The magnitude of heat and salt transfer between the Indian and Atlantic oceans through ‘Agulhas leakage’ is considered important for balancing the global thermohaline circulation1,2,3. Increases or reductions of this leakage lead to strengthening or weakening of the Atlantic meridional overturning and associated variation of North Atlantic Deep Water formation4,5,6. Here we show that modern Agulhas waters, which migrate into the south Atlantic Ocean in the form of an Agulhas ring, contain a characteristic assemblage of planktic foraminifera. We use this assemblage as a modern analogue to investigate the Agulhas leakage history over the past 550,000 years from a sediment record in the Cape basin. Our reconstruction indicates that Indian–Atlantic water exchange was highly variable: enhanced during present and past interglacials and largely reduced during glacial intervals. Coherent variability of Agulhas leakage with northern summer insolation suggests a teleconnection to the monsoon system. The onset of increased Agulhas leakage during late glacial conditions took place when glacial ice volume was maximal, suggesting a crucial role for Agulhas leakage in glacial terminations, timing of interhemispheric climate change7 and the resulting resumption of the Atlantic meridional overturning circulation6.
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The MARE/CLIVARNET project was sponsored by two NWO grants (bilateral cooperation programme NWO-DFG). R.A. acknowledges funding from a NERC grant. R.S. is grateful to the German Science Foundation (DFG) and the IMAGES programme for sponsoring core retrieval and analytical work. We thank M. Segl and P. Mueller for measuring isotopes and alkenones, respectively. We thank the captain and crew of R/V Pelagia and R/V Agulhas as well as the Royal NIOZ technicians. We thank all MARE team members for discussion. This is a publication of the Netherlands Research School of Sedimentary Geology (NSG).
Depth–age relationship for cores GeoB-3603-2 and IMAGES II MD96-2081.
The δ18O of the benthic foraminifer Cibicides wuellerstorfi (red line) of the Cape Basin record after age tuning to the SPECMAP curve.
The planktic foraminifera fragmentation index of the Cape Basin record.
Location of plankton tow stations.
The integrated standing stock per species for nine plankton tow stations.
Blackman–Tukey spectral cross-correlation diagrams.
About this article
Climate Dynamics (2017)