The Response of an Idealized Ocean Basin to Variable Buoyancy Forcing

The response of an idealized ocean basin to variable buoyancy forcing is examined. A general circulation model that employs a Gent–McWilliams mixing parameterization is forced by a zonally constant restoring surface temperature profile, which varies with latitude and time over a period P. In each experiment, 17 different values of P are studied, ranging from 6 months to 32 000 yr. The model’s meridional overturning circulation (MOC) exhibits a very strong response on all time scales greater than 15 yr, up to and including the longest forcing time scales examined. The peak-to-peak values of the MOC oscillations reach up to 125 % of the steady-state maximum MOC and exhibit resonance-like behavior, with a maximum at cen-tennial to millennial forcing periods (depending on the vertical diffusivity). This resonance-like behavior stems from the existence of two adjustment time scales, one of which is set by the vertical diffusion and the other of which is set by the basin width. Furthermore, the linearity of the response as well as its lag with the forcing varies with the forcing period. The considerable deviation from the quasi-equilibrium response at all time scales above 15 yr is surprising and suggests a potentially important role of the ocean circulation for climate, even at Milankovich time scales. 1