On steady and variable buoyancy forcing in the Atlantic : an idealised modelling study
This study examines the response of the thermohaline circulation in the north Atlantic to steady and variable buoyancy forcing. The model used is a version of the MOMA model (Webb, 1996), updated to include a free surface and Gent & McWilliams mixing. The model’s resolution is coarse, 4 x4 degrees with 15 levels in the vertical. In a first instance, the model’s response to 14 different fixed thermal profiles is investigated, by systematically keeping the equator temperature fixed and then the northernmost temperature fixed. The results show that the models response differs for these two sets of experiments as one setup favours stratification while the other favours convection. In a second instance, the restoring field is made to oscillate over 17 different periods, ranging from 6 months to 32,000 years. The model's meridional overturning circulation (MOC) exhibits a very strong response on all timescales greater than 15 years, up to and including the longest forcing timescales examined. The peak-to-peak values of the MOC oscillations reach up to 125% of the steady-state maximum MOC and exhibit resonance-like behaviour, with a maximum at centennial to millennial forcing periods (depending on the vertical diffusivity). This resonance-like behaviour stems from the existence of two adjustment time scales, one of which is set by the vertical diffusion and another, which is set by the basin width. Finally, the study is extended to a double hemisphere basin. Again, the model's MOC exhibits a very strong response on all timescales in both hemispheres, up to and including the longest forcing timescales examined for either set of experiments with the amplitude of the oscillations reaching up to 140% of the steady-state maximum MOC and exhibiting resonance-like behaviour, with a maximum at centennial to millennial forcing periods. This resonance like behaviour is identical to what has been observed in a single hemisphere and occurs for the same reasons. What is novel is that when the forcing in the southern subordinate hemisphere lags that of the northern by half a period, the amplitude of the response is substantially greater for large forcing periods (millennial and above), particularly in the subordinate (southern) hemisphere. This happens because the basin has in effect two sources of deep water. This leads to colder bottom waters and thus greater stratification, which in turn stabilises the water column and thus reduces the value of the minimum overturning. The considerable deviation from the quasi-equilibrium response at all timescales above 15 years for both the single hemisphere and the double hemisphere experiments is surprising and suggests a potentially important role of the ocean circulation for climate even at Milankovich timescales.