INTERDECADAL VARIABILITY IN A ZONALLY AVERAGED OCEAN MODEL - AN ADJUSTMENT OSCILLATOR

A meridional-plane, hemispherical ocean model is developed to study in terdecadal variability of the thermohaline circulation (THC). The mode l differs from previous formulations of zonally averaged ocean models by using a prognostic equation to calculate the meridional velocity. T his 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 salin ity forced by a time-independent surface heat flux. The governing equa tions are linearized about a basic state in order to isolate the effec t of parameter changes on the oscillation. An eigenvalue analysis reve als that the frequency of the oscillation is independent of the advect ive timescale. Instead, the interdecadal timescale of the oscillation is set by the slow adjustment of the overturning intensity to a meridi onal pressure gradient anomaly. The oscillatory instability, on the ot her hand, is dependent on both advective and adjustment processes. Adv ection by the basic state acts locally to reinforce the meridional pre ssure gradient anomaly, whereas the delayed adjustment of the overturn ing intensity to this pressure gradient anomaly modifies the poleward transport of heat, thereby initiating the phase reversal of the pressu re gradient anomaly. Thus, the advection of temperature anomalies by t he basic state provides a positive feedback while the adjustment of th e overturning intensity serves as the phase-switching mechanism. The r elevance of this ''adjustment oscillator'' to the interdecadal variabi lity simulated in idealized ocean general circulation models is discus sed. The results strongly suggest that internal, interdecadal variabil ity of the THC is not an inherently three-dimensional or nonlinear phe nomenon and that this type of variability cannot be conceptualized as a loop oscillator.