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Seasonality of Precipitation Affects the Annual Isotopic (d 18 O) Values across the United States: A Sensitivity Study

R.W. Vachon, J.W.C. White
Institute of Arctic and Alpine Research, University of Colorado at Boulder

J.M. Welker
Environment and Natural Resources Institute & Biology Department, University of Alaska, Anchorage

E. Gutmann
CSES/CIRES, University of Colorado at Boulder

 

Year to year differences in the isotopes of precipitation (d18O) are typically attributed to corresponding interannual changes in mean annual temperature. However, changes in the seasonal distribution of precipitation are one of the main complicating factors in interpreting year to year variability of d18O in precipitation. In this study, we investigate the influence of changes in seasonal distribution of precipitation on annually averaged isotopes using 73 sites across the entire United States. Based on these isotope measurements, and using multi-decadal records of the variance in monthly precipitation amounts, we identify areas within the United States where records of d18O in precipitation are susceptible to changes in the seasonal distribution of precipitation. Our findings indicate that the annual d18O values of precipitation in the Northern Great Plains, the Rocky Mountains, and the Southwest are strongly influenced by changes in the seasonal distribution of precipitation. To the contrary, shifts in the seasonal distribution of precipitation have almost no effect on the annual d18O values in the Eastern United States and along the West Coast. For the three regions most affected by the changes in seasonal distribution of precipitation, seasonality effects could be misinterpreted as 1ºC to 3ºC temperature changes between years. These findings facilitate a more accurate interpretation of climate records recorded in ice cores, tree rings, lake varves, plant segments, corals and speleothems.