RAPID TRANSITIONS OF THE OCEANS DEEP CIRCULATION INDUCED BY CHANGES IN SURFACE-WATER FLUXES

DEEP water in the world's oceans flows predominantly from the northern North Atlantic into the Pacific 1, slowly upwells on the way to become part of the upper warm-water circulation, and returns to the North Atlantic. The stability of this thermohaline conveyor belt has recently been questioned on the basis of palaeoclimatic data from deep-sea sediment and ice cores 2,3. Different modes of deep circulation have been confirmed in numerical ocean models 4-6, and the present-day circulation has been shown to be sensitive to changes in the surface-water budget 7. Here we use an idealized mode 4,5 to examine the hypothesis that small changes in the atmospheric flux of fresh water from the Atlantic to the Pacific could force the thermohaline circulation to switch between two stable modes. Our results indicate that a decrease of this flux can reverse the Atlantic circulation, although the Pacific thermohaline circulation does not change direction. This is consistent with reconstructions of conditions in the Atlantic Ocean during the last glacial obtained from deep-sea cores 8. To reestablish the conveyor belt, the fresh-water flux need be increased only slightly beyond its present value.