Ice-sheet modulation of deglacial North American monsoon intensification


Tripti Bhattacharya
Jessica E. Tierney
Jason A. Addison
James W. Murray


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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a, Core sites (diamonds) and core-top data (circles) with leaf-wax-inferred δD of precipitation (lower colour scale). The coloured contours show the climatological percentage of JAS (monsoon season) rainfall45b, Bayesian regression between core-top leaf-wax-inferred δDprecip and the percentage of JAS rainfall. The shaded envelope represents the 95% confidence interval, the central line is the mean regression relationship.

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