Stability of the interhemispheric thermohaline circulation in a coupled box model

The coupled atmosphere-ocean box model of the interhemispheric thermohaline circulation (THC) formulated by Scott et al. [Scott, J.R., Marotzke, J., S tone, P.H., 1999. Interhemispheric THC in a coupled box model. J. Phys. Oce anogr., 29, 351-365.] is solved analytically, by introducing the approximat ion that the time variations of salinity in the ocean are much slower than the time variations in the temperature. The analytic solution shows that th ere is an unstable limit cycle near the bifurcation where the now becomes u nstable, as suggested by Scott et al.'s numerical solutions. The solution a lso leads to an analytic expression for the conditions under which the inst ability discovered by Scott et al. sets in, which is more general than that found by Scott et al. In particular, it includes the effect of coupling th e THC to the atmospheric meridional transports of heat and moisture. It sho ws that the stability of THC is much more sensitive to the representation o f the atmospheric heat transport, i.e., to how it depends on the meridional temperature gradient, than it is in hemispheric models. In particular, it shows that interhemispheric ocean models that use mixed boundary conditions , or couple the ocean to a diffusive representation of the atmospheric heat transport, are less susceptible to this kind of instability than when the ocean is coupled to a representation of the atmospheric meridional heat tra nsport which is more sensitive to the meridional temperature gradient, as i s implied by observations and theory. (C) 1999 Elsevier Science B.V. All ri ghts reserved.