Deep Decoupling Oscillations of the Oceanic Thermohaline Circulation

Abstract

The polar ice and ocean sediment cores contain a record of large and sudden climate changes around the North Atlantic Ocean. Evidence for ocean circulation changes occurring synchronously with the terrestrial climate changes has been found in forams from sediment cores which show colder sea surface temperatures and increased deep nutrient levels during the cold Younger Dryas period (Boyle and Keigwin, 1987). The increased nutrients have been interpreted as evidence that North Atlantic Deep Water formation was reduced during the Younger Dryas. A natural conclusion is that such a reduction would have contributed to the cold climate by diminished importation of heat to high, northern hemisphere latitudes. The ¹⁸O record from the Greenland ice cores reveals that similar climate oscillations with 500 to 2,000 year warm periods separating the cold periods occurred frequently during the last glaciation (Johnson et al., 1992). These “interstadials” began with abrupt warmings (within decades) followed by gradual or stepwise coolings. Broecker et al. (1990) have suggested that variability of the salinity of the Atlantic Ocean caused by melting ice sheets and water vapor export from the Atlantic forces oscillations in the thermohaline overturning.