AN INTERDECADAL OSCILLATION IN AN IDEALIZED OCEAN-BASIN FORCED BY CONSTANT HEAT-FLUX

The authors report on a regular, interdecadal oscillation in a three-d imensional ocean circulation model. The model is run using box geometr y of size comparable to the North Atlantic and is driven by a constant , zonally uniform, surface heat flux. The meridional overturning in th e 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 mod el. In particular, the SST anomaly pattern during the oscillation is q uite similar to that in the coupled model and also to interdecadal ano maly patterns seen in SST data from the North Atlantic. Since the surf ace flux is constant, the oscillation is due to a balance between conv ergence in the oscillating part of the poleward heat transport and cha nges in local heat storage. A similar balance applies to the coupled m odel where changes in surface heat flux weakly oppose the oscillation. Including salinity, by adding a zonally uniform surface salt flux for cing, acts to weaken the oscillation but does not change its form. Thi s is also consistent with the coupled model. The oscillation is also f ound when the surface heat flux is calculated interactively, by coupli ng the ocean model to a zero-heat-capacity model of the atmosphere. Th e authors suggest that an oscillation of this kind may have played a r ole in the warming of the North Atlantic surface waters during the 192 0s and 1930s and the subsequent cooling in the 1960s.