Steering of westerly storms over western North America at the Last Glacial Maximum


The hydroclimate history of North America includes the formation and desiccation of large inland lakes and the growth and ablation of glaciers throughout the Quaternary period. At the Last Glacial Maximum, expanded pluvial lakes in the south1,2 and aridity in the northwest3 suggest that the winter westerly storm track was displaced southwards and migrated northwards as the Laurentide Ice Sheet waned4. However, lake highstands do not occur synchronously along zonal bands5, in conflict with this hypothesis. Here we compile a network of precipitation proxy reconstructions from lakes, speleothems, groundwater deposits, packrat middens and glaciers from the western and southwestern US, which we compare with an ensemble of climate simulations to identify the controls of regional hydroclimatic change. The proxy records suggest a precipitation dipole during the Last Glacial Maximum, with wetter than modern conditions in the southwest and drier conditions near the ice sheet, and a northwest–southeast trending transition zone across the northern Great Basin. The models that simulate a weaker and south-shifted Aleutian low-pressure system, a strong North Pacific high-pressure system, and a high above the ice sheet best reproduce this regional variation. We therefore conclude that rather than a uniformly south-shifted storm track, a stronger jet that is squeezed and steered across the continent by high-pressure systems best explains the observed regional hydroclimate patterns of the Last Glacial Maximum.

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Figure 1: Modern climatology, LGM boundary conditions, and proxy network by type and LGM precipitation category.
Figure 2: Proxy-model comparison for models with highest κw.
Figure 3: Simulated LGM atmospheric circulation changes.


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We appreciate constructive comments from N. Diffenbaugh and D. Horton. We gratefully acknowledge the international climate modelling groups participating in PMIP2 and CMIP5/PMIP3 for producing and making their model output available for analysis, and the PMIP2 Data Archive (supported by CEA, CNRS, and PNEDC) and PCMDI (supported by the US Department of Energy) for distributing the data. PMIP databases were last accessed in September 2014. D.E.I. and M.J.W. acknowledge support from C. P. Chamberlain. This work was supported by the National Science Foundation grants to J.L.O. (AGS1203701) and K.M. (EAR0921134).

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D.E.I. and J.L.O. conceived the project, compiled the proxy data, and conducted the statistical analyses. D.E.I. and M.J.W. analysed the model output. J.L.O. and D.E.I. synthesized the results and wrote the manuscript with contributions from all authors.

Correspondence to Jessica L. Oster.

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Oster, J., Ibarra, D., Winnick, M. et al. Steering of westerly storms over western North America at the Last Glacial Maximum. Nature Geosci 8, 201–205 (2015) doi:10.1038/ngeo2365

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