Forced and Intrinsic Low-Frequency Variability in the Mediterranean Sea from a Multi-Decadal Ensemble Simulation

This study investigates the low-frequency variability of the Mediterranean Sea using an ensemble of 30 eddy-permitting (1/12°) NEMO-based regional ocean simulations. The ensemble members were slightly perturbed in their initial conditions and forced by the same atmospheric variability during 34 years, allowing us to separate the intrinsic and atmospherically-forced components of the ocean variability.

At interannual timescales, our analysis of sea surface height (SSH) reveals distinct patterns of intrinsic variability across the basin. While the variability of certain circulation features, such as the North Ionian Gyre (NIG), is mostly paced by the atmosphere, low-frequency fluctuations of other features —like in the Algerian Basin— are largely intrinsic and random. The variance decomposition reveals that intrinsic processes control most of the total SSH variability over one-fifth of the basin, highlighting their pivotal role in shaping the interannual fluctuations in the basin.

Inspired by previous studies of the Atlantic Meridional Overturning Circulation (Gregorio et al., 2015; Leroux et al., 2018), we investigate the forced and intrinsic components of the Mediterranean Zonal Overturning Circulation (ZOC) interannual variability, focusing on the eastward flow of Atlantic waters and westward flow of intermediate waters in density coordinates. While the transport in the western basin shows moderate variability, our results reveal an increase in total variability in the Levantine Basin, driven by both forced and intrinsic components. EOF analyses of ZOC fluctuations suggest distinct variability modes east and west of Sicily, which remain to be further investigated.

This work highlights the substantial contribution of intrinsic variability in various features of the Mediterranean's fluctuations, up to decadal timescales. A better understanding of the relative contributions of externally-driven and internally-generated oceanic fluctuations is crucial for accurately interpreting simulated and observed signals, making reliable predictions, and exploring possible impacts on marine ecosystems.