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RATIONALE

Isotope (δ18O and δ2H values) ratios in precipitation have been central to understanding changes in Earth's climate as recorded in ice, coral, speleothems, lake varves and long-lived plants. Understanding how climate phases (i.e. ENSO) affect the spatial and temporal patterns of δ18O and δ2H values in precipitation has, however, been uncertain across the USA.

METHODS

A spatial precipitation isotope network (USNIP) has been established that aims to: (1) characterize the δ18O, δ2H and d-excess values in precipitation across the USA with the highest spatially dense network of measurements yet undertaken; (2) quantify the annual and seasonal patterns of precipitation δ18O and δ2H values that may be affected by ENSO climate phases; and (3) provide a new isotope database for scientific studies that can be incorporated into NEON, BASIN, GNIP, and IsoMAP.

RESULTS

On average, precipitation δ18O and δ2H values are very low in the northern Rocky Mountain region (~ −15‰ δ18O, and ~ −120‰ δ2H), and precipitation δ18O and δ2H values are relatively higher along the Gulf Coast (~ −5‰ δ18O and −10‰ δ2H) and in the Southeast. During El Niño periods the precipitation δ18O and δ2H values are lowest in northwest Montana, with precipitation that is depleted in 18O and 2H extending into northern Colorado, while moisture that is enriched in 18O and 2H continues to dominate the Gulf Coast. The annual average differences between the climate phases generally show especially depleted 18O and 2H in precipitation across the Rocky Mountain region during El Niño, compared with Neutral periods.

CONCLUSIONS

Detailed spatial and seasonal patterns of δ18O, δ2H and d-excess values provide fine-scale resolution not previously recognized. Climate phases of ENSO have major effects on the spatial patterns of δ18O, δ2H and d-excess values, being especially important on a seasonal basis in the Desert Southwest. Copyright © 2012 John Wiley & Sons, Ltd.