Short-term, linear, and non-linear local effects of the tides on the surface dynamics in a new, high-resolution model of the Mediterranean Sea circulation

The tides in the Mediterranean Sea are generally weaker than in other regions of the world ocean, but are locally intensified in passages with complex bathymetry, such as the Gibraltar Strait and the Sicily Channel. To date, a detailed understanding of their effects on the circulation, on the short time scales relevant to forecasting, is still missing, due to the lack of specific observations, and of basin-scale, numerical models explicitly accounting for the tidal forcing. The present investigation attempts to bridge this gap, using a newly developed forecasting model of the circulation of the Mediterranean Sea-Black Sea system that includes the effects of the main diurnal and semidiurnal astronomical tides. After validating the model barotropic tidal dynamics through comparison with the results of a tidal inversion software and historical data, a fully baroclinic run is analyzed, with focus on the Mediterranean Sea dynamics. The run covers the period 19 March-30 April 2018 and is initialized and forced with realistic fields produced by state-of-the-art operational models. After verifying that the modelled circulation and hydrology agree with available remote and in situ observations, attention is focused on short-time effects of the tidal forcing on the local circulation. Tides are found to significantly modulate transports not only across the Strait of Gibraltar and the Sicily Channel, as expected, but also across the Corsica Channel and the Otranto Strait. Tidal effects also modify relevant features of the circulation inside the basin, inducing local modulations of some of the main currents and exciting topographic waves that eventually get trapped over shallow bathymetric features, producing diurnal rotations of the flow patterns. Examples of the latter dynamics are found over the Adventure Bank and the Malta Plateau, in the Sicily Channel, and in the eastern portions of the Corsica Channel and the Otranto Strait. Furthermore, in several locations (Sicily Channel, Corsica Channel, Messina Strait, North Adriatic Sea), spectral analysis of the mean kinetic energy reveals the presence of spectral peaks corresponding to periods of about 8 and 6 h, which can only be interpreted as harmonics (overtides and compound tides) of the diurnal and semidiurnal tidal components, generated through non-linear interactions. This shows that, contrary to a widespread assumption, tidal effects in the Mediterranean cannot be linearly superimposed on the basin circulation.