while it is significantly weaker in summer. High-resolution Local Area Model Italy winds were used to relate the drifter velocities to the wind fields. The surface currents appear to be significantly influenced by the winds. The mean flow during the northeasterly bora regime shows an intensification of the across-basin recirculating currents. In addition, the WAC is strongly intensified both in intensity and in its offshore lateral extension. In the southeasterly sirocco regime, northward flow without recirculation dominates in the eastern half of the basin, while during northwesterly maestro the WAC is enhanced. Separating the data into low and high Po River discharge rates for low-wind conditions shows that the WAC and the velocity fluctuations in front of the Po delta are stronger for high Po River runoff. Lagrangian covariance, diffusivity, and integral time and space scales are larger in the along-basin direction and are maximal in the southern portion of the WAC.Citation: Ursella, L., P.-M. Poulain, and R. P. Signell (2006), Surface drifter derived circulation in the northern and middle Adriatic Sea: Response to wind regime and season,

Basin‐wide hydrographic observations performed in the eastern Mediterranean during the past 2 decades attest changes in the thermohaline circulation as well as new aspects concerning the onset and the follow up of the major transient event that occurred at the beginning of the 1990s, i.e., the change of the dense water formation site from the Adriatic to the Aegean Sea. Since 1999, the upper thermohaline circulation has indicated the restoring of the opposite flows of the Atlantic Water and the Levantine Intermediate Water, which were greatly reduced in the period 1987–1995. In the deep layer the comparison between water mass structures observed in 1995, during the mature status of the transient, and those observed in 1999 shows a damping of the event and a regained role of the Adriatic Sea as a primary source of dense waters. Separate calculations of the salt content in the Ionian and in the Levantine Seas show an overall salt redistribution. During 1987–1995 a salt loss of about 25 × 1012 kg was computed for the upper 800 m, constituting only 27% of the salt gain in the deep layer over most of the eastern Mediterranean. On the contrary, during 1995–1999 the restored upper thermohaline circulation caused a salt redistribution between the two basins of about the same amount, but in the opposite sense, while an extra quantity of 12 × 1012 kg was deposited in the deep layer. In addition, calculations of the salt concentration in the convection region of the southern Adriatic reveal a remarkable amount of the salt, not yet totally transferred into the deep layers by its interior dynamics because of mild winters.

Under the emerging features of interannual-to-decadal ocean variability, the periodical reversals of the north ionian Gyre (niG), driven mostly by the mechanism named Adriatic-ionian Bimodal oscillating System (BioS), are known as impacting on marine physics and biogeochemistry and potentially influencing short-term regional climate predictability in the Eastern Mediterranean. Whilst it has been suggested that local wind forcing cannot explain such variability, aspects of the alternative hypothesis indicating that niG reversals mainly arises from an internal ocean feedback mechanism alone remain largely debated. Here we demonstrate, using the results of physical experiments, performed in the world's largest rotating tank and numerical simulations, that the main observed feature of BioS, i.e., the switch of polarity of the near-surface circulation in the niG, can be induced by a mere injection of dense water on a sloping bottom. Hence, BioS is a truly oceanic mode of variability and abrupt polarity changes in circulation can arise solely from extreme dense water formation events.

A number of recent studies based on hydrographic observations and modelling simulations have dealt with the major climatic shift that occurred in the deep circulation of the Eastern Mediterranean. This work presents hydrographic observations and current measurements conducted from 1997 to 1999, which reveal strong modifications in the dynamics of the upper, intermediate and deep layers, as well as an evolution of the thermohaline characteristics of the deep Aegean outflow since 1995. The reversal of the circulation in the upper layer of the north/central Ionian is worthy of note. The observations indicate a reduction of Atlantic Water in the northern Ionian with an increase on the eastern side of the basin. In the intermediate layer, the dispersal path of the Levantine Intermediate Water (LIW) is altered. Highly saline (> 39.0) and well-oxygenated intermediate waters were found near the Western Cretan Arc Straits. They flow out from the Aegean, thus interrupting the traditional path of the LIW, and spread prevalently northwards into the Adriatic Sea. In the deep layer, dense waters, exiting from the Adriatic (s q~2 9.18 kg´m ±3 ), flow against the western continental margin in the Ionian Sea at a depth of between 1000±1500 m. Dense waters of Aegean origin (> 29.20 kg´m ±3 ), discharged into the central region of the Eastern Mediterranean during the early stages of the transient, propagate prevalently to the east in the Levantine basin and to the west in the northern Ionian Sea. Near-bottom current measurements conducted in the Ionian Sea reveal unforeseen aspects of deep dynamics, suggesting a new configuration of the internal thermohaline conveyor belt of the Eastern Mediterranean. U. S.