OCEAN CLIMATE DRIFT AND INTERDECADAL OSCILLATION DUE TO A CHANGE IN THERMAL DAMPING

The authors investigate the effect of a change in the rate of thermal damping upon the climate of an ocean general circulation model. Initia lly, the thermal forcing condition is that proposed by Haney, that is, restoring the model surface temperature to a climatology. The restori ng condition represents a strong damping. When a steady state is reach ed, the thermal damping is switched to a weaker one, but the atmospher e-ocean heat exchange is adjusted so that at the moment of the switch the heat flux is identical to that prior to the switch. It is found th at interdecadal oscillations and climate drift occur as a result of th e switch, regardless of the forcing condition for salinity. The cause for the variability and drift can be traced to the spinup. During the spinup, the surface climatology of the model ocean is forcefully ''nud ged'' toward that of the climatology, regardless of whether or not the internal dynamics of the model ocean can maintain the climatology. Th is leads to intermittent convections in the spinup state. When the the rmal damping becomes weaker, the system chooses a convective pattern ( the location and intensity of the convection) more compatible with the internal dynamics. An implication of these results is that drift and variability in a coupled model may be caused by the mechanism. Effects of flux corrections in coupled models are discussed.