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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