Critical decade for freshwater discharge into the Adriatic Sea

Located in the central Mediterranean Sea, the Adriatic Sea experiences complex water circulation patterns driven by saltwater inflow from the Ionian Sea through the Strait of Otranto and the outflow through the same strait but richly charged by fresh and dense waters formed in the northern Adriatic Sea. These circulation patterns place the unique hydrodynamics of the Adriatic Sea as the mainframe in shaping its diverse marine ecosystem, making it a primary region in dense water formation within the Mediterranean Sea. However, in recent decades, the region has continually recorded longer and more intense periods of drought. Some studies account for the loss of about 80 billion tons of freshwater during the 2021-2022 drought only at the Po River basin. To explore this matter further, the present work aims to analyse the last decade (2013-2022) of river discharges into the Adriatic Sea and frame the impacts of recent drought events in the current climatological period. To this end, the hydrological data reconstructed with the European Flood Awareness System (EFAS) were analysed for the period 1991-2022, quantifying river discharges separately in the four subregions of the Adriatic Sea: Shallow Northern Adriatic Sea (SNAd), Northern Adriatic Sea (NAd), Central Adriatic Sea (CAd), and Southern Adriatic Sea (SAd). Over the past 32 years, river discharges have shown different trends along the Adriatic Sea subregions, where a delicate balance between dry seasons in some subregions has been slightly balanced by flood seasons in others and vice versa. This delicate balance, combined with the diversity of its river basins, prevents us from estimating a trend with statistical significance for the Adriatic Sea. However, the river discharge trends are forthright when computed individually for each subregion, balancing slightly negative trends in the northern subregions (-0.6% and -1.0% per year in SNAd and NAd) with intriguingly positive trends in the southern subregions (+0.4% and +1.3% per year in CAd and SAd). When the analysis window narrows to the last decade (2013-2022), this balance breaks down, and a strong negative trend emerges across the entire Adriatic Sea, without exception, indicating reductions of -4.2% per year in freshwater input throughout the river basin. As suggested by the Standardised Flow Index (SFI) results, a climate indicator used to estimate the long-term impact of drought and flood periods on river discharges, 2022 was crucial for the last negative decadal trend. During this year, the northern Adriatic experienced the driest period in the last 32 years, while the southern Adriatic experienced river discharge reductions during flood months. Nevertheless, the most worrying element about the extreme drought of 2022 is that this year is part of a drought cycle that has continuously reduced freshwater availability in the Adriatic Sea every 4-5 years since 2008.