The 8k event: cause and consequences of a major Holocene abrupt climate change

Because of its relevance for the global climate the Atlantic meridional overturning circulation (AMOC) has been a major research focus for many years. Yet the question of which physical mechanisms ultimately drive the AMOC, in the sense of providing its energy supply, remains a matter of controversy. Here we review both observational data and model results concerning the two main candidates: vertical mixing processes in the ocean's interior and wind-induced Ekman upwelling in the Southern Ocean. In distinction to the energy source we also discuss the role of surface heat and freshwater fluxes, which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state. We conclude that both wind-driven upwelling and vertical mixing are likely contributing to driving the observed circulation. To quantify their respective contributions, future research needs to address some open questions, which we outline.

/pdf/on-the-driving-processes-of-the-atlantic-meridional-4ze95ojzss.pdf

On the driving processes of the Atlantic meridional overturning circulation

The extent of climate variability during the current interglacial period, the Holocene, is still debated. Temperature records derived from central Greenland ice cores show one significant temperature anomaly between 8,200 and 8,100 years ago, which is often attributed to a meltwater outflow into the North Atlantic Ocean and a slowdown of North Atlantic Deep Water formation--this anomaly provides an opportunity to study such processes with relevance to present-day freshening of the North Atlantic. Anomalies in climate proxy records from locations around the globe are often correlated with this sharp event in Greenland. But the anomalies in many of these records span 400 to 600 years, start from about 8,600 years ago and form part of a repeating pattern within the Holocene. More sudden climate changes around 8,200 years ago appear superimposed on this longer-term cooling. The compounded nature of the signals implies that far-field climate anomalies around 8,200 years ago cannot be used in a straightforward manner to assess the impact of a slowdown of North Atlantic Deep Water formation, and the geographical extent of the rapid cooling event 8,200 years ago remains to be determined.

/pdf/centennial-scale-climate-cooling-with-a-sudden-cold-event-v62jjvj3eu.pdf

Centennial-scale climate cooling with a sudden cold event around 8,200 years ago

Evidence from a North Atlantic deep-sea sediment core reveals that the largest climatic perturbation in our present interglacial, the 8200-year event, is marked by two distinct cooling events in the subpolar North Atlantic at 8490 and 8290 years ago. An associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the Atlantic meridional overturning circulation. The existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying Laurentide Ice Sheet margin.

http://science.sciencemag.org/content/sci/312/5782/1929.full.pdf

Surface and deep ocean interactions during the cold climate event 8200 years ago.

Precise timing and characterization of abrupt climate change 8200 years ago from air trapped in polar ice

meltwater pulse is assumed to have a volume of 1.6 10 14 m 3 and a period of discharge of 2 years on the basis of glaciological modeling of the decay of the Laurentide Ice Sheet (LIS). We present a possible mechanism which can explain the centennial duration of the 8.2 ka cold event. The mechanism is related to the existence of an additional equilibrium climate state with reduced North Atlantic Deep Water (NADW) formation and a southward shift of the NADW formation area. Hints at the additional climate state were obtained from the largely varying duration of the pulse-induced cold episode in response to overlaid random freshwater fluctuations in Monte Carlo simulations. The model equilibrium state was attained by releasing a weak multicentury freshwater flux through the St. Lawrence pathway completed by the meltwater pulse. The existence of such a climate mode appears essential for reproducing climate anomalies in close agreement with paleoclimatic reconstructions of the 8.2 ka event. The results furthermore suggest that the temporal evolution of the cold event was partly a matter of chance. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 4255 Oceanography: General: Numerical modeling; 4532 Oceanography: Physical: General circulation; 4215 Oceanography: General: Climate and interannual variability (3309); 9325 Information Related to Geographic Region: Atlantic Ocean;

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004PA001030

E. Bauer

Papers

Simulation of the cold climate event 8200 years ago by meltwater outburst from Lake Agassiz