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Abstract
Recent studies of decadal/interdecadal climate variability suggested two main classes of mechanisms: selfsustained (supercritical) oscillations due to the internal nonlinearity of the ocean and linear (subcritical) thermohaline oscillations driven by stochastic atmospheric forcing. The authors use a coupled ocean‐atmosphere meridional box model to carefully examine these two alternatives. It is shown that a weakly nonlinear relation between the north‐south density gradient in the ocean and the meridional ocean transport can lead to selfsustained oscillations. A nonlinear relation between the SST and the air‐sea heat flux can also lead to selfsustained oscillations, although indications are given that the air‐sea heat flux depends linearly on the SST for a wide range of SST perturbations. It is thus concluded that, if interdecadal climate variability is due to selfsustained oscillations, the necessary nonlinearity must be related to internal ocean dynamics rather than to the air‐sea interaction or to nonlinear atmospheric dynamics. The box model results are used to discuss a simple criterion, based on the probability distribution function of the meridional circulation time series, for differentiating between self-sustained and linear variability. This criterion could not rule out either the linear or nonlinear hypotheses for the thermohaline variability in the GFDL coupled general circulation model run of Delworth, Manabe, and Stouffer. This may indicate that the variability in the coupled general circulation model is near critical.
Highlights
Since the work of Bjerknes (1964) it is generally believed that the oceanic thermohaline circulation (THC) plays an important role in interdecadal climate variability
The main objective of this study was to critically examine two possible classes of interdecadal thermohaline variability mechanisms, namely, linear oscillations excited by stochastic atmospheric forcing and nonlinear self-sustained oscillations
We have shown that a nonlinear thermohaline oscillation arises from the combination of two factors: a linearly unstable thermohaline oscillation mechanism and a nonlinearity that limits the growth of the unstable oscillations and leads to a limitcycle behavior
Summary
Since the work of Bjerknes (1964) it is generally believed that the oceanic thermohaline circulation (THC) plays an important role in interdecadal climate variability. There is a most interesting body of work on climate stability and variability that is based on a variety of ocean-only models from box models to ocean general circulation models (GCMs), mostly under mixed boundary conditions (Bryan 1986) These studies may be roughly divided into two groups differing in the energy source of the oscillations. This includes a nonlinear relation between the oceanic meridional transport and the meridional density gradient, and a nonlinear relation between the SST and air–sea heat fluxes. Since this response appears to be linear over a wide range of parameters, we conclude that nonlinear THC oscillations are most likely to be a result of internal ocean nonlinearities.
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