INTERDECADAL VARIABILITY IN AN OCEAN MODEL-DRIVEN BY A SMALL, ZONAL REDISTRIBUTION OF THE SURFACE BUOYANCY FLUX

An idealized, three-dimensional model, of size comparable to the Atlan tic, is used to study interdecadal variability of the thermohaline cir culation (THC). In most of the model experiments, salinity is kept uni form and constant, the model being driven by surface heat flux only. W hen the model is driven by the surface heat flux diagnosed from a rest oring spinup experiment, no oscillations occur. Driving the model by a time-independent, surface heat flux, obtained by applying a ''small'' zonal redistribution to the diagnosed flux, leads to strong interdeca dal oscillations; ''small'' means that the modification to the diagnos ed flux is within the error bars on estimates of surface heat flux bas ed on observations. The model sea surface temperature (SST) anomalies are similar to the observed pattern of SST anomalies in the North Atla ntic and to the SST anomalies associated with the interdecadal oscilla tion in the GFDL fully coupled ocean-atmosphere model. For redistribut ions that weaken the east-west variation of the flux, the mean THC, an d the amplitude/period of the oscillation do not depend strongly on th e amount of redistribution, once the threshold, beyond which oscillati ons occur, has been reached. If the east-west variation is enhanced, t hen the mean THC and the oscillation amplitude/period are very sensiti ve to the amount of redistribution. Coupling to a simple model of the atmosphere, it is found that redistributing the divergence of the atmo spheric heat transport diagnosed from a spinup can lead to interdecada l oscillations. An experiment is included that incorporates freshwater flux, wind forcing, and idealized, non-flat bottom topography to show the robustness of our results. This case exhibits interdecadal variab ility in the vertically integrated transport of the model Gulf Stream. The transport variability is apparently driven by bottom pressure tor que variations induced by the variable thermohaline circulation. Great batch et al, have suggested that the transport of the Gulf Stream was reduced in 1970-74 compared to 1955-59 and that the reduction in trans port was driven by bottom pressure torque.