  Within a simple model context, the sensitivity and stability of the
thermohaline circulation to finite amplitude perturbations is studied. A new
approach is used to tackle this nonlinear problem. The method is based on the
computation of the so-called Conditional Nonlinear Optimal Perturbation (CNOP)
which is a nonlinear generalization of the linear singular vector approach
(LSV). It is shown that linearly stable thermohaline circulation states can
become nonlinearly unstable and the properties of the perturbations with
optimal nonlinear growth are determined. An asymmetric nonlinear response to
perturbations exists with respect to the sign of finite amplitude freshwater
perturbations, on both thermally dominated and salinity dominated thermohaline
flows. This asymmetry is due to the nonlinear interaction of the perturbations
through advective processes.
