Rapid Climate Change: Heinrich/Bolling-

Allerod Events and the Thermohaline

Circulation

By: Andy Lesage

April 13, 2010

Atmos. 6030

Outline

Background

Heinrich Event I/Bolling-Allerod Transition (Liu et al.)

Thermohaline Circulation Sensitivity (Lorenzo et al.)

Conclusions

BackgroundThree major climate events

(Heinrich, Bolling-Allerod,

Younger Dryas), occurred

between 19k and 11k before

present.

(Liu et al. model covers 22k-14k)

Heinrich - very cold phase at end

of most recent glaciation period

Bolling-Allerod - rapid temperature

transition, warmer phase

Younger Dryas - less extreme

cold phase

Proxy records used to determine

multiple variables.

Timmerman and Menviel (2009)

Proxy SourcesSediment cores

Ice cores (N, Ar)

Reef corals (tropics)

Deep-sea corals (O, C)

http://www.arcticice.org/hotraxweb/arctic_basin/images/field_photos/ice_core2.jpg

http://www.mun.ca/geog/images/content/102-1812845044.jpg

Liu et al. Study

First study using a coupled atmosphere-ocean

general circulation model. (NCAR Atmospheric

Research Community Climate System Model)

Spans from Last Glacial Maximum (~20ka) to

Bolling-Allerod warming (~14ka).

Forcings added based on known observations

(insolation, greenhouse gases, ice sheets,

coastlines, meltwater flux over N. America)

Thermohaline Circulation

Surface water flows northward.

Cooler, denser (higher salinity)

water sinks at poles.

Upwelling at lower latitudes.

Fresh water reduces overturning

circulation.

http://www.liv.ac.uk/physocean/schematics/thc.gif

Heinrich Event

22ka-19ka model has

little temperature change

(mostly insolation effects)

17.5ka enters Heinrich

Event

Meltwater flux increased

in N. Atlantic and Gulf of

Mexico matching with

observations.

Results: increase in sea

level and a decrease in

North Atlantic deep water

production.

BA-Bolling-Allerod

H1-Heinrich Event I

insolation

meltwater

sea level rise red-model

data

gray-

observations

deep water production

Meltwater Flux19ka-17ka increase at rate consistent with

sea-level rise. (peak flux of 20m/ky)

Leads to decrease in overturning circulation.

Freshwater anomaly confined to upper

Atlantic then gets transported in upper

ocean, later into deep ocean by Bolling-

Allred (15ky).

Meltwater flux is then reduced under two

scenarios: 1) linear decrease to 0 at 14.2ky

2) sudden shut-off 14.67ky

Overturning circulation increases when

meltwater flux is reduced, peaks at ~19

sverdrups (1 sverdrup = 10e6 cubic

meters/second), ~6 sverdrups above glacial

level

Differs from previous studies which showed

a two stable layer system where a gradual

change in meltwater flux can induce sudden

dramatic warming.

BA-Bolling-Allerod

H1-Heinrich Event I

meltwater reduction

simulations

effect on overturning

Temperature

Evolution

Heinrich Event - North Atlantic cools

dramatically, Southern Hemisphere

little change.

Caused by decrease in northward heat

transport of overturning circulation.

Bolling-Allerod - warming globally,

especially North Atlantic (up to 20C)

~15C over Greenland, 5C from

overturning circulation recovery from

glacial state, 10C from CO2 warming

and overturning overshoot.

Heinrich -

Glacial Surface

Temperature

Change

Bolling-Allerod

- Heinrich

Bolling-Allerod

- Glacial

Temperature

EvolutionClose relation between model results

and ice core data for Greenland and

Antarctica.

Also, close relation between model

results and Iberian Margin (off of

portugal) and Cariaco Basin (off of

Venezuela)

BA-Bolling-Allerod

H1-Heinrich Event I

Greenland ice core

Antarctic ice core

red/blue-model data

gray-observations

Atlantic Meridional Overturning

Circulation

Graphs for the gradual drop in

meltwater flux scenario

Strength of overturning in

Sverdrups

Heinrich event corresponds with

decrease in overturning.

Bolling-Allerod corresponds with

an increase in overturning.

Glacial temperature Heinrich - glacial Bolling-Allerod - glacial

Thermohaline Circulation

Sensitivity (Lorenzo et al.)

Thermohaline circulation has had a large effect in past, may be useful for future

projections.

Lorenzo et al. studied decadal and multi-decadal variability. (Atmospheric-

Ocean General Circulation Models) Resolution = few hundred km.

Stochastic forcing (random noise) inserted.

Other studies have shown, including the IPCC report, a slowing of the

thermohaline circulation under global warming and increased likelihood of

abrupt change.

Model Design

3-D coupled atmospheric-ocean-sea ice model.

Atmospheric: 3-level quasi-geostrophic model, parameterized diabatic

processes.

Ocean: sophisticated vertical mixing parameterization

Sea ice: sensible/latent heat storage in snow/ice, changes in snow/ice

thickness.

Random fluctuations added in freshwater flux (Greenland).

Results - Thermohaline

CirculationModel run in steady state, near collapse of THC with a .8Sv forcing in

Greenland. (approximately that of deglaciation period).

Overturning stream-function fell from 30 to ~2Sv, North Atlantic cooling.

Multi-decadal freshwater forcings lead to dramatic changes in THC strength.

Thermohaline circulation with

continuum discharge of .8Sv in

Greenland-Iceland-Norwegian Sea

decadal

variability

multi-decadal

variability

no freshwater

forcing

Results - Surface Temperature

Temperature changes

most dramatic in North

Atlantic.

Cooling around

Greenland except

northeast (Greenland

Sea). Warming is result

of increased surface

heat flux.decadal variability

in surface temp.

multi-decadal (70 yr)

variability

in surface temp.

Results - Other changes

500hPa stream function (polar jet) weakened due to thermohaline

circulation weakening.

Decrease in precipitation and evaporation over Northern Hemisphere,

particularly ~60 degrees North (-2cm/yr)

ITCZ shifts, leads to change in rainfall near equator.

SST increases or near zero change globally except in North Atlantic.

Salinity decreases significantly near western Greenland.

Conclusions

Atlantic meridional overturning circulation can be connected to rapid climate

change in the recent past (geologic scales).

2 stable state pattern may not be a feature of overturning circulation.

Reconstructions of meltwater flux before Bolling-Allerod would improve results.

Decadal or multidecadal signals (rapid melting of Greenland ice via global

warming could qualify) can induce weakening of the thermohaline circulation

and lead to changes in global climate.

Model improvement (resolution of smaller scales) necessary for greater

accuracy of future studies.

Sources

Liu et al. (2009), Transient Simulation of Last Deglaciation with a New Mechanism for Bolling-Allerod

Warming. Science, Vol. 325. no. 5938, pp. 310 - 314. DOI: 10.1126/science.1171041

Lorenzo et al. (2009). Sensitivity of thermohaline circulation to decadal and multidecadal variability. ICES

Journal of Marine Science, Vol. 66: pp. 1439–1447.

Severinghaus and Brook (1999), Abrupt Climate Change at the End of the Last Glacial Period Inferred

from Trapped Air in Polar Ice. Science, Vol. 286. no. 5441, pp. 930 - 934. DOI:

10.1126/science.286.5441.930

Smith et al. (1997), Rapid climate change in the North Atlantic during the Younger Dryas recorded by

deep-sea corals. Nature, 386, 818 - 820 (24 April 1997); doi:10.1038/386818a0

Timmermann and Menviel (2009), What Drives Climate Flip-Flops? Science, 325, 273 Science Vol. 325.

no. 5938, pp. 273 - 274. DOI: 10.1126/science.1177159

Extra Figures - Stream-function

Extra Figures - Precipitation

Extra Figures - SST/Salinity