| 03 Nov 2025
Externally-forced and intrinsic variability of the Mediterranean surface and overturning circulations
Abstract. Part of Mediterranean Sea variability is forced and paced by external drivers (e.g. atmosphere, river runoffs, Atlantic inflow); the other part has a random phase and spontaneously emerges due to chaotic intrinsic variability (CIV). This study quantifies across time scales the imprints of both variability components on the surface and zonal overturning circulations within the basin, from a 39-year 30-member ensemble ocean 1/12° simulation. We find that most of SSH variance is intrinsic over 17 % of the basin, in particular in the southern Ionian and Levantine basins, and most notably in the Algerian basin where CIV explains 80 % of the SSH variance at periods greater than 4 months. In contrast, 75 % of its interannual to decadal variance of the North Ionian Gyre circulation is paced by the atmosphere, suggesting an external triggering of the Adriatic-Ionian Bimodal Oscillating System (BiOS) reversal. Other gyres such as Rhodes, Bonifacio, and Alboran shows more balanced contribution of CIV. Fluctuations of the density-coordinate zonal overturning circulation (ZOCσ) and of associated transports are mostly paced by the forcing over most of the basin. However, CIV tends to explain a larger fraction of these transports variance in the intermediate and bottom layers near deep convection sites, in particular in the Levantine basin where this fraction exceeds 50 % between 27 and 30° E at submonthly periods, and 20–30 % at periods reaching 20 years locally. This partly random character of the multi-scale Mediterranean variability has consequences for evaluating model simulations, and the design of observation systems targeting the long-term monitoring of the basin.
Héron et al. analyse a 39-year, 30-member, 1/12° ensemble simulations of the Mediterranean Sea to distinguish intrinsic from forced variability in surface and zonal overturning circulation. Ensemble statistics and temporal scale decomposition reveal that SSH variability is predominantly intrinsic over about 17% of the basin at both timescales, with hotspots in the Algerian, Levantine, and Ionian Seas. Zonal overturning variability is largely atmosphere-driven, though intrinsic processes remain important in intermediate and deep layers near convection sites, particularly in the Levantine basin. Using relative vorticity, the authors further show that Mediterranean gyres span a continuum from strongly forced (e.g., North Ionian Gyre) to largely intrinsic (e.g., Algerian Gyre).
The manuscript is clearly written, with some minor structural aspects that could be refined, and supported by robust results. It addresses a relevant scientific question within the broader context of detection and attribution. Although the study is primarily descriptive, it targets a region where such characterization is still limited and provides insights that can meaningfully advance our understanding of Mediterranean Sea dynamics.
I recommend acceptance with the following minor revisions:
Technical corrections: