Low-frequency variability of the quasi-adiabatic overturning circulation

Intrinsic thermohaline variability arises spontaneously in idealized ocean circulation models if dissipation is sufficiently weak. Low-frequency oscillations have been mostly analyzed in the context of diffusive overturning circulations; here we report a series of idealized numerical experiments at very low diffusivity under mixed surface boundary conditions which support robust, nearly periodic thermohaline oscillations at multidecadal and multicentennial time scales.

The multidecadal oscillation is a basin-wide fluctuation in the strength of the overturning circulation wherein anomalies form in the northern subpolar gyre and propagate southward. The mechanism usually thought to produce decadal variability—large-scale baroclinic instability—is suppressed under mixed boundary conditions. Instead, the observed decadal oscillation is maintained by a convectively coupled thermohaline instability in which correlations between temperature and salinity anomalies in the surface mixed layer generate buoyancy variance despite the damping due to surface thermal relaxation.

The multicentennial mode is characterized by a strengthening and weakening of the overturning circulation which is nearly synchronous with latitude. The overturning anomalies are associated with sea surface salinity (SSS) anomalies which have components which are both symmetric and antisymmetric with respect to the equator. The antisymmetric component of SSS anomalies is in phase with the overturning anomalies while the symmetric component of SSS anomalies leads the overturning anomalies by a quarter period. These oscillations are interpreted using a novel modification of the Howard-Malkus-Welander loop model.