Using deuterium excess to track annual and interannual contribution of tropical moisture to the west coast of the United States

Jessica Zaiss1, Lowell Stott2 and Nikolaus Buenning3

As global temperatures rise, water stressed regions along the western coast of the United States could become increasingly drier due to projected northward shifts in storm trajectories over the 21st century. Assessing the variations in moisture source and delivery that could accompany these atmospheric changes will greatly benefit future water management practices. To determine if shifts in moisture transport can be detected in isotopic tracers, we are continuously measuring the isotopic composition of precipitation collected at 18 NADP sites along the west coast of the United States. Sites with longer records offer continuous sampling as far back as 2001. Oxygen and hydrogen isotopes in precipitation provide a record of variability within the hydrological cycle at all spatial and temporal scales via fractionation effects. Additionally, a second order variable, deuterium excess (d-excess = δD – 8*δ18O), records changes in the source of the water vapor. Results from these measurements show no significant trends in δD or δ18O over the past 15 years. However, deuterium excess values do vary systematically on both seasonal and interannual time scales. The seasonal cycle consists of higher deuterium excess values in the winter and lower deuterium excess values in the summer. Interannual variability is expressed as an increase in deuterium excess values over approximately four water years followed by a marked decrease for one water year. Because moisture originating in the tropics carries an isotopically heavier signal than does extratropical moisture, we hypothesize that both the annual and interannual variability described here is indicative of changing proportions in the amount of tropically sourced moisture. Interannual variations in meridional wind patterns are consistent with this hypothesis.

 

1University of Southern California , zaissbow@usc.edu
2University of Southern California, stott@usc.edu
3University of Southern California, buenning@usc.edu