The Adriatic-Ionian Bimodal Oscillating System: relevance, phenomenology, reproducibility, impact

In the 1950s it has been recognized that the Adriatic Sea, the northernmost basin of the Mediterranean Sea, exhibits substantial quasi-decadal oscillations of its thermohaline properties. The nature of the oscillations remained unknown till 2010, when they were related to circulation regimes of the adjacent northern Ionian Sea. Within the northern Ionian, circulation is either cyclonic, resulting in advection of highly-saline ultraoligotrophic waters from the Eastern Mediterranean, or anticyclonic, resulting in advection of less saline and nutrient-rich waters from the Western Mediterranean into the Adriatic. Type of water which is advected into the Adriatic subsequently influences density of the Adriatic dense waters generated through either wintertime cooling of shallow northern Adriatic shelf waters or through open ocean convection in the 1200 m deep Southern Adriatic Pit. Thus generated dense waters outflow from the Adriatic Sea, and, depending on their density, change vorticity of the northern Ionian Sea. This mechanism is known as the Adriatic-Ionian Bimodal Oscillating System (BiOS). Since 2010, there were several studies which detail the BiOS regimes with conclusions that (i) there is an asymmetry between BiOS circulation patterns, (ii) the regimes may change either rapidly or in a few years, (iii) the BiOS regimes are interconnected with circulation patterns of the whole Eastern Mediterranean. It has further been documented that the BiOS is a dominant process governing biogeochemical properties and impacting organisms in most of the Adriatic. Reproduction of BiOS regimes has been examined by multidecadal atmosphere-ocean simulations and coupled climate models of O(10 km) resolution. This reproduction is successful at the qualitative level only. For that reason, state-of-the-art high-resolution (O(3 km)) multidecadal simulations are under execution. Future evolution of BiOS regime has so far been examined from a single regional climate model, pointing to a shift towards anticyclonic BiOS regime in the future climate for all scenarios, with strengthening of both cyclonic and anticyclonic BiOS regimes in RCP8.5 scenario. Changes of BiOS pattern may have a significant impact to the Adriatic Sea water masses, thermohaline circulation and biogeochemical properties in the future climate.