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), while 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 sea surface height (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 the interannual-to-decadal SSH 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. CIV exerts a larger influence on the Rhodes, Bonifacio, and Alboran gyre circulations. Fluctuations of the density-coordinate zonal overturning circulation (ZOCσ) and of associated transports are mostly paced by the forcing over 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, 30 % at subannual periods, and 20 % at longer periods (reaching 20 years locally). This partly random character of the multi-scale Mediterranean variability has implications for evaluating model simulations and designing long-term observation systems, which are finally discussed.