Abstract
Abstract. Paleorecords from distant locations on the globe show rapid and large amplitude climate variations during the last glacial period. Here we study the global climatic response to different states of the Atlantic Meridional Overturning Circulation (AMOC) as a potential explanation for these climate variations and their possible connections. We analyse three glacial simulations obtained with an atmosphere-ocean coupled general circulation model and characterised by different AMOC strengths (18, 15 and 2 Sv) resulting from successive ~0.1 Sv freshwater perturbations in the North Atlantic. These AMOC states suggest the existence of a freshwater threshold for which the AMOC collapses. A weak (18 to 15 Sv) AMOC decrease results in a North Atlantic and European cooling. This cooling is not homogeneous, with even a slight warming over the Norwegian Sea. Convection in this area is active in both experiments, but surprisingly stronger in the 15 Sv simulation, which appears to be related to interactions with the atmospheric circulation and sea-ice cover. Far from the North Atlantic, the climatic response is not significant. The climate differences for an AMOC collapse (15 to 2 Sv) are much larger and of global extent. The timing of the climate response to this AMOC collapse suggests teleconnection mechanisms. Our analyses focus on the North Atlantic and surrounding regions, the tropical Atlantic and the Indian monsoon region. The North Atlantic cooling associated with the AMOC collapse induces a cyclonic atmospheric circulation anomaly centred over this region, which modulates the eastward advection of cold air over the Eurasian continent. This can explain why the cooling is not as strong over western Europe as over the North Atlantic. In the Tropics, the southward shift of the Inter-Tropical Convergence Zone appears to be strongest over the Atlantic and Eastern Pacific and results from an adjustment of the atmospheric and oceanic heat transports. Finally, the Indian monsoon weakening appears to be connected to the North Atlantic cooling via that of the troposphere over Eurasia. Such an understanding of these teleconnections and their timing could be useful for paleodata interpretation.
Highlights
Since the discovery of abrupt oceanic and climate changes in marine records from the North Atlantic (Heinrich, 1988) and glaciological records from Greenland (Dansgaard et al, 1993), numerous studies have contributed to better describe this large amplitude millennial scale variability during glacial periods
Bond et al (1993) suggested the simultaneity between the abrupt changes in the North Atlantic surface conditions and the Greenland ice core records. These authors showed that glacial millennial variability is organised as follows in this area: after a cold period associated with a massive iceberg discharge from the Laurentide ice-sheet to the mid-latitude North Atlantic Ocean (a “Heinrich event”), an abrupt and large amplitude warming event (a “DansgaardOeschger event”) occurs, followed by a progressive slow cooling (a “Greenland interstadial”) ended by an abrupt cooling leading to a cold period (a “Greenland stadial”)
LGMa and LGMb present strong and stable Atlantic Meridional Overturning Circulation (AMOC), with strengths of 18 and 15 Sv respectively, and strong North Atlantic Deep Water (NADW) formation that reaches the bottom of the North Atlantic Ocean in the northern midlatitudes, the AMOC collapses to 2 Sv by the end of simulation LGMc (Figs. 1 and 2)
Summary
Since the discovery of abrupt oceanic and climate changes in marine records from the North Atlantic (Heinrich, 1988) and glaciological records from Greenland (Dansgaard et al, 1993), numerous studies have contributed to better describe this large amplitude millennial scale variability during glacial periods. Bond et al (1993) suggested the simultaneity between the abrupt changes in the North Atlantic surface conditions and the Greenland ice core records. These authors showed that glacial millennial variability is organised as follows in this area: after a cold period associated with a massive iceberg discharge from the Laurentide ice-sheet to the mid-latitude North Atlantic Ocean (a “Heinrich event”), an abrupt and large amplitude warming event (a “DansgaardOeschger event”) occurs, followed by a progressive slow cooling (a “Greenland interstadial”) ended by an abrupt cooling leading to a cold period (a “Greenland stadial”). Our main objective is to analyse the mechanisms for climate changes related to AMOC differences over 3 regions where the model compares qualitatively well with data: the northern extra-tropics, the tropical Atlantic and Indian monsoon region
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