In high-resolution ocean general circulation models (OGCMs), as in process-oriented models, a substantial amount of interannual to decadal variability is generated spontaneously by oceanic nonlinearities: that is, without any variability in the atmospheric forcing at these time scales. The authors investigate the temporal and spatial scales at which this intrinsic oceanic variability has the strongest imprints on sea level anomalies (SLAs) using a 1 /128 global OGCM, by comparing a ''hindcast'' driven by the full range of atmospheric time scales with its counterpart forced by a repeated climatological atmospheric seasonal cycle. Outputs from both simulations are compared within distinct frequency-wavenumber bins. The fully forced hindcast is shown to reproduce the observed distribution and magnitude of low-frequency SLA variability very accurately. The small-scale (L , 68) SLA variance is, at all time scales, barely sensitive to atmospheric variability and is almost entirely of intrinsic origin. The high-frequency (mesoscale) part and the low-frequency part of this small-scale variability have almost identical geographical distributions, supporting the hypothesis of a nonlinear temporal inverse cascade spontaneously transferring kinetic energy from high to low frequencies. The large-scale (L , 128) low-frequency variability is mostly related to the atmospheric variability over most of the global ocean, but it is shown to remain largely intrinsic in three eddy-active regions: the Gulf Stream, Kuroshio, and Antarctic Circumpolar Current (ACC). Compared to its 1 /48 predecessor, the authors' 1 /128 OGCM is shown to yield a stronger intrinsic SLA variability, at both mesoscale and low frequencies.

The low-frequency variability of the Atlantic meridional overturning circulation (AMOC) is investigated from 2, 1 /48, and 1 /128 global ocean-sea ice simulations, with a specific focus on its internally generated (i.e., ''intrinsic'') component. A 327-yr climatological 1 /48 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 /48 and 1 /128 resolutions); R barely exceeds 5%-10% in the subpolar gyre but reaches 30%-50% at 348S, up to 20%-40% near 258N, and 40%-60% near the Gulf Stream. About 25% of the meridional heat transport interannual variability is attributed to intrinsic processes at 348S and near the Gulf Stream. Fourier and wavelet spectra, built from the 327-yr 1 /48 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 358S and 258N (15-30 yr periods) and throughout the whole basin (50-90-yr periods).

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