Internal and forced ocean variability in the Mediterranean Sea

Two types of variability are discernible in the ocean: a response to the atmospheric forcing and the so-called internal/intrinsic ocean variability, which is associated with internal instabilities, nonlinearities, and the interactions between processes at different scales. Producing an ensemble of 20 multiyear ocean simulations of the Mediterranean Sea, initialized with different realistic initial conditions but using the same atmospheric forcing, the study examines the intrinsic variability in terms of its spatial distribution and seasonality. In general, the importance of the external forcing decreases with depth but dominates in extended shelves such as the Adriatic Sea and the Gulf of Gabes. In the case of temperature, the atmospheric forcing plays a major role in the uppermost 50 m of the water column during summer and the uppermost 100 m during winter. Additionally, intrinsic variability displays a distinct seasonal cycle in the surface layers, with a prominent maximum at around 30 m depth during the summer connected to the summer thermocline formation processes. Concerning current velocity, the internal variability has a significant influence at all depths.