Dynamics and predictability of Stommel’s box model. A phase-space perspective with implications for decadal climate variability

The dynamics and predictability of Stommel’s box model of the thermohaline circulation isstudied. This nonlinear model with idealized geometry of the North Atlantic is solved exactly. A phase space analysis of the model reveals that the optimal perturbation affecting long-termclimate variability is provided by high-latitude haline forcing in the Atlantic ocean, althoughthis perturbation has little resemblance with the most unstable mode of the system and theleading EOF. Furthermore, the predictability problem is investigated by means of singularvector analysis and the evolution of the probability distribution function. Uncertainties in theoceanic initial conditions do increase in the phase space of the model. In the stochasticallyforced box model with identical oceanic initial conditions, the climate predictability is examinedfor the damped persistence forecast. We find that the loss of the predictability is related to thedifferent stages of the variance evolution which is also measured by the relative entropy. Ouranalysis shows that the non-normal system matrix of Stommel’s model does affect the dynamicsand predictability of the system which is useful for the interpretation of long-term climatevariability and predictability.