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

We demonstrate in a simple climate model that there exist nonlinear fe edbacks between the atmosphere, ocean, and ice sheets capable of produ cing century/millennium timescale internal oscillations resembling tho se seen in the paleoclimate record. Feedbacks involve meridional heat and salt transports in the North Atlantic, surface ocean freshwater fl uxes associated with melting and growing continental ice sheets in the northern hemisphere and with Atlantic to Pacific water vapor transpor t. The positive feedback between the production of North Atlantic Deep Water (NADW) and the meridional salt transport by the Atlantic thermo haline circulation tends to destabilize the climate system, while the negative feedback between the freshwater flux, either to or from the c ontinental ice sheets, and meridional heat flux to the high-latitude N orth Atlantic, accomplished by the thermohaline circulation, stabilize s the system. The thermohaline circulation plays a central role in bot h positive and negative feedbacks because of its transport of both hea t and salt. Because of asymmetries between the growth and melt phases the oscillations are, in general, accompanied by a growing or decreasi ng ice volume over each cycle, which in the model is reflected by incr easing or decreasing mean salinity. The magnitude of the oscillations can be as large as the fluctuations observed in the meltwater fluxes d uring the last deglaciation, suggesting that the feedback mechanisms t hemselves may have been actively involved in the deglaciation process. The feedbacks could potentially play a causal role in the Dansgaard-O eschger events. They are also candidates for driving the apparent majo r 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.