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Synoptic and mesoscale controls on the isotopic composition of precipitation in the western United States |
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| Authors: | M Berkelhammer L Stott K Yoshimura K Johnson A Sinha |
| Institution: | 1. Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA 2. Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA 3. Center for Climate System Research, University of Tokyo, Tokyo, Japan 6. Scripps Institute of Oceanography, University of California, San Diego La Jolla, CA, USA 4. Earth Systems Science, University of California, Irvine, CA, USA 5. Department of Earth Sciences, California State University, Dominguez Hills, CA, USA |
| Abstract: | We present a new event-scale catalog of stable isotopic measurements from 5?years of storm events at 4 sites in southern California, which is used to understand the storm to storm controls on the isotopic composition of precipitation and validate the event-scale performance of an isotope-enabled GCM simulation (IsoGSM) (Yoshimura et?al. 2008). These analyses are motivated to improve the interpretation of proxy records from this region and provide guidance in testing the skill of GCMs in reproducing the hydrological variability in the western US. We find that approximately 40% of event-scale isotopic variability arises from the percentage of precipitation that is convective and the near surface relative humidity in the days prior to the storms landfall. The additional isotopic variability arises from the fact that storms arriving from different source regions advect moisture of distinct isotopic compositions. We show using both field correlation and Lagrangian trajectory analysis that the advection of subtropical and tropical moisture is important in producing the most isotopically enriched precipitation. The isotopic catalog is then used along with satellite-derived δD retrievals of atmospheric moisture to benchmark the performance of the IsoGSM model for the western US. The model is able to successfully replicate the observed isotopic variability suggesting that it is closely reproducing the moisture transport and storm track dynamics that drive the large storm-to-storm isotopic range. Notably, we find that an increase in moisture flux from the central tropical Pacific leads to a convergence of isotopically enriched water vapor in the subtropics and consequently an increase in δ18O of precipitation at sites along the entire west coast. Changes in poleward moisture flux from the central Tropical Pacific have important implications for both the global hydrological cycle and regional precipitation amounts and we suggest such changes can be captured through instrumental and proxy-reconstruction of the spatiotemporal isotopic patterns in the precipitation along the west coast of the US. |
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