| he influence of the deep thermohaline circulation (THC) of the global ocean on late-Pleistocene climate variability, for timescales ranging from centuries to millennia, has been investigated through development of a reduced model of the THC which has been successfully coupled to a pre-existing climate model.;Firstly, a single (Atlantic) basin version of the model is developed, and investigations using it are shown to clearly establish that reduced models of this kind are able to provide a good approximation to the known properties of the modern circulation. Very interesting results are also obtained to the effect that the temporal characteristics of the internal variability of the THC are a strong function of net surface freshening. Depending on the net freshwater input at the surface, the THC is shown to oscillate in strength with a period that ranges from a century to several millennia, the latter being commensurate with the timescale of the so-called Dansgaard-Oeschger oscillations that are so apparent in the GRIP and GISP2 ice cores from Summit, Greenland.;A multi-basin version of the model has also been developed in a second stage of analysis by connecting three individually two-dimensional basins (Atlantic, Indian, and Pacific) through the Southern basin, with parameterizations added of a number of additional processes (sea ice, the wind-driven circulation). This model, too, produces both the century timescale oscillations, characteristic of the Holocene, and the millennium timescale oscillations, characteristic of the full glacial state. Both the single basin and multi-basin versions of the model have also been employed in simulations intended to mimic the Younger-Dryas event, and are shown to successfully produce a history of shutdowns and onsets of the THC which reasonably fit the observations.;In the final part of the thesis, the multi-basin version of the THC model is coupled to an existing global climate model, itself consisting of coupled atmospheric energy balance and continental ice-sheet components. Simulations of the last glacial-interglacial cycle are performed with this model, and the impact of the heat flux from the ocean to the atmosphere thereby shown to be significantly more important than has been believed on the basis of previous studies. In this fully coupled model the millennium timescale Dansgaard-Oeschger oscillation is shown to be accompanied by a variability in atmospheric temperature local to the region of Greenland that matches extremely well the signal inferred on the basis of the... |
