Hydrological Cycle Scenarios, Deep Ocean Circulation, and Century/Millennium Climate Change: A Simulation Study Using an Ocean-Atmosphere-Ice Sheet Model

Abstract

Rapid climate changes (Dansgaard-Oeschger events) during most of the last glaciation and during the last deglaciation, seen in the Greenland ice cores and a deep sea sediment core in the North Atlantic, are likely caused by dynamical instability of the climate system itself. Here we present, with a simple coupled ocean-atmosphere-ice sheet energy-salt balance model (ESBM), oscillatory feedback mechanisms to explain these climate changes. The major physical mechanisms active in the model are latitudinal heat and salt transports by the thermohaline circulation 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 primary positive feedback is between the production of North Atlantic Deep Water (NADW) and the meridional salinity flux to the high latitude North Atlantic Ocean. The principal negative feedback is 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.

Water vapor for growing ice sheets is assumed to be transported from the subtropical ocean to the ice sheets around the periphery of the North Atlantic by a ‘baroclinic mass flux’ process-a parameterization of the mechanism of feeding water vapor to continental ice sheets by intense cyclone eddies seen in GCM simulations of the last glacial maximum (Manabe and Broccoli, 1985). Meltwater from the ice sheets is returned to the subtropical Atlantic or to the high-latitude North Atlantic.

Model oscillations are characterized by periods on the order of a few hundred to a thousand years, alternating rates of, NADW production, heat flux from ocean to atmosphere in the high-latitude North Atlantic (and hence the air temperature), and the strength of the global thermohaline circulation as measured by deep outflow from the Atlantic basin to the Southern Ocean. Robust over a wide range of model parameters, the oscillations tend to substantiate the conceptual salt oscillator model proposed by Broecker et al. (1990a) and Birchfield and Broecker (1990).

Inter-basin transport of water vapor in the simulations acts, through the salinity balance for the Atlantic and the basic feedback mechanisms, as a major control process, not only on the climate oscillatory state but on the longer time scale waxing and waning of the continental ice sheets themselves. For a net water vapor flux from the Atlantic to Pacific Ocean, the oscillatory feedback mechanism results in a ‘deglaciation’ scenario in which the entire volume of Northern Hemisphere ice sheets can potentially be dissipated within a few oscillations. If the flux is from the Pacific to Atlantic Ocean, a ’glaciation’ scenario ensues in which large increases in mean ice volume can occur.