Climate Dynamics of Tropical Africa: Capturing Paleoclimate Changes Bette Otto-Bliesner National Center for Atmospheric Research, Boulder, Colorado Zhang et al., GMD, 2012 CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] The Sahel Since 1950, the Sahel has seen persistent anomalies of precipitation: • 1950s – 1960s, persistent excessive rainfall • 1970s – 1980s, excessive drought Partial recovery, rise in extreme events Satellite observations indicate a strong increase in seasonal greenness since the early 1980s. Proposed cause are numerous, including: • Improved agricultural practices • Increasing GHGs, direct and indirect (SST) effects on Sahel rainfall. CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Past and Future − Data and Models CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Data − Glacial-Interglacial Measurements: siliciclastic grain size variations in a deep-sea core off western Africa and alkenone measurements from two sediment cores near Portuguese margin Interpretation: African “humidity index” displays eccentricity-modulated precessional cycles (~20 kyr), with millennial variations correlated with North Atlantic SSTs deMenocal, Science, 2008 CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Models − Glacial-Interglacial: Orbital Forcing CLIMBER2 AOV Model Simulation Sahara box Interpretation: Sahel box Recurrence of humid periods when precession-forced orbital monsoon forcing is strong. Wet Greening of Sahel and Sahara associated with precipitation Dry increases. Tjallingii et al., Science, 2008 CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Models − Glacial-Interglacial: Millennial Events Holocene LGM AHP Measurements: grain size and elemental ratios in marine core GeoB9508-5, on continental slope just west of Senegal River mouth. Interpretation: Data: Strong decreases in Al/Si and Fe/K ratios during Heinrich stadials are associated with much lower sediment discharge from Senegal River è arid West African Sahel. Climate model: Cold North Atlantic during meltwater events induces a CCSM2 AO “Hosing” Simulation southward shift of West African P-E (m/yr) monsoon trough and mid- tropospheric African Easterly Jet. Mulitza et al., Paleoceanography, 2008 CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Last 21ka – Deglaciation and Holocene Abrupt, synchronous initiation of the African Humid Period across Africa? African The DATA Humid Period Street-Perrott & Perrott, 1993 deMenocal et al., 2000; Gasse, 2000; Tierney et al. 2008 CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Role of Climate Forcings TraCE (Transient Climate Evolution) Project to simulate 21ka to present AHP Community Climate System Model 3 • Atmosphere model, 3.75° lat-lon • Land model with predicted vegetation • Ocean model, ~3°, 25 levels • Sea ice model, dynamic & 20°N JJA thermodynamic 10°S DJF Three simulations Ice sheets: ICE-5G • TraCE – all forcings: GHG, orbital, NH meltwater SH meltwater ice sheets, and meltwater • TraCE – orbital-only* • TraCE – GHG-only* * All else held at 17ka Otto-Bliesner et al., Science, 2014 CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Figure 1, x27 picas, Otto-Bliesner/Science TraCE simulation versus Proxy Data 22 21 20 19 18 17 16 15 14 13 12 11 18 A AHP 0.05 14 0.06 Th 230 10 0.07 / AMOC (Sv) Atlantic ocean Pa 6 0.08 overturning circulation 231 2 21 ka 0.09 0.3 600 B −0.1 400 −0.5 NA Precip (mm/year) −0.9 200 Humidity Index 4 40 3 C 45 TLAI 2 50 Proxy data 55 1 Terrigenous NA TLAI 0 60 1 TraCE simulation 1200 −130 D 0.8 −110 x wa 0.6 1000 −90 D δ 0.4 BIT index −70 SEA Precip (mm/year) 10 ka 0.2 800 2200 −160 Data and TraCE simulation 2100 E −150 • Relatively dry at LGM (~20ka) x −140 wa 2000 • Even drier from ~18 to 15ka D δ −130 1900 • Abrupt increase in moisture at ~14.7ka Congo basin Precip (mm/year) −120 22 21 20 19 18 17 16 15 14 13 12 11 Age (ka) OttoΔ SST-Bliesner et al., Science, 2014 CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Figure 2, x27 picas, Otto-Bliesner/Science A EOF1, proxy data PC1, proxy data Figure 2, x27 picas, Otto-Bliesner/SciencePC amplitude Figure 2, x27 picas, Otto-Bliesner/Science B EOF1, TraCE PC1, TraCE Dominant Pattern of Hydroclimate Variability Figure 2, x27 picas, Otto-Bliesner/Science EOF1, proxy data PC amplitude PC1, proxy data A A EOF1, proxy data PC1, proxy data 21 ka C EOF2, proxy data PC2, proxy data EOF1, proxy data PC1, proxy data A PC amplitude PC amplitude EOF1, TraCE PC1, TraCE B PanPC amplitude -African coherent changes PC amplitude TLAI • Dry from LGM (20ka) to 17ka • Increase in moisture from 17ka to 11ka B EOF1, TraCE PC1, TraCE • Some drying at Younger Dryas period (~12ka) D EOF2, TraCE PC2, TraCE EOF1, TraCE PC1, TraCE B PC amplitude 10 ka C EOF2, proxy data PC2, proxy data PC amplitude PC amplitude PC amplitude Age (ka) EOF2, proxy data PC2, proxy data Δ SST C PC amplitude Otto-Bliesner et al., Science, 2014 EOF2, proxy data PC2, proxy data C CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] D EOF2, TraCE PC2, TraCE PC amplitude PC amplitude PC2, TraCE D EOF2, TraCE PC amplitude Age (ka) D EOF2, TraCE PC2, TraCE PC amplitude Age (ka) PC amplitude Age (ka) Figure 3, x28p6 picas, Otoo-Bliesner/Science Sensitivity to GHG and Orbital Forcings Deglacial change (11ka minus 17ka) of A D model precipitation Monsoon nature of rainfall: • North Africa 21– NH summer months ka • SE equatorial Africa – SH summer months TraCE simulation with all forcings shows B E summer rainfall increases from 17ka to 11ka in both regions. TLAI Only orbital forcing: • Sahel: Increased summer rainfall plus northward expansion. • SE equatorial Africa: Decreased summer rainfall. C F Only GHG forcing: • Sahel: Increased summer rainfall. • SE equatorial Africa: Increased summer rainfall. Otto-Bliesner et al., Science, 2014 Δ SST CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Figure 3, x28p6 picas, Otoo-Bliesner/Science Sensitivity to GHG and Orbital Forcings A D 21 ka B E Annual Annual TraCE – all TLAI forcings GHG only C F 10 ka Orbital only OttoΔ SST-Bliesner et al., Science, 2014 CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Figure 2, x27 picas, Otto-Bliesner/Science A EOF1, proxy data PC1, proxy data Figure 2, x27 picas, Otto-Bliesner/Science Figure 2, x27 picas, Otto-Bliesner/Science PC amplitude A EOF1, proxy data PC1, proxy data A EOF1, proxy data PC1, proxy data B EOF1, TraCE PC1, TraCE PC amplitude PC amplitude B EOF1, TraCE PC1, TraCE PC amplitude B EOF1, TraCE PC1, TraCE Meltwater and North Atlantic InfluencePC amplitude C EOF2, proxy data PC2, proxy data PC amplitude C EOF2, proxy data PC2, proxy data 21 ka C EOF2, proxy data PC2, proxy data PC amplitude PC amplitude PC amplitude D EOF2, TraCE PC2, TraCE Meltwater to North Atlantic Slowdown of thermohaline circulation D EOF2, TraCE PC2, TraCE D EOF2, TraCE PC2, TraCE PC amplitude 10 ka PC amplitude Age (ka) PC amplitude Age (ka) Δ SSTOtto-Bliesner et al., Science, 2014 Age (ka) CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Meltwater and North Atlantic Influence LGM to Heinrich 1 change (17ka minus 20ka) • Adding meltwater to the North Atlantic, 21 ka cooling the SSTs, and slowing the ocean conveyor circulation, • With the ocean transporting less heat northward, the South Atlantic warms, • Shifting rainfall south, drying North Africa. TLAI • Ocean circulation takes too long to transmit to SE equatorial Africa, • Instead the cooling is transmitted westward over Europe, then southward 10 ka over the Indian Ocean by the atmosphere, • Shifting rainfall south, drying SE equatorial Africa. When meltwater to North Atlantic ceases, rainfall pattern abruptly flips back Δ SST CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] A Challenge: Green Sahara during the AHP Poorly-represented positive land- atmosphere feedbacks? 21 ka § Vegetation • (As opposed to desert) influences surface albedo, evapotranspiration. § Soil • Composition and color affect soil albedo and soil water retention. TLAI § Groundwater hydrology • Groundwater for recharging large lakes in mid- Holocene accumulated during Early Holocene. 10 ka § Dust • More vegetated land surface with greater soil moisture and enlarged lakes −> decreased dust production. IPCC AR5 WG1 report, Chapter 9, 2013 Δ SST CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected] Concluding Remarks • GHG changes, particularly atmospheric CO2, played an important role in the pan-African rainfall response during the deglaciation, • Onset of African Humid Period synchronized by state of Atlantic 21 ka Meridional Overturning Circulation. TLAI Precipitation scaled by global temperature (% per °C) 10 ka Δ SST IPCC AR5 WGI, 2013 CSDMS Annual Meeting 2016: Capturing Climate Change Bette Otto-Bliesner May 2016 [email protected].