Abrupt onset and termination of
the African Humid Period:
Rapid climate response to gradual
insolation forcing
Abstract
A detailed (ca. 100 yr resolution) and well-dated (31 AMS 14C dates to
24 cal. ka BP) record of latest Pleistocene-Holocene variations
in terrigenous (eolian) sediment deposition at ODP Site 658C off
Cap Blanc, Mauritania documents very abrupt, large-scale changes
in subtropical North African climate. The terrigenous record exhibits
a well-defined period of low influx between 14.8-5.5 cal. ka BP
associated with the African Humid Period, when the Sahara was
nearly completely vegetated and supported numerous perennial lakes;
an arid interval corresponding to the Younger Dryas Chronozone
punctuates this humid period. The African Humid Period has been
attributed to a strengthening of the African monsoon due to gradual
orbital increases in summer season insolation. However, the onset
and termination of this humid period were very abrupt, occurring
within decades to centuries. Both transitions occurred when summer
season insolation crossed a nearly identical threshold value,
which was 4.2% greater than present. These abrupt climate responses
to gradual insolation forcing require strongly non-linear feedback
processes, and current coupled climate model studies invoke vegetation
and ocean temperature feedbacks as candidate mechanisms for the
non-linear climate sensitivity. The African monsoon climate system
is thus a low-latitude corollary to the bi-stable behavior of
high-latitude deep ocean thermohaline circulation, which is similarly
capable of rapid and large-amplitude climate transitions.
Reference:
deMenocal, P.B., J. Ortiz, T. Guilderson, J. Adkins, M. Sarnthein,
L. Baker, and M. Yarusinsky, Abrupt onset and termination of the
African Humid Period: Rapid climate responses to gradual insolation
forcing, Quat. Sci. Rev., 19, 347-361, 2000. (PDF)
| A SeaWIFS image of a
summer eolian dust plume event emanating from northwest Africa.
The Sahara and Sahel regions of NW Africa are largest sources
of mineral aerosol, supplying nearly 500 million tons of dust
to the ocean annuallly. During the summer months, turbulent easterly
waves entrain fine particles from the Sahara and Sahel regions
which are convectively lifted to mid-tropospheric levels and
transported westward by the African Easterly Jet. Geochemical
and mineralogical studies of marine sediments off west Africa
indicate that this eolian vector is the dominant supply of terrigenous
sediment to eastern Atlantic sediments. ODP Site 658 (2200 m)
is located off the Mauritanian coast, just south of the lower
left of the image. |
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Site Location: ODP Hole 658C off
West Africa (20°N, 18°W, 2200m)
- Year-round upwelling occurs off West Africa
due to coastal divergence resulting from the NE wind field.
- The site is also positioned directly below
the mid-troposphere summer African dust plume.
- High accumulation rates (22 cm/ka) result
from high biogenic and terrigenous sediment fluxes.
- Sea-surface temperature seasonality is
high here (18-24°C; 21°C mean) due to the winter penetration
of the southward Canary Current and summer advance of warmer
tropical waters.
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Age
Control
- Age control was established with 31 AMS
radiocarbon dates on G. bulloides and G. inflata
to 24 cal. ka BP. (R=-500 yrs).
- Sampling interval was 2 cm or ~50-150
years.
- Terrigenous (eolian) percent calculated
as residual after measurement of biogenic carbonate and opal
percentages.
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Abrupt Onset and Termination of the African
Humid Period:
Non-Linear Climate Response to Gradual Insolation Forcing
- The terrigenous percent and flux records
document the "African Humid Period" when the now-hyperarid
Sahara Desert was fully vegetated and dotted with numerous perennial
lakes (Jolly et al., 1998).
- The African Humid Period has been attributed
to orbital increases in boreal summer insolation which strengthened
the West African monsoon (Kutzbach, 1981).
- The transitions into and out of this wet
phase were extremely abrupt, having been completed within centuries
(deMenocal et al., 2000).
- Both transitions occurred when summer
insolation was 4.2% greater than today.
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Monsoon Sensitivity to Insolation Forcing:
Climate Model-Data Comparisons
- Claussen et al. (1999) examined the sensitivity
of a fully coupled ocean-atmosphere-ocean model to prescribed
gradual changes in summer insolation from 9-0 ka (Panel A).
- They observed abrupt West African monsoon
responses to gradual insolation forcing (Panel B).
- The non-linear response was attributed
to the vegetation-albedo feedback, where initial rainfall decreases
led to reduced vegetation cover, which further reduced rainfall
(Panel C).
- The abrupt response was not observed when
the vegetation feedback was disabled.
- The abrupt response was centered at 5.5±0.1
ka for ten experiments.
- The Hole 658C terrigenous percentage and
flux record suggests a similar timing and rate-of-change (Panel
D; deMenocal et al., 2000).
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Non-Linear Monsoonal Climate Dynamics -
Figure
- Prell and Kutzbach (1987) observed that
monsoonal climate response to orbital insolation forcing was
linear using an atmosphere-only GCM.
- The Hole 658C paleoclimate data and the
Claussen et al. (1999) coupled model results suggest a highly
non-linear response.
- The West African monsoon climate system
is thus a low-latitude corollary to the bi-stable behavior of
high-latitude deep ocean thermohaline circulation, which is similarly
capable of rapid and large-amplitude climate transitions.
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