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Ice sheet runoff and Dansgaard-Oeschger Cycles Ian Hewitt*, Eric - - PowerPoint PPT Presentation
Ice sheet runoff and Dansgaard-Oeschger Cycles Ian Hewitt*, Eric - - PowerPoint PPT Presentation
Ice sheet runoff and Dansgaard-Oeschger Cycles Ian Hewitt*, Eric Wolff, Andrew Fowler, Chris Clark, Geoff Evatt, Helen Johnson, David Munday, Ros Rickaby, Chris Stokes Universities of Oxford, Cambridge, Limerick, Sheffield, Manchester, Durham,
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Age [ka b2k] (GICC05 extended)
20 40 60 80 100 120
δ18O [ppt]
- 46
- 44
- 42
- 40
- 38
- 36
- 34
MIS 1 MIS 2 MIS 3 MIS 4 MIS 5
Age [ka b2k]
30 32 34 36 38 40
δ18O [ppt]
- 46
- 44
- 42
- 40
- 38
Dansgaard-Oeschger cycles
8 7 6 5
~10 C
NGRIP
Time
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Distinctive features
Rapid warming at onset (‘D-O event’) non-linear feedbacks Quasi-periodic - cycles repeat without obvious trigger Global temperature change obeys bipolar see-saw AMOC important No D-O cycles during interglacials, nor during coldest glacial periods (LGM, MIS4) Heinrich events occur when climate already cold
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Distinctive features
Rapid warming at onset (‘D-O event’) non-linear feedbacks Quasi-periodic - cycles repeat without obvious trigger self-sustaining oscillations ? Global temperature change obeys bipolar see-saw AMOC important No D-O cycles during interglacials, nor during coldest glacial periods (LGM, MIS4) Heinrich events occur when climate already cold
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Distinctive features
Rapid warming at onset (‘D-O event’) non-linear feedbacks Quasi-periodic - cycles repeat without obvious trigger self-sustaining oscillations ? Global temperature change obeys bipolar see-saw AMOC important No D-O cycles during interglacials, nor during coldest glacial periods (LGM, MIS4) ice sheets important ? Heinrich events occur when climate already cold
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Distinctive features
Rapid warming at onset (‘D-O event’) non-linear feedbacks Quasi-periodic - cycles repeat without obvious trigger self-sustaining oscillations ? Global temperature change obeys bipolar see-saw AMOC important No D-O cycles during interglacials, nor during coldest glacial periods (LGM, MIS4) ice sheets important ? Heinrich events occur when climate already cold Heinrich events not important ?
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Background
Many models exist - most invoke changes in ocean circulation to help explain global pattern
Sudden freshwater sources to North Atlantic - e.g. Clark et al 2001, Ganapolski & Rahmstorf 2001 Salt oscillators - Broecker et al 1990, Birchfield & Broecker 1990, Peltier & Vettoretti 2014 Ice shelf growth and sea ice - Petersen et al 2013 Sea ice and North Atlantic stratification - Dokken et al 2013, Jensen et al 2016 Atmospheric-sea ice-ocean feedbacks caused by changing height of Northern hemisphere ice sheets - e.g. Zhang et al 2014
Meltwater routing through the Arctic has most effect
- n AMOC Condron & Winsor 2012
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AMOC strength Q Freshwater forcing F
Hysteresis in ocean circulation
Warm Cool
Stommel 1961, Ganapolski & Rahmstorf 2001, Rahmstorf et al 2005
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Oscillation mechanism
Effect of runoff on ‘freshwater’ delivery is buffered by changes in Arctic Ocean salinity Strong AMOC produces warmer Northern hemisphere
warming accentuated by sea ice - albedo feedback
Leads to more runoff from ice sheets This freshwater sends AMOC onto weaker branch Cooling reduces runoff and starves ocean of fresh water Sends AMOC back to strong branch
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Model schematic
X RA Q RA RN E SN SF SD K
Atlantic Arctic Ice
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Arctic Ocean
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X RA Q RA RN E SN SF SD K
Atlantic Arctic Ice
bτ dQ dt = f(Q) − F. RA = R0 + λ (Q − Q0) ;
Model equations
AMOC Runoff
− ∝ Q = F
Freshwater Salinity
F ≈ F∗ + (X + RA) ✓ 1 − SD SN ◆
VD dSD dt ≈ X(SN − SD) − RASD
Non-linear dynamical system - relaxation oscillation Parameters estimated using current day values
phenomenological model of hysteresis in ocean models amplification by ocean and sea ice rolled into
+ λ
effective freshwater flux through Fram Strait salt balance for deep Arctic (fresher surface layer evolves more rapidly)
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Oscillations
F [ Sv ]
- 0.02
0.02
Q [ Sv ]
14 16 18 20 22 24 26
Ocean equilibrium curve Decreasing salinity Salinity equilibrium curve
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Oscillations
F [ Sv ]
- 0.02
0.02
Q [ Sv ]
14 16 18 20 22 24 26
Ocean equilibrium curve Decreasing salinity
∆ T [ K ]
- 5
5
Time [years]
1000 2000 3000 4000 S [ ppt ] 26 28 30 32 34 36
Salinity equilibrium curve Time
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Oscillations
F [ Sv ]
- 0.02
0.02
Q [ Sv ]
14 16 18 20 22 24 26
Ocean equilibrium curve Decreasing salinity
∆ T [ K ]
- 5
5
Time [years]
1000 2000 3000 4000 S [ ppt ] 26 28 30 32 34 36
Salinity equilibrium curve
Time scale controlled by Fram Strait exchange To get ~1000 years, exchange flow around 10 times smaller than present day
Time
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Can we rationalise observed variability?
F [ Sv ]
- 0.02
0.02
Q [ Sv ]
14 16 18 20 22 24 26
F [ Sv ]
- 0.02
0.02
Q [ Sv ]
14 16 18 20 22 24 26
Interglacials (no ice) ? LGM (Arctic melt pathway blocked) A slow change of parameters can alter period of cycles, or produce steady (but ‘excitable’) states.
Reduced sensitivity of runoff to AMOC Increased background runoff Clark et al 2001
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