Atmospheric circulation controls on precipitation isotope-climate relations in western Canada

摘要

delta O-18 and delta H-2 time-series of monthly composite precipitation (July 1975-June 1982) for three stations located in western Canada were examined to characterize the influence of atmospheric circulation on modern isotope-climate relations in the region. Spatially coherent trends in long-term isotope and temperature anomalies were evident among the three stations, with isotope and temperature anomalies showing the strongest correlations when weighted to reflect precipitation amount. Strong correlations were also found between unweighted isotope anomalies and the Pacific-North American (PNA) index, which is a key descriptor of air-mass circulation patterns across North America. Positive delta anomalies and variable temporal delta-temperature relations having relatively shallow slopes occur during periods of intensified meridional circulation (PNA+), especially during winter. Periods of stronger zonal circulation (PNA-), in contrast, are marked by negative delta anomalies and steeper delta-temperature relations, consistent with increased distillation and deepening of the isotope shadow in the lee of the Canadian Cordillera. Significant positive correlations with pressure-height and precipitable-water anomalies located over western Canada provide additional confirmation that seasonal and interannual variability in the strength of the PNA ridge-and-trough pattern profoundly influences the isotopic evolution of moisture reaching the interior. Analogous circulation-dependent shifts in the precipitation isotope-temperature relation in western Canada also occur over much longer timescales, as inferred from isotope palaeorecords in various natural archives.