Kilometer-Scale Assessment of the Adriatic Dense Water Multi-Decadal Dynamics

The North Adriatic Dense Water (NAddW)-the densest Mediterranean water generated by extreme cooling during wintertime hurricane-strength winds-drives the thermohaline circulation, ventilates the deep layers, and changes the biogeochemical properties of the Adriatic Sea. However, modeling the dynamical properties of such dense water at the climate scale has been a challenge for decades due to the complex coastal geomorphology of the Adriatic basin not properly reproduced by existing climate models. To overcome these deficiencies, a 31-year-long simulation (1987-2017) of the Adriatic Sea and Coast (AdriSC) kilometer-scale atmosphere-ocean model is used to analyze the main NAddW dynamical phases (i.e., generation, spreading and accumulation). The study highlights four key results. First, during winter, the NAddW densities are higher in the shallow northern Adriatic shelf than in the deeper Kvarner Bay-where 25%-35% of the overall NAddW are found to be generated-due to a median bottom temperature difference of 2 degrees C between the two generation sites. Second, the NAddW mass transported across most of the Adriatic peaks between February and May, except along the western side of the Otranto Strait. Third, for the accumulation sites, the bottom layer of the Kvarner Bay is found to be renewed annually while the renewal occurs every 1-3 years in the Jabuka Pit and every 5-10 years in the deep Southern Adriatic Pit. Fourth, the NAddW cascading and accumulation is more pronounced during basin-wide high-salinity conditions driven by circulation changes in the northern Ionian Sea. The densest water in the Mediterranean Sea known as the North Adriatic Dense Water (NAddW) forms during severe winter wind events in the northern Adriatic. This phenomenon which brings oxygen-rich waters to the sea bottom, plays a vital role in sustaining life in the Adriatic Sea. However, due to the complex geography of the Adriatic, accurate representation of NAddW is very challenging and requires fine-resolution atmosphere-ocean models such as the Adriatic Sea and Coast (AdriSC) model. Here, the AdriSC historical simulation is used to study NAddW generation, spreading and accumulation. The findings reveal that about a third of NAddW is produced in the Kvarner Bay which is a deeper and warmer than the Northern Adriatic shelf. Further, it is found that NAddW spreads over most of the Adriatic between February and May bringing dense waters to the bottom of the accumulation sites, which are renewed annually in the northern Adriatic, up to every 3 years in the middle Adriatic pit, and up to every decade in the Southern Adriatic Pit. Lastly, the study highlights that NAddW spreading and accumulation are significantly enhanced during periods of higher salinity across the Adriatic, a phenomenon driven by water exchanges with the Ionian Sea. The kilometer-scale dense water generation, spreading and accumulation is quantified in the Adriatic Sea during the 1987-2017 period About a third of the dense water is generated within the Kvarner Bay which is deeper and warmer than the well-studied main generation site The bottom layer renewal of the accumulation sites is annual in the Kvaner Bay and up to decadal in the deepest Adriatic pits