A simple model of the Atlantic thermohaline circulation: Internal and forced variability with paleoclimatological implications

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Abstract
We describe the development and application of a simple two‐dimensional model of the thermohaline circulation in the Atlantic basin. The nondimensional parameters of the model consist of thermal and haline Rayleigh numbers, Lewis number, and ratios of horizontal and vertical diffusion coefficients for heat, salt, and momentum. Under mixed boundary conditions and with a convective adjustment scheme applied to represent the subgrid scale convective downwelling that drives the process of deepwater formation, the model support two modes of oscillatory behavior. One of these is on a timescale that ranges from subdecadal to decadal and appears as noisy fluctuations driven by the convective adjustment scheme. The second physical mode has a characteristic timescale of the order of centuries when the net influx of freshwater into the basin is near zero, a mode of variation which is similar to that found previously in full three‐dimensional ocean general circulation model analyses of the modern circulation. We demonstrate, however, that the timescale of this physical mode is strongly dependent on the net freshwater forcing: the weaker this forcing, the shorter the timescale, the stronger the forcing, the longer the timescale. This mode therefore provides a possible explanation both for the century timescale variations observed in the modern climate record and for the millennium timescale Dansgaard‐Oeschger oscillations that were characteristic of the late glacial climate state. The response of the model to freshwater anomalies similar to those known to have developed during the late glacial state of the ice age and during deglaciation seems to provide a straightforward explanation of the climatic variations recently revealed in δ18O time series for ice cores drilled at Summit, Greenland. This is especially true of the experiments that we have performed in an effort to better understand the Younger‐Dryas type climatic fluctuations that occurred during the late glacial and the last deglaciation periods. Our experiments strongly support the hypothesis that the Atlantic thermohaline circulation played a very important role in these intense events.