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


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 (11,500-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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Figure 1: Location and regional climate of the study site.
Figure 2: Lake Challa climate-proxy data in context.

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This research was sponsored by funding agencies in Belgium (FWO-Vlaanderen), Denmark (Danish National Research Council), Germany (DFG) and The Netherlands (NWO) through the European Science Foundation (ESF) EuroCORES programme, EuroCLIMATE. Fieldwork was conducted with permission of the Kenyan Ministry of Education, Science and Technology (MOEST) and the Tanzania Commission for Science and Technology (COSTECH). We thank A. Alcantara, M. Kohler, S. Lauterbach, R. Niederreiter and S. Opitz for assistance with core collection and processing; D. Engstrom for 210Pb dating, A. Berger, D. Fleitmann, J. Kutzbach and Z. Liu for discussion, and E. Odada for support. J.M. acknowledges the support of the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen).

Author Contributions D.V. initiated and coordinated the research, and wrote the paper. J.S.S.D. was responsible for the production of the biomarker data, J.M. interpreted the seismic-reflection data, I.K. constructed the composite sediment sequence and depth scale, M.B. developed the sediment age model, and M.F. supervised core sampling and managed the project database. G.H.H. and all CHALLACEA members contributed their expertise to improving the discussed data sets and/or the manuscript.

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Correspondence to Dirk Verschuren.

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Verschuren, D., Sinninghe Damsté, J., Moernaut, J. et al. Half-precessional dynamics of monsoon rainfall near the East African Equator . Nature 462, 637–641 (2009).

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