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

Abstract

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).

Thermally Forced Transients in the Thermohaline Circulation

“…They found a strong response with a resonancelike behavior for periods longer than 15 yr. Delworth and Greatbatch (2000) used a low-resolution but otherwise realistic global model to show that the dominant variability of the MOC was externally forced by changes in air-sea heat fluxes. Grégorio et al (2015) came to a similar conclusion, particularly at high latitudes, with an eddy-resolving Atlantic sector general circulation model forced by realistic atmospheric fluxes.…”

Development of a probabilistic ocean modelling system based on NEMO 3.5: application at eddying resolution

“…These temperature anomalies were computed by first removing the long-term non-linear trend of the time series derived from a local regression model (as in Grégorio et al, 2015). This detrending step acts as a non-linear high-pass temporal filter with negligible end-point effect also called local regression (LOESS) detrending (e.g.…”

“…6, along with their simulated counterpart from E-ORCA025. They were computed in geopotential coordinates as in Zhang (2010) and Grégorio et al (2015), and are shown after LOESS detrending and after removing the mean seasonal cycle.…”

“…forced with the same repeated annual cycle from year to year. This purely intrinsic variability has a significant imprint on many ocean variables, especially in eddyactive regions, and develops on spatio-temporal scales ranging from mesoscale eddies up to the size of entire basins, and from weeks to multiple decades (Penduff et al, 2011;Grégorio et al, 2015;Sérazin et al, 2015). The evolution of this chaotic ocean variability under repeated climatological atmospheric forcing is sensitive to initial states.…”

Interannual Variability in the North Atlantic Ocean’s Temperature Field and Its Association with the Wind Stress Forcing

“…The present method complements, therefore, the common approach of comparing multiple realizations of an ocean general circulation model, driven with different atmospheric forcings (e.g., Grégorio et al 2015;Sérazin et al 2015).…”