examples from past abrupt climate changes - kurs.uib.nokurs.uib.no/acdc/filer/229.DCveMh.pdf ·...

Stability of past ocean circulation: examples from past abrupt climate changes

Kerim Hestnes Nisancioglu ([email protected])

Bjerknes Centre for Climate Research, Norway

ACDC 2009, Espegrend

Part 2 - Past Abrupt Climate ChangeAdvanced Courses in the Dynamics of Climate, 2009

Past rapid climate changes as seen in ice cores:

• δ18O records of Greenland and Antarctica.

D-O and H-events of the last glacial cycle:

• Main characteristics and timing of events.

Models of past rapid climate changes:

• Crowley’s temperature seesaw;

• mechanisms behind D-O events.

New conceptual model of D-O events:

• role of the Fennoscandian ice sheet.

Synopsis of Part II

146 V. Masson-Delmotte et al.: Past temperature from ice cores

Fig. 1. Maps of Greenland and Antarctica showing first and most recent deep ice core sites. For each drilling site are indicated the name of

the deep drilling, the year of drilling completion, the final depth and an estimate of the length of the recovered climate and environmental

records.

have been showed to have impacts on the formation of these

deep and salty waters, but also to influence the hemispheric

atmospheric circulation (Dethloff et al., 2006; Yuan andMar-

tinson 2000; Chiang and Bitz, 2005). Atmospheric circula-

tion has been shown to link polar locations with other lati-

tudes (Liu et al., 2004; Marshall et al., 2001) and southern

latitudes (Guo et al., 2004; Yuan, 2004; Turner, 2004). Sev-

eral processes at play in polar regions are involved in climate

feedbacks: dynamical transport effects; atmospheric content

of water vapour (Solomon, 2006); polar cloud cover; surface

snow and ice albedo; sea-ice extent (Holland and Bitz, 2003).

Polar regions north and south are also involved in the car-

bon cycle via the role of the southern ocean and deep ocean

circulation (Kohler et al., 2005) and the soil carbon storage

in permafrost areas (Davidson and Janssens, 2006). Today,

some areas of the polar regions experience the largest ob-

served warming over the instrumental records (IPCC, 2001).

There is therefore a strong interest to obtain past temperature

reconstructions from these polar regions.

The Greenland and Antarctic polar ice sheets represent

!70% of the freshwater on Earth. Because of their volume

and time constants, these ice sheets are the slow components

of the climate system. It is for instance suggested that the av-

erage age of ice inside the Greenland ice sheet is 41±2 kyrs(respectively, 125±5 kyrs and 44±3 kyrs for the East andWest Antarctic ice sheets) (Lhomme et al., 2005). Huge

Clim. Past, 2, 145–165, 2006 www.clim-past.net/2/145/2006/

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Figure 6.3. Variations of deuterium (!D; black), a proxy for local temperature, and the atmospheric concentrations of the greenhouse gases CO2 (red), CH4 (blue), and nitrous oxide (N2O; green) derived from air trapped within ice cores from Antarctica and from recent atmospheric measurements (Petit et al., 1999; Indermühle et al., 2000; EPICA com-munity members, 2004; Spahni et al., 2005; Siegenthaler et al., 2005a,b). The shading indicates the last interglacial warm periods. Interglacial periods also existed prior to 450 ka, but these were apparently colder than the typical interglacials of the latest Quaternary. The length of the current interglacial is not unusual in the context of the last 650 kyr. The stack of 57 globally distributed benthic !18O marine records (dark grey), a proxy for global ice volume fl uctuations (Lisiecki and Raymo, 2005), is displayed for comparison with the ice core data. Downward trends in the benthic !18O curve refl ect increasing ice volumes on land. Note that the shaded vertical bars are based on the ice core age model (EPICA community members, 2004), and that the marine record is plotted on its original time scale based on tuning to the orbital parameters (Lisiecki and Raymo, 2005). The stars and labels indicate atmospheric concentrations at year 2000.

Reconstructed Antarctic temperature and atmospheric greenhouse gas concentrations follow global sealevel.

Antarctic Ice Cores

Greenland Ice Cores & δ18O-ratio

Greenland temperatures as recorded in the δ18O-ratio of ice show evidence of millennial scale abrupt climate changes during the last glacial period.

warm

cold-46

-44

-42

-40

-38

-36

-34

-32

020,00040,00060,00080,000100,000120,000

NGRI

P δ1

8 O (

‰)

NGRIP Age (years BP)

δ18O of Ice Explained

The δ18O value recorded in ice cores vary with changes in local temperature of precipitated snow, changes in source area of water vapor, and δ18O in the source regions.

The effect of global ice volume on δ18O in ice is relatively small.

Ruddiman, Earth’s Climate, 2001

Historical PerspectiveWilly Dansgaard discovered link between stable isotopic composition of precipitation and atmospheric temperature by accident in 1952.

“An investigation will be undertaken as soon as the opportunity offers” - Dansgaard (1954)

Analyzed first Greenland ice core at Camp Century in 1969 drilled by Chester Langway.

Historical PerspectiveWilly Dansgaard discovered link between stable isotopic composition of precipitation and atmospheric temperature by accident in 1952.

“An investigation will be undertaken as soon as the opportunity offers” - Dansgaard (1954)

Analyzed first Greenland ice core at Camp Century in 1969 drilled by Chester Langway.

Millenial scale abrupt (“violent”) events were first discovered in 1981 after drilling Dye-3.

Hans Oeschger found “drastic climatic changes” in glacial lake deposits outside Bern matching the millennial scale events in Dye-3.

The glacial period on Greenland is punctuated by a series of rapid warming events (D-O events) with temperature changes on Greenland on the order of 10-15°C.

D-O events are most pronounced during MIS3.

Rahmstorf, Nature, 2002

Dansgaard-Oeschger Events

The glacial period on Greenland is punctuated by a series of rapid warming events (D-O events) with temperature changes on Greenland on the order of 10-15°C.

D-O events are most pronounced during MIS3.

Rahmstorf, Nature, 2002

10-15°C


Time (years since start of event)

δ18O

(‰

)

Te

mp

era

ture

(oC

)


Each DO-event is characterised by a rapid shift from a cold stadial climate to a warm interstadial climate.

The return from a warm interstadial to a cold stadial climate is slower.

Ganopolski & Rahmstorf, Nature, 2001

δ18

O (

‰)

SS

T (

°C)

GISP2 Age (Ka BP)

Greenland Summit

NW Indian Ocean Monsoon:stronger/Earlier onset during warm stadial

Carriaco Basin:increased Precip/runoff during warm stadial

Santa Barbara Basin:more ocean ventilation during warm stadial China Caves:

lower δ18O during warm stadial

Global Imprint of D-O events

Heinrich EventsThe marine geologist Hartmut Heinrich (1988) was the first to discover major episodes of ice rafting (IRD) in the North Atlantic separated by 5000 years or more.


Ruddiman’s IRD belt

What causes DO-events?

(1) Natural oscillations inherent to the internal behavior of NH ice sheets.



(2) A response to solar variations external to the climate system.




➡ A possible pacemaker?





(3) The result of internal interactions among several parts of the climate system (seaice, AMOC, deep ocean).





(3) The result of internal interactions among several parts of the climate system (seaice, AMOC, deep ocean).

➡ Most (all) theories involve freshwater perturbations to the North Atlantic.


(1) Natural oscillations of ice margins

A: ice sheets have very long response times, however the margins are thin and capable of fast changes.

B: small amounts of heat can destabilize the ice margin causing the ice to surge and release icebergs.

C: as the ice margins thicken, the bedrock is depressed, and the ice is lifted and floated off by sea-water.


(2) External variations in Insolation

Changes in the production of 14C can be used as a record of the Sun’s strength in the past. The major cycle found is at 420 year. There is no significant cycle near 1500-years, and the link to climate is faint.


Broecker et al., A salt oscillator in the Glacial Atlantic?, Paleoceanography, 1990:

“we suggest that the Greenland climate changes are driven by oscillations in the salinity of the Atlantic Ocean which modulate the strength of the Atlantic's conveyor circulation.”

(3) Interactions within the climate system

Broecker et al., A salt oscillator in the Glacial Atlantic?, Paleoceanography, 1990:

“we suggest that the Greenland climate changes are driven by oscillations in the salinity of the Atlantic Ocean which modulate the strength of the Atlantic's conveyor circulation.”

Problems:

• most climate models require a large freshwater perturbation (0.1 - 1.0 Sv).

• cold stadial conditions correspond to complete AMOC shutdown.

• unknown source of freshwater on D-O timescales.

(3) Interactions within the climate system

Hysteresis of the AMOC

Response of AMOC to freshwater in simple models:e.g. Stommel (1961); Bryan (1986); Manabe & Stouffer (1988); Stocker & Wright (1991); Rahmstorf (1995).

Rahmstorf, 2002

Model Response to Freshwater

Coupled models with simplified oceans (zonal mean or idealized bathymetry)

Coupled models with 3D oceans

Rahmstorf et al., GRL, 2005

Bipolar Temperature SeesawT. J. Crowley, North Atlantic deep water cools the southern hemisphere, Paleoceanography, 1992 (see also Broecker, 1998; Stocker, 1998):

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Perturbations in northern high latitudes trigger transitions in the state of the AMOC. This gives a large signal in the North Atlantic as well as a weak opposite signal in the south due to changes in PHT.

Vellinga & Wood, Climatic Change, 2002:

HadCM3 perturbed with freshwater input to the North Atlantic between 50N and 90N.

Pure white noise freshwater forcing with a standard deviation of 0.03 Sv.

White noise freshwater forcing with periodic signal of amplitude 0.01Sv.

Simulated temperature response.


Stochastic forcing and DO-events

Ganopolski & Rahmstorf, Physical Review Letters, 2002

Stochastic forcing

Pure white noise freshwater forcing with a standard deviation of 0.03 Sv.

White noise freshwater forcing with periodic signal of amplitude 0.01Sv.



Stochastic forcing and DO-events

Ganopolski & Rahmstorf, Physical Review Letters, 2002

Stochastic forcing + weak periodic forcing

Questioning the Role of the Ocean in DO-events

generates detectable signals at great distances in latitude andlongitude. The extent to which it is generated in the oceanictropics, as opposed, for example, to being largely governed bystochastic westerly wind bursts of continental origin, is thesubject of great debate (e.g., Neelin et al., 1998). Thus for D–Oevents, even the detectability of a signal remotely need not leadto a deduction of its local importance, nor to inferences aboutgeneration—a subject taken up next.

North Atlantic circulation control

Our focus now changes to the separate issue of the cause (or‘‘trigger’’) of the rapid changes seen in central Greenland.Consider the widely accepted scenario that Greenland D–O

events are a direct consequence of a major shift in the NorthAtlantic meridional overturning circulation. Some confusionoccurs at the outset because of a failure to specify whichelements of that circulation are supposed to generate the climatechange (Wunsch, 2002). Often, the focus is on the mass fluxassociated with the meridional overturning, and the mass flux isindeed central to the dynamics of the ocean. But in terms of theimpact on the climate system, it is the oceanic poleward heat fluxthat has the most immediate consequences for the atmosphere.Alternatively, sea-surface temperatures are most often used todetermine how the ocean is affecting the atmospheric state,although these will be in large part a consequence of the heat fluxdivergence (exchange of enthalpy with the atmosphere).

Figure 10 displays the estimated net meridional transport ofheat by the combined ocean–atmosphere system, as well asseparate estimates of the oceanic and atmospheric contributions(Wunsch, 2005). A number of features stand out in this figure.First, the oceanic Northern Hemisphere contribution polewardof about 25-N falls very rapidly as heat is transferred to theatmosphere through the intense cyclogenesis in the mid-latitudestorm belts, and as the relative oceanic area rapidly diminishes.By 40-N, the oceanic contribution is less than 25% of theatmospheric contribution. Of this 25%, most is in the NorthAtlantic (e.g., Ganachaud and Wunsch, 2002). The assumptionthat a fractional change in this comparatively minor contributionto the global heat flux is the prime mover of abrupt climatechange is not very appealing, if there is any alternativepossibility (one will be proposed below). Furthermore, air masstrajectories circling the globe at high latitudes are in contact withthe North Atlantic Ocean for only a very short time compared tothe North Pacific, Asia and North America. The oceanic tail maynot necessarily be wagging the meteorological dog.

Figure 10. Total meridional heat transport of the combined ocean/atmosphere

system estimated from Earth Radiation Budget Experiment (ERBE) satellites

(solid), direct ocean measurements (dashed), and atmospheric contribution as a

residual (dash-dot line). Taken from Wunsch (2005). Standard error bars are

shown.

Figure 9. Power densities (a, b) and coherences (c– f) for the GRIP CH4 and y18O records. Both records were normalized to unit variance before computation so that

the power densities are dimensionless power/cycle/kyr. Coherence is plotted on both linear and logarithmic frequency scales. An approximate 95% confidence level

of no significance for the coherence magnitude is shown as a dashed line. Coherence magnitudes are above the level-of-no-significance only at low frequencies.

C. Wunsch / Quaternary Research 65 (2006) 191–203196

Carl Wunsch, Quat. Res., 2006:

Oceanic PHT decreases rapidly poleward of 25°N: at 40°N the oceanic contribution (mostly N. Atl.) is less than 25% of that of the atmosphere.

“The oceanic tail may not necessarily be wagging the meteorological dog.”

Note: changes in the AMOC and in the mass flux of the ocean do not correspond to changes in PHT and climate.

The total PHT is governed by the combined atmosphere and ocean transports which is anti-symmetric about the equator.

If this anti-symmetry is maintained as the climate system shifts, a reduction of the North Atlantic PHT would be compensated by an increase in atmospheric PHT (Stone, 1978).

Unresolved Issues with DO-events

• Discharge of ice (H-events) is focused towards the end of the cold stadials and does not occur for every DO-event.

• Unclear if changes in the AMOC have a significant impact on the total PHT at high latitudes.

• Warming event at the start of a DO is extremely rapid (cannot be explained as a direct response to AMOC recovery).

Proxy record from the Norwegian Seas

Trond Dokken et al., in prep, 2009

Figure 3

IS8 IS7 IS6 IS5a)

b)

-38

-40

-42

-44

c)

8

6

4

2

d)

e)

0.0

-0.5

-1.0

-1.5

-2.0

40 39 38 37 36 35 34 33 32 3141

5.0

4.5

4.0

3.5

3.0

4000

3000

2000

1000

IS9

40 39 38 37 36 35 34 33 32 3141

No data

Figure 1

Greenland

temperature

Subsurface

ocean temp.

time (GICC05 b2k)

Subsurface

ocean salinity

Ice Rafted

Detritus (IRD)

Brine release

due to sea ice

formation

NG

RIP

!1

8O

Te

mp

. (°

C)

Pla

nk

ton

ic

!1

8O

(s

mo

w)

IRD

> 0

.5m

m/g

Be

nth

ic !

18O

A Conceptual Model of D-O Events

Greenland temperature

Nordic Seas sea ice

Nordic Seas subsurface temperature

Fennoscandia freshwater discharge

time

A

B

C

D

1 32 4

time


stadial

time


stadialinterstadial

Response to Changes in Sea Ice

Camille Li et al., GRL, 2005

• Reducing winter sea ice cover in the North Atlantic has a large impact on temperatures in the region.

• Simulated warming on Greenland Summit is close to 7°C.

Changes in the Jet Streamcold state

warm state

0 10 20eddy heat flux 850mb (Km/s)

Li & Battisti, JoC, 2008

Under glacial conditions, the atmosphere can exhibit different circulation regimes

Contour: DJMF winds at 200mb, Colours: DJFM heat flux by storms at 850mb (Km/s)

When do we expect D-O events?

D-O events do not occur when sea level is below the shelf break:

• less efficient brine formation. It is not possible to form sufficient sea ice in the Nordic Seas and the halocline is not maintained.

• atmospheric jet stream is more zonal and steadier during LGM (less precipitation in the Nordic Seas)

figure by Camille Li, BCCR

Time (Ka)

Summary• The δ18O-ratio of ice in Greenland and Antarctic ice cores records

past temperatures over the past 650Ka.

• On Greenland there is evidence for millennial scale rapid changes in climate called Dansgaard-Oeschger events during the last Glacial.

• D-O events occur roughly every 1500 years and are characterized by an abrupt warming of up to 15°C, followed by a slower cooling.

• Based on work with box models and simple climate models it was believed that these D-O events where caused by a hysteresis in the AMOC in response to freshwater discharges form the Laurentide ice sheet.

• Recent work suggests that changes in sea ice and atmospheric circulation are a more likely explanation of D-O events.

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