Letter

Nature 462, 637-641 (3 December 2009) | doi:10.1038/nature08520; Received 10 January 2009; Accepted 15 September 2009

Half-precessional dynamics of monsoon rainfall near the East African Equator

Dirk Verschuren1, Jaap S. Sinninghe Damsté2,3, Jasper Moernaut4, Iris Kristen5, Maarten Blaauw6, Maureen Fagot1, Gerald H. Haug7,8 & CHALLACEA project members

  1. Limnology Unit, Department of Biology, Ghent University, Ledeganckstraat 35, 9000 Gent, Belgium
  2. Faculty of Geosciences, Utrecht University, PO Box 80021, 3508 TA Utrecht, The Netherlands
  3. Department of Marine Organic Biogeochemistry, NIOZ Royal Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, The Netherlands
  4. Renard Centre of Marine Geology, Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, 9000 Gent, Belgium
  5. GeoForschungsZentrum Potsdam, Sektion 3.3 Klimadynamik und Sedimente, Telegrafenberg, D-14473 Potsdam, Germany
  6. School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, BT9 6AX Belfast, UK
  7. Geological Institute, Department of Earth Sciences, ETH Zürich, CH-8092 Zürich, Switzerland
  8. DFG Leibniz Center for Earth Surface Process and Climate Studies, Potsdam University, D-14476 Potsdam, Germany
  9. Institute for Biodiversity and Ecosystem Dynamics, Research Group Paleoecology and Landscape Ecology, Faculty of Science, Universiteit van Amsterdam, 1098 SM Amsterdam, The Netherlands.
  10. Renard Centre of Marine Geology, Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, 9000 Gent, Belgium.
  11. Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK.
  12. Atmospheric and Environmental Sciences, State University of New York, Albany, New York 12222, USA.
  13. GeoBiosphere Centre, Department of Geology, Lund University, SE-223 62 Lund, Sweden.
  14. Department of Geology, University of Nairobi, PO Box 30197, Nairobi, Kenya.
  15. Paleoecological Environmental Assessment and Research Laboratory, Department of Biology, Queen's University, Kingston, ON K7L 3N6, Canada.
  16. GeoForschungsZentrum Potsdam, Sektion 3.3 Klimadynamik und Sedimente, Telegrafenberg, D-14473 Potsdam, Germany.
  17. Limnology Unit, Department of Biology, Ghent University, Ledeganckstraat 35, 9000 Gent, Belgium.
  18. Faculty of Geosciences, Utrecht University, PO Box 80021, 3508 TA Utrecht, The Netherlands.
  19. Center for Isotope Research, Groningen University, 9747 AG Groningen, The Netherlands.
  20. Department of Palynology, National Museums of Kenya, Museum Hill, Nairobi, Kenya.
  21. Geological Sciences, Brown University, Providence, Rhode Island 02912, USA.
  22. Department of Geology, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium.
  23. Centre de Physique du Globe, Royal Meteorological Institute, 5670 Dourbes, Belgium.
  24. Climate System Research Center, Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003-9297, USA.
  25. NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK.
  26. Lists of participants and affiliations appear at the end of the paper.

Correspondence to: Dirk Verschuren1 Correspondence and requests for materials should be addressed to D.V. (Email: dirk.verschuren@UGent.be).

External climate forcings—such as long-term changes in solar insolation—generate different climate responses in tropical and high latitude regions1. Documenting the spatial and temporal variability of past climates is therefore critical for understanding how such forcings are translated into regional climate variability. In contrast to the data-rich middle and high latitudes, high-quality climate-proxy records from equatorial regions are relatively few2, 3, 4, especially from regions experiencing the bimodal seasonal rainfall distribution associated with twice-annual passage of the Intertropical Convergence Zone. Here we present a continuous and well-resolved climate-proxy record of hydrological variability during the past 25,000 years from equatorial East Africa. Our results, based on complementary evidence from seismic-reflection stratigraphy and organic biomarker molecules in the sediment record of Lake Challa near Mount Kilimanjaro, reveal that monsoon rainfall in this region varied at half-precessional (approx11,500-year) intervals in phase with orbitally controlled insolation forcing. The southeasterly and northeasterly monsoons that advect moisture from the western Indian Ocean were strengthened in alternation when the inter-hemispheric insolation gradient was at a maximum; dry conditions prevailed when neither monsoon was intensified and modest local March or September insolation weakened the rain season that followed. On sub-millennial timescales, the temporal pattern of hydrological change on the East African Equator bears clear high-northern-latitude signatures, but on the orbital timescale it mainly responded to low-latitude insolation forcing. Predominance of low-latitude climate processes in this monsoon region can be attributed to the low-latitude position of its continental regions of surface air flow convergence, and its relative isolation from the Atlantic Ocean, where prominent meridional overturning circulation more tightly couples low-latitude climate regimes to high-latitude boundary conditions.

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