Journal of Physical Oceanography: Vol. 29, No. 5, pp. 865–892.

Interdecadal Variability of the Thermohaline Circulation in Box-Ocean Models Forced by Fixed Surface Fluxes

Thierry Huck and Alain Colin de Verdière
Laboratoire de Physique des Océans, Université de Bretagne Occidentale, Brest, France

Andrew J. Weaver
School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada

(Manuscript received 1 July 1997, in final form 28 May 1998)

ABSTRACT
Intrinsic modes of decadal variability are analyzed using box-geometry ocean models forced by constant surface fluxes. An extensive parameter sensitivity analysis of the oscillatory behavior is carried out with respect to the spherical/Cartesian geometry, the beta-effect, the Coriolis parameter, the parameterization of momentum dissipation and associated boundary conditions, the vertical and horizontal diffusivities, the convective adjustment algorithm, the horizontal and vertical model resolution, the forcing amplitude, and the basin width. The oscillations stand out as a robust geostrophic feature whose amplitude is mainly controlled by the horizontal diffusivity. Unsuccessful attempts to reproduce the variability in zonally averaged 2D models suggest that the 3D adjustment processes are necessary. However, the beta-effect is not necessary for decadal variability to occur, and therefore classical Rossby waves play no fundamental role in the mechanism. Various experiments with different geometry and forcing are conducted and do not support the necessity of viscous numerical boundary waves or any boundary in sustaining the oscillations. The models show two types of oscillatory behavior: 1) temperature anomalies propagating geostrophically westward in the eastward jet (northern part of the basin) and inducing an opposite anomaly in their wake and 2) stationary temperature anomalies in the northwest quarter that respond to the western boundary current transport changes and reinforce geostrophically this change until the opposite temperature anomaly built on the east finally reverses the meridional overturning anomaly. The analysis of the transition from steady to oscillatory states (using heat fluxes diagnosed at the equilibrium under restoring boundary conditions) and the comparison of the variability under various forcing fields suggest that the oscillations are triggered in the regions of strongest surface cooling. Finally, a simple box-model analogy is proposed that captures the crucial phase shift between meridional overturning and north–south density gradient anomalies on these decadal timescales.