The meridional overturning circulation (MOC) of the ocean is viewed as an important component of the climate system owing to its attendant poleward heat flux. A popular theory ascribes rapid climate changes of the geological past to variations in the MOC caused by glacial meltwater input to the northern North Atlantic. The theory, however, is currently challenged by studies based on idealized models, showing that the response of the MOC to surface forcing depends qualitatively on the representation of vertical mixing in the models: if the mixing is represented with a fixed, apparent diffusivity, the MOC decreases with decreased equator-to-pole density contrast at the surface (the usual result), whereas if the diffusivity depends on vertical density stratification (perhaps a more plausible assumption), the MOC increases with decreased density contrast. Here we will determine whether a similar result holds for the time-dependent response of the circulation to surface freshening in a realistic model of the global ocean. To meet our goal we will conduct a set of numerical experiments with an ocean general circulation model, which rely on different assumptions about vertical mixing, and compare their results with paleoceanographic data obtained from sediments. Our work plan includes three major tasks. First, the model will be set up to represent the global ocean with realistic bathymetry. Second, a series of model equilibrium states assuming constant or stability-dependent vertical diffusivity will be generated. Third, these states will be perturbed by applying an anomaly in the surface freshwater flux. Comparing results obtained from different assumptions about vertical mixing will provide a test of our hypothesis. Comparing model results with paleoceanographic data will enable us to assess the extent to which these data could contribute to an outstanding question in physical oceanography, related to the role of vertical mixing in the MOC.
