A series of numerical experiments have been carried out to explore the
variability of the thermohaline circulation and the energetics of hal
ocline catastrophe. It is found that when the amplitude of surface fre
shwater flux is smaller than a critical value, the thermohaline circul
ation is in a thermal mode, with deep water formed in the north. When
the freshwater flux amplitude is supercritical, the thermohaline circu
lation does not reach a single steady state. Instead, the model ocean
is in a continuous transition between a slow, quasi-steady saline mode
and an energetic, unsteady thermal mode. For cases with no wind stres
s, the saline mode is characterized by sinking along the equator. For
cases with wind stress, the saline mode is characterized by intermedia
te water formation at midlatitude. During the saline mode phase, the d
eep water gradually becomes warm and salty. Thus at the end of the sal
ine mode phase, within the northern basin there is cold and relatively
fresh water lying on top of warm and salty water. Such a vertical str
ucture is potentially very unstable because small perturbations can gr
ow, supported by the release of potential energy during strong cooling
. A quantity called the diabatic available potential energy index is i
ntroduced as an indicator for such convective instability. Thus a fina
l equilibrium in the saline mode cannot be reached; instead, the model
ocean flips to a very energetic thermal mode in which violent overtur
ning destroys the vertical stratification. After the energy is release
d, the saline cell supported by precipitation in the subpolar basin ad
vances southward and the model ocean returns to the saline mode. The w
hole cycle will be repeated.