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Ice-sheet modulation of deglacial North American monsoon intensification


The North American monsoon, the dominant source of rainfall for much of the arid US Southwest, remains one of the least understood monsoon systems. The late Pleistocene evolution of this monsoon is poorly constrained, largely because glacial changes in winter rainfall obscure summer monsoon signatures in many regional proxy records. Here, we develop deglacial records of monsoon strength from isotopic analyses of leaf wax biomarkers in marine sediment cores. Reconstructions indicate a regional decrease in monsoon rainfall during the Last Glacial Maximum, and that the deglacial trajectory of the North American monsoon closely tracks changes in North American ice cover. In climate model simulations, North American ice cover shifts the westerlies southwards, favouring the mixing of cold, dry air into the US Southwest. This process, known as ventilation, weakens the monsoon by diluting the energy fluxes required for convection. As the ice sheet retreats northwards, the monsoon strengthens, and local ocean conditions may play a larger role in regulating its intensity. We conclude that on glacial–interglacial timescales, ice-sheet-induced reorganizations of atmospheric circulation have a dominant influence on the North American monsoon.

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Fig. 1: Relationship between leaf-wax-inferred δD and monsoon rainfall.
Fig. 2: Leaf-wax-based reconstructions of JAS rainfall at the Guaymas Basin and Mexican Margin sites.
Fig. 3: Sensitivity of monsoon reconstructions to winter rainfall changes.
Fig. 4: Atmospheric and oceanic drivers of glacial NAM changes.


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Support for this research comes from the David and Lucile Packard Foundation Fellowship in Science and Engineering to J.E.T. and NSF grant OCE-1651034 to J.E.T. We thank A. Orchard and the junior docents at the Arizona-Sonora Desert Museum for assistance with leaf collection and J. Case-Gonzalez, N. Montiel, P. Murphy and P. Zander for assistance with the preparation and analysis of the Sonoran desert plant samples. Access to core samples and coretops was facilitated by BOSCORF, the Geological and Oceanographic Collections at the Scripps Institution of Oceanography, and the Marine and Geology Repository at OSU (NSF OCE-1558679). We thank P. Valdes at University of Bristol for providing access to the HadCM3 timeslice simulations. We thank J. Barron and S. Praetorius (USGS) for their guidance with age models and insights on the Gulf of California. J.A.A. is supported by the USGS Climate Research and Development Program. On publication, the proxy records contained in this data will be archived in the NOAA Paleoclimatology Database.

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T.B. and J.E.T. designed the study, collected the data and performed the data analysis. J.A.A. and J.W.M. provided access to samples and assisted in data collection. T.B. and J.E.T. wrote the manuscript.

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Correspondence to Tripti Bhattacharya.

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Bhattacharya, T., Tierney, J.E., Addison, J.A. et al. Ice-sheet modulation of deglacial North American monsoon intensification. Nature Geosci 11, 848–852 (2018).

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