Inferring precipitation-anomaly gradients from tree rings

Long-term information on gradients in precipitation-anomaly over tens to hundreds of km is important to hydroclimatology for improved understanding of the spatiotemporal variability of moisture-delivery systems and runoff. Site-centered reconstructions of cool-season (Nov–Apr) precipitation at 36 Quercus douglasii tree-ring sites in the Central Valley of California, USA, are generated, regionalized, and evaluated for ability to track north–south gradients in precipitation-anomaly. Event series are constructed for overall-wet (W), overall-dry (D), wetter-to-north (W/D) and wetter-to-south (D/W) conditions, 1557–2001. Interesting features of the event series are clustering of W events in the 1780–1790s and a three-year run of D/W events in 1816–1818 (coincidentally following the eruption of Tambora in 1815). The most recent 25 years of the event series stand out for a high frequency of W and D events and low frequency of events associated with strong gradients in precipitation-anomaly. The five strongest W events in this period and seven of the nine W events since 1934 match El Nino years. Recent changes in the event series may be a Central-Valley footprint of a well-documented post-1976 change in the atmosphere-ocean climate system over the North Pacific. Similar studies may prove useful in other geographical areas where networks of tree-ring data sufficiently sensitive to precipitation are available.