Z. Sirkes et E. Tziperman, Identifying a damped oscillatory thermohaline mode in a general circulation model using an adjoint model, J PHYS OCEA, 31(8), 2001, pp. 2297-2306
A damped oscillatory mode of the thermohaline circulation (THC), which may
play a role in interdecadal climate variability, is identified in a global
primitive equation model. This analysis is done under mixed boundary condit
ions using an adjoint of the primitive equation model.
The linearized versus nonlinear stability behavior of the model is studied
by comparing the adjoint analysis to runs of the fully nonlinear model. It
is shown that a steady-state solution obtained under larger amplitude fresh
water surface forcing (and hence with a weaker North Atlantic overturning)
is unstable, while a steady-state solution with stronger THC is stable. In
a certain intermediate parameter regime it is found that the full nonlinear
model state may be unstable, while the linearized analysis indicates that
the model state is stable. It is proposed that this may be because either t
he instability mechanism at this intermediate regime is nonlinear or, while
the model is linearly stable at this regime, it allows for temporary growt
h of small perturbations due to the non-normal nature of the problem.
A clear signal of variations is not found in the amplitude of the horizonta
l gyre circulation, possibly indicating that the gyre effect that was found
in THC oscillations in some previous studies may not be essential for the
existence of the THC oscillation. The long timescale of the oscillation in
the present model also seems to indicate that the gyre effect may not be a
main active participant in the thermohaline oscillation mechanism.