Sub-Seasonal Tree-Ring Reconstructions for More Comprehensive Climate Records in US West Coast Watersheds

Tree-ring records of preinstrumental hydroclimate, which contribute needed context for understanding recent drought and flood events, typically provide one value per year that represents the entire year or one particular season. This may introduce a seasonal bias to the records or omit seasonally variable moisture. Here, I use tree-ring records to reconstruct precipitation and runoff in 28 U.S. West Coast watersheds for running 1, 3, 6, and 12-month intervals. When compared on a yearly basis, the Monthly and Four-Season models have higher overall skill and better extreme capture in most basins than the Cool-Warm and Annual models. The Monthly and Four-Season versions also decrease model error in years with more intense precipitation and retain more variability in the preinstrumental period. Improved capture of year-round moisture can provide a more complete representation of the preinstrumental past and strengthen capacity to study shorter-duration and season-specific events like atmospheric rivers. Plain Language Summary Records from long-lived trees help us learn what the Earth's climate was like before we had thermometers and rain gauges. These records are useful for understanding how and why our current climate is different than it was in the past, information that can help us plan for the future. Most tree-ring records provide one value per year, which might miss precipitation that falls outside the season that most influences tree growth or that does not arrive in the same months each year. In this study, I used tree-ring widths to produce records of precipitation and runoff in watersheds along the U.S. West Coast over 1, 3, 6, and 12-month time periods for the past 200 years. I compared all four versions on a yearly basis and found that the 1 and 3-month versions replicated the instrumental data more closely than the 6 and 12-month records and more accurately identified the very wet and very dry years in most watersheds. This research will help us learn more about past climate patterns by better describing the entire year and by increasing our ability to study a particular season of interest. Key Points . New annual, seasonal, and monthly precipitation and runoff reconstructions provide year-round moisture records along the U.S. West Coast Monthly and Four-Season reconstructions increase model skill, extreme capture, and variability retained in most watersheds Higher temporal resolution can help characterize shorter-duration, season-specific events like atmospheric rivers in the paleoclimate past