Intrinsic Variability of the Atlantic Meridional Overturning Circulation at Interannual-to-Multidecadal Time Scales

The low-frequency variability of the Atlantic meridional overturning circulation (AMOC) is investigated from 2, 1/4 degrees, and 1/12 degrees global ocean-sea ice imulations, with a specific focus on its internally generated (i.e., intrinsic) component. A 327-yr climatological 1/4 degrees simulation, driven by a repeated seasonal cycle (i.e., a forcing devoid of interannual time scales), is shown to spontaneously generate a significant fraction R of the interannual-to-decadal AMOC variance obtained in a 50-yr fully forced hindcast (with reanalyzed atmospheric forcing including interannual time scales). This intrinsic variance fraction R slightly depends on whether AMOCs are computed in geopotential or density coordinates, and on the period considered in the climatological simulation, but the following features are quite robust when mesoscale eddies are simulated (at both 1/4 degrees and 1/12 degrees resolutions); R barely exceeds 5%-10% in the subpolar gyre but reaches 30%-50% at 34 degrees S, up to 20%-40% near 25 degrees N, and 40%-60% near the Gulf Stream. About 25% of the meridional heat transport interannual variability is attributed to intrinsic processes at 34 degrees S and near the Gulf Stream. Fourier and wavelet spectra, built from the 327-yr 1/4 degrees climatological simulation, further indicate that spectral peaks of intrinsic AMOC variability (i) are found at specific frequencies ranging from interannual to multidecadal, (ii) often extend over the whole meridional scale of gyres, (iii) stochastically change throughout these 327 yr, and (iv) sometimes match the spectral peaks found in the fully forced hindcast in the North Atlantic. Intrinsic AMOC variability is also detected at multidecadal time scales, with a marked meridional coherence between 35 degrees S and 25 degrees N (15-30 yr periods) and throughout the whole basin (50-90-yr periods).