CENTURY MILLENNIUM INTERNAL CLIMATE OSCILLATIONS IN AN OCEAN-ATMOSPHERE-CONTINENTAL ICE-SHEET MODEL

We demonstrate in a simple climate model that there exist nonlinear feedbacks between the atmosphere, ocean, and ice sheets capable of producing century/millennium timescale internal oscillations resembling those seen in the paleoclimate record. Feedbacks involve meridional heat and salt transports in the North Atlantic, surface ocean freshwater fluxes associated with melting and growing continental ice sheets in the northern hemisphere and with Atlantic to Pacific water vapor transport. The positive feedback between the production of North Atlantic Deep Water (NADW) and the meridional salt transport by the Atlantic thermohaline circulation tends to destabilize the climate system, while the negative feedback between the freshwater flux, either to or from the continental ice sheets, and meridional heat flux to the high-latitude North Atlantic, accomplished by the thermohaline circulation, stabilizes the system. The thermohaline circulation plays a central role in both positive and negative feedbacks because of its transport of both heat and salt. Because of asymmetries between the growth and melt phases the oscillations are, in general, accompanied by a growing or decreasing ice volume over each cycle, which in the model is reflected by increasing or decreasing mean salinity. The magnitude of the oscillations can be as large as the fluctuations observed in the meltwater fluxes during the last deglaciation, suggesting that the feedback mechanisms themselves may have been actively involved in the deglaciation process. The feedbacks could potentially play a causal role in the Dansgaard-Oeschger events. They are also candidates for driving the apparent major ice sheet rapid collapses (in approximately 300 yrs.) that appear to have occurred at intervals of 7 ky to 10 ky, identified in the North Atlantic sediment record as Heinrich events.