EVIDENCE FOR DECADAL VARIABILITY IN AN OCEAN GENERAL-CIRCULATION MODEL - AN ADVECTIVE MECHANISM

A series of numerical experiments involving long-time integrations are conducted using the Bryan-Cox Ocean General Circulation Model under mixed surface boundary conditions (i.e. a Newtonian restoring surface boundary condition on temperature and a specified flux boundary condition on salinity). Under steady forcing the system oscillates with significant energy at decadal period. This oscillation is shown to be an advective phenomenon, associated with the propagation of salinity and temperature anomalies from the region between the subtropical and subpolar gyres, where they are generated, to the eastern boundary, where deep water is formed. Furthermore, the oscillation is characterized by the fluctuation of the thermohaline circulation between a state in which deep water is formed and a collapsed state in which no deep water is formed. Over the period of the oscillation the poleward heat transport changes by as much as a factor of 3 at certain latitudes. The anomalies are initially formed by the upwelling of warm, saline waters that are being transported polewards by a western boundary current that has separated from the coast. The observed decadal variability is robust in that it is present in all numerical experiments (12 and 33 vertical-level models; one and two hemisphere models; synchronous and asynchronous integrations). Crucial to the existence of the variability is the use of a low vertical eddy viscosity coefficient.