On the near-inertial resonance of the Atlantic meridional overturning circulation

Florian Sévellec, Joël J.-M. Hirschi and Adam T. Blaker

The Atlantic Meridional Overturning Circulation (AMOC) is a crucial component of the global climate system. It is responsible for around a quarter of the Global northward heat transport and contributes to the mild European climate. Observations and numerical models suggest a wide range of AMOC variability. Recent results from an Ocean General Circulation Model (OGCM) in a high-resolution configuration (1/4°) suggest the existence of super-inertial variability of the AMOC. In this study we test the validity of this result in a theoretical framework. At low Rossby number and in presence of Rayleigh friction, it is demonstrated that, unlike a typical forced damped-oscillator (which shows sub-inertial resonance), the AMOC undergoes both super- and sub-inertial resonances (except at low latitudes). A dimensionless number, Sr, measuring the ratio of the ageostrophic forcing to the geostrophic one (i.e. the zonal vs. meridional pressure gradients), indicates which of these resonances dominates. If Sr<<1, the AMOC variability is mainly driven by geostrophic forcing and shows sub-inertial resonance. Alternatively and consistent with the recently published 1/4°-OGCM experiments, if Sr>>1, the AMOC variability is mainly driven by the ageostrophic forcing and shows super-inertial resonance. In both regimes, a forcing of ±1 K induces an AMOC variability of ±10 Sv through these near-inertial resonance phenomena. It is also shown that, as expected from numerical simulations, the spatial structure of the near-inertial AMOC variabilities corresponds to equatorward propagating waves equivalent to baroclinic Poincaré waves. Long time average of this resonance phenomenon, raising and depressing the pycnocline, could contribute to the mixing of the ocean stratification.