A systematic approach to determine thresholds of the oceanʹs thermohaline circulation

A systematic approach is proposed to determine thresholds in freshwater flux perturbations related to abrupt changes in the oceanʹs thermohaline circulation. The typical problem considered is the response of a thermohaline driven flow to a localized change, of specified strength and duration, in the surface freshwater flux. The initial transient response due to the freshwater anomaly is considered as a finite amplitude perturbation. An estimate of this response can be obtained by using ideas from dynamical systems theory. Central quantity to determine whether such a perturbation leads to instability (i.e. a ʹcollapsedʹ state) is the sign of the tendency of a specific energy functional. The approach is first illustrated with a simple box model and then shown to give good results in a global ocean general circulation model.