An Interdecadal Oscillation in an Idealized Ocean Basin Forced by Constant Heat Flux

Abstract

The authors report on a regular, interdecadal oscillation in a three-dimensional ocean circulation model. The model is run using box geometry of size comparable to the North Atlantic and is driven by a constant, zonally uniform, surface heat flux. The meridional overturning in the model exhibits a peak to peak oscillation of 7 Sv about a mean of 15 Sv. The period is 50 years. The oscillation has many similarities to that found by Delworth et al. in the GFDL coupled ocean-atmosphere model. In particular, the SST anomaly pattern during the oscillation. is quite similar to that in the coupled model and also to interdecadal anomaly patterns seen in SST data from the North Atlantic. Since the surface flux is constant, the oscillation is due to a balance between convergence in the oscillating part of the poleward heat transport and changes in local heat storage. A similar balance applies to the coupled model where changes in surface heat flux weakly oppose the oscillation. Including salinity, by adding a zonally uniform surface salt flux forcing, acts to weaken the oscillation but does not change its form. This is also consistent with the coupled model. The oscillation is also found when the surface heat flux is calculated interactively, by coupling the ocean model to a zero-heat-capacity model of the atmosphere. The authors suggest that an oscillation of this kind may have played a role in the warming of the North Atlantic surface waters during the 1920s and 1930s and the subsequent cooling in the 1960s.