Intrinsic low-frequency variability in long multilayer simulations of the Mediterranean Sea

In multi-centennial integrations of a multilayer shallow-water model of the Mediterranean Sea driven solely by constant volume transports representing Atlantic inflow and intermediate outflow, and with no atmospheric forcing or imposed temporal variability, coherent intrinsic low-frequency fluctuations emerge. These signals propagate predominantly westward at very slow phase speeds, of order 10–30 km/yr, and exhibit horizontal scales of O(100 km). The variability is robust across different dissipation closures and numerical configurations and is most clearly visible at internal density interfaces, particularly in regions of strong bathymetric influence. Comparison with multi-decadal satellite altimetric records reveals significant and spatially coherent correlations in selected areas of the basin, suggesting that the simulated intrinsic variability may contribute to observed sea-level fluctuations. This research is supported by the Italian INVMED-P.R.I.N. project.