Atmospheric circulation is reflected in precipitation isotope gradients over the conterminous United States

The stable isotopic (delta O-18 and delta H-2) composition of meteoric precipitation integrates information on the history of water fluxes to, from, and between air masses as they traverse the continents. The development of new methods relating isotopic data to water cycle processes will increase our ability to understand this system and its response to natural and anthropogenic forcing. Here we present an analysis of 2 years (1992-1993) of precipitation isotope data from 73 sites across the conterminous United States. We focus on patterns in the spatial precipitation isotope gradients (rate and direction of isotopic change in space), a metric which we suggest reflects two factors: (1) variation in the balance of rainout and land-surface recycling for air masses moving across the continent and (2) the spatial juxtaposition of air masses carrying moisture of differing origin and rainout history (e. g., Pacific versus Gulf of Mexico moisture). We demonstrate that the position of zones of particularly high precipitation isotope slopes correspond to the time-averaged position of air mass boundaries and to regions where prevailing moisture transport trajectories interact with orographic barriers. Differences in the location of high-slope zones between winter and summer seasons and between the same seasons in 1992 and 1993 can be related to differences in circulation and weather patterns. These results suggest new opportunities for the interpretation of precipitation, vapor, and paleoclimate water isotope data in the context of regional climate dynamics through spatial analysis.