Insights into the Moisture Budget for the Western US Using Isotopic Measurements from the NADP Sample Archive

Max Berkelhammer and Lowell Stott
Department of Earth Sciences
University of Southern California
Los Angeles, CA

The isotopic composition of precipitation (δ18O and δD) is a useful but complex tracer of both local meteorological conditions and broader synoptic and mesoscale climate. One of the impediments to a richer understanding of the multi-scale controls on the isotopic composition of precipitation has been the reliance on collection sites that are sparsely distributed and have only seasonal to monthly temporal resolution. Because the NADP sampling and archiving protocol has been shown to be adequate for preserving an unadulterated record of the isotopic composition of precipitation, a handful of researchers have begun to use these samples for the development of a dense highresolution (event-scale) network of the isotopic composition of precipitation for the United States. We discuss one facet of this effort, which has been in the development of an isotopic catalog for storms that struck the west coast of the United States from 2001-2010. This work is motivated by a demand to improve our understanding of the relationship between atmospheric circulation patterns and drought in the western United States. We show using a lagrangian analysis how the relative contributions of isotopically enriched moisture from the tropics and isotopically depleted waters from the high latitudes can explain the wide range of isotopic variability between storms. The results suggest that remote moisture sources provide a critical contribution to the annual precipitation budget and that through careful monitoring of the isotopic composition of precipitation from coastal sites, we can elucidate how dynamical climate behavior such as El Nino events or evolving changes in the mean latitude of the jet stream influence moisture convergence to the western United States. A related application of this dataset that will be discussed involves a unique approach to benchmarking the performance of Global Climate Models (GCMs), which have been fitted with numerical routines for isotope tracers. The ability for a GCM with isotope tracers to reproduce the empirical results from the isotopic network provides one of the most rigorous tests available to benchmark the skill of a climate model. We highlight specifically the robustness of the Experimental Climate Prediction Center's Global Spectral Model and suggest that similar benchmarking efforts are crucial to improving hydrological forecasts for the western United States.