ROLE OF THE THERMOHALINE CIRCULATION IN THE ABRUPT WARMING AFTER HEINRICH EVENTS

EVIDENCE of rapid climate oscillations during the last glacial period has been identified in climate records from Greenland ice cores(1,2) and ocean sediments in the North Atlantic(3,4). These records show that periods of gradual cooling are terminated by abrupt warming events(5), with the coldest periods coinciding with the deposition of ice-rafted debris (so-called Heinrich events) throughout the North Atlantic. Heinrich events are thought to be a signature of massive iceberg discharges owing to collapse of the Laurentide ice sheet; Bond et al.(5) have proposed that the decrease in meltwater flux following collapse and retreat of the ice sheet enhanced the ocean's thermohaline circulation: thereby increasing advection of heat from the tropics and giving rise to abrupt climate warming. Here we test this idea using a simple ocean model coupled to a model of a periodically surging ice sheet. We find that massive discharges of icebergs first stop the thermohaline circulation because of the consequent freshwater influx, cooling the North Atlantic region. This is followed by a rapid restart of the circulation, leading to abrupt warming. Thus our model can reproduce the qualitative features of the climate oscillations seen in the ice-core and ocean records.