| The mechanism of interdecadal variability of the thermohaline circulation (THC) is examined with a zonally averaged ocean model (ZAOM), an ocean general circulation model (OGCM), and a linear perturbation ocean model. The ZAOM developed for this study differs from previous formulations in that a slower adjustment timescale is specified for the meridional velocity. This allows the adjustment of the overturning intensity to serve as a phase-transition mechanism, thereby inducing interdecadal variability. The OGCM configuration is highly idealized since it is forced only by a surface heat flux, has simplified basin geometry, and uses crude parameterizations for subgrid-scale processes. Three separate interdecadal oscillations are documented with the feature that distinguishes the simulations being the zonal location of deep-water formation. The three- dimensional, linear perturbation ocean model is developed to examine how the basic state influences the oscillation characteristics. A wide parameter range is explored including the sensitivity of the interdecadal mode to variations in the basic-state circulation intensity, density gradient magnitude, stratification, location and frequency of deep-ocean convection, and strength of a wind-driven gyre circulation.; An eigenvalue analysis reveals that the oscillatory mechanisms of both the ZAOM and OGCM are dependent on coupled upper-/deep-ocean interactions. Two distinct feedbacks link the density anomalies (rho') in the upper (surface to 1000 m) and deep (1000 to 4000 in) ocean. The upper-ocean rho ' directly impacts the deep-ocean rho' by being mixed downwards in regions of deep-water formation, whereas the deep-ocean rho ' influences the upper-ocean rho' by causing an anomalous advection of the basic-state density gradients. These two interactions are primarily responsible for determining the interdecadal timescale of oscillation. The interdecadal variability is sustained against the damping caused by mixing processes by the basic-state advection of density anomalies. The basic-state advection provides a positive feedback by reinforcing the upper-ocean baroclinic pressure anomaly near the sinking region. While the mechanism is adequately resolved by a one-component fluid, a linear feedback that links the temperature and salinity anomalies through the THC adjustment can influence the stability of the oscillations. |
