African Monsoon Variability over Past Millennia

Peter B. deMenocal Jennifer Arbuszewski Jess AdkinsZhengyu Liu

Overview

The African monsoon and orbital forcing

The early Holocene African Humid Period

Climate model simulations:… mechanisms for abrupt change

Sahel Drought (1960s onward)

Giannini et al. (2003)

The African Monsoon and Tropical Atlantic Upwelling

Cool SSTsMaximum upwellingSteep thermocline slope

W. Ruddiman, “Earth's Climate Past and Future”

Orbital Precession and the Monsoon Kutzbach (1981)

W. Ruddiman, “Earth's Climate Past and Future”

Linear (?) monsoon sensitivity

Precessional monsoon variability

June Insol.(W/m2)

900 10500

50

100

150

200

250

Age(kyr)

18 20 22 24

Foram SST(°C)

20 40 60

F. profunda(%)

0 200 400 600

Melosira(#/g x105)

(McIntyre et al., 1989; Molfino and McIntyre, 1993; Pokras and Mix, 1985; Rossignol-Strick, 1983)

Tropical Upwelling

Med.Sapropels

African Climate

…persisted for millions of years!

Miocene Mediterranean Sapropels (Sicily; Frits Hilgen)

NW African dust export

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Site 658

African Humid Period

AHP

deMenocal et al., 2000

African Humid Period

Africa was much wetter between ca. 15-5 kyr BP.

Most of Saharan Desert vegetated, wetland fauna.

North Africa wet during early Holocene

Paleolake sediments in Mali (NW Africa)

Climate-Controlled Occupation of the Eastern Sahara

Kuper and Kroepelin, 2006

Faiyum

Mirimde

Badari

Nagada

Egyptian Neolithic(6.5-4.5 ka BP)

AHP: A pan-North African event

Liu et al., 2007

Claussen et al., 1999; deMenocal et al., 2000

Albedo-Vegetation feedback?

Fully coupled transient

O-A-Veg GCM

Liu et al. (QSR in press)

Climate Model insight

African climate very sensitive to orbital radiation changes.

Abrupt responses to gradual forcing due to some aspect of vegetation feedback.

Humid-arid transitions occurred within centuries.

SST influence uncertain but present in historical records.

Annual sea-surface temperature

Annual Evap - Precip

Meridional ITZ displacements:LGM water-hosing experiments

Change in surface temperature (°C)Change in P-E (m/yr)

Vellinga and Wood (2002)

LGM Collapse of zonal SST Gradient

SST gradients along transect

Fresh

Salty

Large and abruptDeglacial ITCZ Swings

Summary

Past African climate changes were large.

Moisture budget changes were abrupt.

Likely importance of vegetation feedbacks, perhaps SSTs as well.