The 8.2Kyr eventJulia 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 3oC-6oC)• 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 coreFigure from Alley and Ágústsdóttir 2005
The 8.2Kyr event globally?
Timing and structure in Greenland(δ18O 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
• 151,000km3 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
δ18O inprecipitation
precip
δ18O inseawater
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 HadCM3Expt 1 Expt2
Timing Instantaneously Over one year
Depth Top 800m Top 10m
Area Near Labrador Sea (84 gridboxes)
N. Atlantic (50oN-70oN)
(103 gridboxes)
δ18O 0‰ -30‰
Other issues Spin up not completed
HadCM35Sv added over North Atlantic for 1 year
First 10 years of model runshow cooling over much of the Northern hemisphere however δ18O signal is more noisy.
Atlantic MOC
Temperature Changes δ18O 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
