Tidal Resonance in the Adriatic Sea: Observational Evidence

Tides in the Adriatic Sea are anomalously strong in comparison with most of other parts of the Mediterranean Sea. Consequently, tides play a fundamental role in the hydrodynamics of this sea and in the formation of destructive floods in its northernmost part. The resonant character of the Adriatic tides is evident, but the exact mechanism of their formation has never been thoroughly examined. In the present study, we used multidecadal tide gauge data from 10 stations located along the basin's coastline to examine the spectral properties of the Adriatic Sea and to provide high-resolution tidal analysis. Our results demonstrate that both diurnal and semidiurnal tidal harmonics are resonantly amplified due to the proximity of tidal periods to the periods of the fundamental (21.5 hr) and first (10.9 hr) Adriatic eigen modes, respectively. The constructed resonance models were found to closely describe specific features of the observed tides in the Adriatic Sea, including both major and minor harmonics, and therefore help to explain the dominant physical mechanism driving the floods in the northernmost part of the basin, including the Venice Lagoon. Plain Language Summary The northern Adriatic Sea is an exceptional Mediterranean subbasin where tides are several times larger than in other parts of the Mediterranean Sea. For that reason, the tides play a principal role in the hydrodynamics of the Adriatic Sea, which includes the flooding of coastal regions, such as "acqua alta" events in the city of Venice. In practice, a coincident storm surge peak and high tide can double the flooded area of Venice. In this paper we used multidecadal sea level observations to examine the physical basis of large tides in the Adriatic Sea. We demonstrated that both diurnal and semidiurnal tidal harmonics in this sea are resonantly amplified due the proximity of these tidal periods to the periods of the fundamental (21.5 hr) and first (10.9 hr) eigen modes, respectively. The constructed resonance models were found to appropriately describe specific features of the observed tides in the Adriatic Sea.