| This thesis suggests and explores mechanisms relevant to millennial scale climate variability during glacial and interglacial periods. In particular, we present the results of model studies which are able to reproduce many aspects of observed glacial climate variability (Dansgaard-Oeschger oscillations) without external forcing and which provide a natural explanation for the prevalence of high-amplitude variability in glacial climates and the relative stability in the present Holocene. We show that the role of sea ice is critical to cold climate variability because of the effective reductionin the surface temperature gradient resulting from sea ice expansion, and its associated role in inhibiting heat flux from the ocean to atmosphere in high latitudes. Thus, as sea-ice expands in a cooler climate, the thermohaline circulation weakens and the sinking regions move equatorward, leading to shallower and weaker deep circulation. Because high latitude heat loss is inhibited (by sea ice) this circulation is unstable, and intermittent high-amplitude oscillations occur on a time-scale and with a spatial structure very similar to Dansgaard-Oeschger cycles. Consistent results are found using both a three-dimensional ocean-circulation model coupled to a simpler atmospheric model, and with a much simpler four-box ocean box model. In general, freshening plays a secondary role in the weakening of the North Atlantic thermohaline during cold periods. Significant freshening required to alter the stable northern deep water formation that occurs in a war climate such as today's Holocene. Once this freshening threshold is achieved the thermohaline circulation shifts to reverse overturning with sinking in the tropics which there is no evidence for in the geologic record for the last glacial period or the Holocene. We show that the thermohaline circulation is most sensitive to the to the temperature or salinity gradient at high latitudes and in some cases high-latitude freshening in a cold climate can cause the thermohaline circulation to strengthen rather than weaken as expected. |
