Buoyancy-driven flow and nature of vertical mixing in a zonally averaged model

Marchal, Olivier; Jackson, Charles; Nilsson, Johan; Paul, André; Stocker, Thomas F. (2007). Buoyancy-driven flow and nature of vertical mixing in a zonally averaged model. In: Schmittner, Andreas; Chiang, John C. H.; Hemmings, Sidney R. (eds.) Ocean Circulation: Mechanisms and Impact. Geophysical Monograph Series: Vol. 173 (pp. 33-52). Washington: American Geophysical Union AGU 10.1029/173GM05

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The consequences for the meridional overturning circulation (MOC) of fundamentally different assumptions about the vertical effective diffusivity of heat and salt (Kv) are examined in a zonally averaged model of the buoyancy-driven flow in one- and two-hemisphere basins. First, we replicate results obtained in earlier studies from a zonally averaged model based on a less elaborate closure for the zonal pressure difference. For a single-hemisphere basin, the equilibrium response of the MOC to freshwater forcing (salt addition at low latitudes and salt extraction at high latitudes) depends qualitatively on the nature of vertical mixing: if the diffusivity is constant (a common assumption), the MOC decreases with increased forcing, whereas if it depends on vertical density stratification (at least an equally plausible assumption) the MOC increases with increased forcing. For a two-hemisphere basin, on the other hand, the equilibrium response of the MOC in the dominant hemisphere to increased freshwater forcing (symmetric about the equator) is an amplification for both mixing representations. Second, we investigate the instability of the flow at large freshwater forcing. For both basins, the flow is more stable to the forcing if Kv varies with vertical stratification. For a single-hemisphere basin, self-sustained oscillations of the flow that are quasi-periodic (e.g., millennial) are found for both fixed and stability-dependent Kv. For a two-hemisphere basin, such oscillations are found only when Kv is stability-dependent. For both basins, the occurrence and period of the oscillations are partly determined by the energy available for vertical mixing if Kv varies with vertical stratification. A possible analogy with the relaxation oscillations of van der Pol is presented.

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