The 8.2Kyr event Julia Tindall Freshwater hosing experiments Ron Kahana

The 8.2Kyr event Introduction and motivation The 8.2Kyr event in data Cause of the event Modelling the 8.2Ka event using other models Modelling the 8.2Ka event using HadCM3L

The 8.2Ka event in Greenland ice cores Largest rapid climate change event of the Holocene (cooling of 3 o C-6 o C) Useful for understanding the sensitivity of the climate and the likelihood of a similar future event The ideal test for climate models

Data from Greenland ice core Figure from Alley and Ágústsdóttir 2005

The 8.2Kyr event globally?

Timing and structure in Greenland (δ 18 O from GRIP(red) and GISP2(black)) From Thomas et al 2006

Recent review (Morrill et al. 2005) found a statistically significant signal at 8.2Ka in 40% of records considered in both the Northern Hemisphere and the tropics Important to separate a clear ~150yr 8.2Ka signal from millennial scale variability in the Holocene Was sharp 150year event superimposed on a longer (millennial scale) weaker event No evidence for event over Southern Hemisphere, or southward shift of ITCZ Some evidence of a slowdown in NADW formation at 8.2Ka, although this evidence is weak as many proxy records contain no signal Summary of evidence for 8.2Ka event globally

Cause of the 8.2Ka event

Details of outflow from Glacial Lake Agassiz 151,000km 3 of freshwater 5.2Sv over 6months/1year Reasonably well dated and occurred at 8.45Ka

Legrande et al 2006 GISS (model E) Ensemble of experiments with 2.5Sv – 5.0Sv added over 6 months to 1 year Large differences between ensemble members All ensemble members, had a full recovery of the THC within 30 years although sometimes there were secondary shutdowns. Temperature δ 18 O in precipitation precip δ 18 O in seawater

Modelling the 8.2Ka event using other models Wiersma et al 2006 ECBilt-Clio model (intermediate complexity) Flood equivalent to 5.2Sv Without baseline flux With baseline flux of 0.172Sv

Other of previous modelling results NCAR model has full recovery in ~10 years (Carrie Morill 8.2Ka workshop) Vellinga and Wood 2001; HadCM3 forced with ~16Sv years – recovery in ~120years Bauer et al 2004: CLIMBER-2, multi-century weak freshwater pulse (0.04Sv) required (associated with melting of LIS)

Experiments with HadCM3 Expt 1Expt2 TimingInstantaneouslyOver one year DepthTop 800mTop 10m AreaNear Labrador Sea (84 gridboxes) N. Atlantic (50 o N-70 o N) (103 gridboxes) δ 18 O0‰0‰-30‰ Other issuesSpin up not completed

HadCM3 5Sv added over North Atlantic for 1 year First 10 years of model run show cooling over much of the Northern hemisphere however δ 18 O signal is more noisy.

Atlantic MOC

Temperature Changes δ 18 O Changes First 10 years Next 10 years Last 20 years (yr 57-yr 77)

What could improve model results? 8.2Ka boundary conditions Extra freshwater forcing (e.g. preflood=0.055Sv, flood=2.5Sv, routing=0.172Sv rerouting=0.104Sv ????) Other initial conditions

Summary 8.2Ka event is the largest rapid climate change event to have occurred in the Holocene. It is a good test for climate models and could provide information about the sensitivity of the climate and hence the likelihood of future rapid climate change Attributed to the final drainage of Lake Agassiz which released ~5.2Sv of freshwater into the North Atlantic for 6months-1year at 8.45Ka Focus of a number of modelling studies, but often models need more than the suggested amount of water to produce a realistic response. Initial experiments with HadCM3 suggest that the model will be able to reproduce a realistic representation of the 8.2Ka event, although difficulties may occur with accurately representing the duration and the 250year lag.