Isotopes in Precipitation in Northwestern North America

Abstract

Regional analyses of oxygen and deuterium isotope abundances in precipitation from selected stations of the International Atomic Energy Agency-World Meteorological Organization global netwok are presented as background for a more detailed study of the origin and history of atmospheric water in western Canada east of the Rocky Mountains. Departures from Dansgaard's regression between oxygen-18 concentrations and surface temperatures are attributed mainly to differences in intial water vapor isotope concentrations at inland stations in western Canada. Values of δO18 in rain at Edmonton are best correlated with 800 mb temperatures. However, snow data showed little variation in correlation with height up to 800 mb, and larger unexplained variance than rain, despite the fact that evaporation and isotope exchange effects are small for snow. Using simultaneous upper air temperature and wind data the δO18 variations in snow are attributed both to large condensation temperature variations that can occur in winter storms, and to the condensation of water vapor of different origin and history. Isotope concentrations characteristic of precipitation derived from re-evaporated water are associated with strong westerly flow below cloud level. On the other hand isotope concentrations characteristic of coastal station precipitation are associated with easterly low-level flow or light winds. The magnitude of the differences in concentrations in the two situations is so large that isotope measurements should be useful in the study of the structure of such storms. Previous studies have attributed the small slopes of δO18-δD regression lines at Fort Smith, Bethel, and Whitehorse to the effects of rapid evaporation of precipitation below cloud base. An alternative approach using a fixed slope of 8 and varying intercepts suggests that the observed isotope concentrations can be accounted for by variations from near-equilibrium evaporation of North Pacific Ocean water in summer to rapid evaporation of the ocean water in winter. This explanation appears to be more consistent with the climate characteristics of these stations.