Interdecadal Variability in a Zonally Averaged Ocean Model: An Adjustment Oscillator

A meridional-plane, hemispherical ocean model is developed to study interdecadal variability of the thermohaline circulation (THC). The model differs from previous formulations of zonally averaged ocean models by using a prognostic equation to calculate the meridional velocity. This allows the incorporation of an adjustment timescale comparable to the advective timescale of the meridional overturning. An interdecadal oscillation is documented for an idealized ocean of homogeneous salinity forced by a time-independent surface heat flux. The governing equations are linearized about a basic state in order to isolate the effect of parameter changes on the oscillation. An eigenvalue analysis reveals that the frequency of the oscillation is independent of the advective timescale. Instead, the interdecadal timescale of the oscillation is set by the slow adjustment of the overturning intensity to a meridional pressure gradient anomaly. The oscillatory instability, on the other hand, is dependent on both advective and adjustment processes. Advection by the basic state acts locally to reinforce the meridional pressure gradient anomaly, whereas the delayed adjustment of the overturning intensity to this pressure gradient anomaly modifies the poleward transport of heat, thereby initiating the phase reversal of the pressure gradient anomaly. Thus, the advection of temperature anomalies by the basic state provides a positive feedback while the adjustment of the overturning intensity serves as the phase-switching mechanism. The relevance of this ‘‘adjustment oscillator’’ to the interdecadal variability simulated in idealized ocean general circulation models is discussed. The results strongly suggest that internal, interdecadal variability of the THC is not an inherently three-dimensional or nonlinear phenomenon and that this type of variability cannot be conceptualized as a loop oscillator.