Four-Season and Monthly Tree-Ring Reconstructions for More Complete Records of Important Precipitation Events in U.S. West Coast Watersheds

Abstract:

Tree-ring records from western North America have been used to reconstruct hydroclimate over a variety of time steps, most commonly the October-September water year, a six-month cool season, or a seasonal index such as the June-July-August Palmer Drought Severity Index. These reconstructions are robust for those time steps, but they can have muted or absent climate signals in key parts of the year such as the autumn, when atmospheric rivers begin bringing important precipitation to the region. Such seasonal biases may be the source of discrepancies between dendroclimate reconstructions and those derived from historical and other proxy records. In this study, precipitation was reconstructed in 28 major river basins in Washington, Oregon, and California using point-by-point regression over multiple time slices: annual, cool/warm seasons, four meteorological seasons, and monthly. All 28 basins had successful four-season reconstructions, and 27 could be reconstructed monthly. Results show that the four season and monthly reconstructions better capture dry extremes in 71% of the basins and wet extremes in 60% of the basins. Model residuals were larger in the annual and cool/warm season models when atmospheric rivers were prevalent, particularly in the northern coastal region. These results have the potential to more clearly resolve patterns of past moisture delivery over multiple seasons and to support comparisons with historical climate information and other proxy records.