Theoretical investigation of the Atlantic multidecadal oscillation
Florian Sevellec and Thierry Huck
submitted to Journal of Physical Oceanography, 7May2014

A weakly damped mode of multidecadal variability was found in the North Atlantic through the linear stability analysis of an ocean General Circulation Model in a global realistic configuration. A simple 2-levels model was proposed to rationalize both its period and damping rate. This model is extended here to 3-levels to investigate how the mode could draw energy from the mean flow and lead to multidecadal oscillations, as found in various ocean and coupled models. A linear stability analysis in this 3-levels model shows that the positive growth rate of the oscillatory mode depends on the zonally-averaged isopycnal slope. This mode corresponds to a westward propagation of density anomalies in the pycnocline. The most unstable mode corresponds to the largest scale one (at least for low isopycnal slope). The mode can be described in two phases: One inducing an intensification of the AMOC followed by one inducing a global warming of the North Atlantic. These two phases are followed by their corresponding opposite phase (reduction of the AMOC and cooling of the North Atlantic) to induce a full oscillation. A criterion is developed to test, in oceanic datasets or numerical models, whether this multidecadal oscillation is an oceanic internal mode of variability or is externally forced. Consistently with classical theory of baroclinic instability, this criterion depends on the vertical structure of the mode in the pycnocline. If the upper pycnocline signature is in advance with the deeper pycnocline one, the mode is unstable and could be explained in an endogenous paradigm as an oceanic internal mode of variability.