Is There a Unique Mechanism for Abrupt Changes in the Climate System?

T. F. Stocker (Climate and Environmental Physics, University of Bern, 3012 Bern, Switzerland; ph. +41-31-631-4462; fax +41-31-631-4405; Internet: stocker@climate.unibe.ch)

In the Greenland ice core records, about 24 Dansgaard/Oeschger cycles have been recorded; their typical timescales range from a few hundred years to a few millennia. A characteristic feature is their asymmetric evolution in time: cooling phases initiate immediately after a very brief interstadial, they evolve over a few millennia at which time, a faster transition to full glacial conditions occurs. In addition, the cooling phases may be punctuated by short cold spells. The climate remains in the cold state for a few centuries to millennia and then jumps back to a milder state within less than 100 years. About a third of these events are stronger, with larger anomalies in the oxygen isotope ratio, significantly longer cooling phases and distinct signals in the atmospheric CO2 concentration.

The past record of atmospheric methane exhibits all of these Dansgaard/Oeschger cycles and can therefore be used to synchronise ice cores from Antarctica with those form Greenland. Wiggle matching would clearly fail since no a priori assumption about the phase relationship must be made. We found recently, that the pronounced Dansgaard/Oeschger events 1 (15 kyr BP), 8 (36 kyr BP), and 12 (44 kyr BP) have a concomittant in the south. This consists of a slow warming which precedes the abrupt transition in the north by a few thousand years. At the time of abrupt warming, a cooling starts in the south. This suggests an antiphase coupling between the northern and the southern hemisphere. Although superimposed by deglaciation, this is also observed during the Bolling/Allerod/Younger Dryas sequence. On the other hand, coupling between north and south is weaker or absent during the numerous short D/O events.

Various coupled atmosphere-ocean models exhibit such antiphase coupling of the hemispheres during abrupt switches on the Atlantic thermohaline circulation (THC). An active Atlantic THC extracts heat from the southern ocean: a sudden reinitiation of the THC leads to a cooling in the south. We propose this as a mechanism for D/O events 1, 8 and 12. For the other events, the records suggest that north-south coupling was weaker. This is the case for a weaker THC which is basically confined to the Atlantic basin. Shut-downs of the THC would then be registered only in the Atlantic region with associated effects in the northern hemisphere due to atmospheric teleconnections. A picture thus emerges in which the Atlantic THC can reside in 3 modes: (i) strong, active overturning in the Atlantic with export of deep water into the southern ocean; (ii) weak overturning confined to the Atlantic and close to a stability threshold; (iii) collapsed thermohaline circulation with strongly reduced meridional heat transport. Transitions between these modes are probably triggered by meltwater discharge from ice sheets. Whether or not a transition of THC occurs and to which mode is highly uncertain, since the response of the ocean is very sensitive to the volume and the transient evolution of the discharge.