HR: 10:50h
AN: PP22A-03 INVITED     [Abstracts]
TI: Bipolar Seesaw Concepts: Do we Understand North-South Climate Connections?
AU: * Stocker, T F
EM: stocker@climate.unibe.ch
AF: Climate and Environmental Physics, Physics Institute, University of Bern Sidlerstrasse 5, Bern, 3012 Switzerland
AU: Knutti, R
EM: knutti@climate.unibe.ch
AF: Climate and Environmental Physics, Physics Institute, University of Bern Sidlerstrasse 5, Bern, 3012 Switzerland
AU: Fl\"uckiger, J
EM: flueckiger@climate.unibe.ch
AF: Climate and Environmental Physics, Physics Institute, University of Bern Sidlerstrasse 5, Bern, 3012 Switzerland
AB: Recent ice core results from Greenland bring the number of Dansgaard/Oeschger (D/O) events to 25 [NorthGRIP Members, 2004]. It is surprising that the earliest D/O event occurs around the beginning of glaciation. Model simulations have shown that collapses and rapid onsets of the Atlantic thermohaline circulation explain many features of abrupt change found in different paleoclimatic archives. In this lecture we give an overview of the hierarchy of models describing the north-south climate connection during abrupt climate change. Changes in the Atlantic thermohaline circulation have a far-field effect which was first described by a simple bipolar seesaw [Crowley, 1992; Broecker, 1998] and later by the thermal bipolar seesaw [Stocker and Johnsen, 2003]. The latter concept has resolved the controversy of lead and lag between climate signals in the northern and southern hemispheres. However, its major shortcoming was that the southern damping time scale, derived from high-resolution, synchronised ice core records from Greenland and Antarctica, was about 1000 years and hence much longer than the typical adjustment times of the Southern Ocean of about 100 years. Using a coupled model of reduced complexity, but including a comprehensive ocean circulation component, we could show that the major north-south response to a shut down of the thermohaline circulation is well described by an extended bipolar seesaw concept with physically plausible time scales. Changes in the density structure, caused by the freshwater discharges, induce anomalous ocean circulations in the South Atlantic and modify the heat and water exchange with the Southern Ocean [Knutti et al., 2004]. This {\it thermal freshwater seesaw}, in which both temperature and freshwater anomalies contribute to the changes in southern temperature is able to explain most of the millennial variability in different paleoclimatic records during the last ice age. Open questions regard the trigger and location of thermohaline shut-downs and what the influence of southern freshwater releases on the climatic signals in Greenland and Antarctica might be. \scriptsize Broecker, W.S., {\it Paleoceanogr., 13}, 119-121, 1998. Crowley, T.J., {\it Paleoceanogr., 7}, 489-497, 1992. Knutti, R., et al., {\it Nature, 430}, 851-856, 2004. NorthGRIP Members, {\it Nature, 431}, 147-151, 2004. Stocker, T.F., and S.J. Johnsen, {\it Paleoceanogr., 18}, 1087, 2003.
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 1836 Hydrologic budget (1655)
DE: 1600 GLOBAL CHANGE (New category)
DE: 1620 Climate dynamics (3309)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting