Mechanisms of Thermohaline Mode Switching with Application to Warm Equable Climates

A three-box model of haline and thermal mode overturning is developed to study thermohaline oscillations found in a number of ocean general circulation models and that might have occurred in warm equable paleoclimates. By including convective adjustment modified to represent the localized nature of deep convection, the box model shows that a steady haline mode circulation is unstable. For certain ranges of freshwater forcing/ vertical diffusivity, a self-sustained oscillatory circulation is found in which haline–thermal mode switching occurs with a period of centuries to millennia. It is found that mode switching is most likely to occur in warm periods of earth’s history with, relative to the present climate, a reduced Pole–equator temperature gradient, an enhanced hydrological cycle, and somewhat smaller values of oceanic diffusivities.