California Precipitation Whiplash Events: A 450-Year Evaluation and Dynamic Relationships to Circulation and Ocean Conditions

A combination of paleoclimate reconstructions and instrumental data is used to identify strong year-to-year flips in California precipitation going back to 1571 CE, providing observations for a period over 3.5 times as long as the instrumental record. Such "whiplash" events have been a systematic characteristic of both statewide and regional precipitation throughout this period; they are linked to overall precipitation variability, but the linkage is not strong and varies temporally. Dry-to-wet flips [from below the 30th percentile in year (t) to above the 70th percentile in year (t+1)] occur more frequently than wet-to-dry flips [from above the 70th percentile in year (t) to below the 30th percentile in year (t+1)] across the state, exhibiting a systematic gradient of increasing dry-to-wet proportion from north to south. In the southern California coastal and interior desert regions this predominance is significant at the p < 0.05 level relative to an equal-probability binomial null model. "Grand flips" that entrain the entire state show the same bias - over 60% of these occurrences are dry-to-wet events, which is also significant at the p < 0.05 level. Current work explores the dynamical circulation and ocean conditions that are associated with these phenomena, based on long-term paleoclimate reconstructions of sea level and 200 hPa pressure and wind fields, along with Pacific Ocean sea surface temperatures. Related results are generally consistent with anomalous high pressure in the northeastern Pacific during extreme dry years in California and anomalous low pressure during extreme wet years. However, reconstructed western North America precipitation also indicates no-analog spatial patterns (relative to the instrumental period) associated with subtle differences within these pressure composites, suggesting the paleo-record will yield heretofore unevaluated complexes of circulation and ocean conditions associated with extreme precipitation switching in California. Potential multi-centennial-scale changes in oceanic and circulation conditions associated with a notable shift in overall precipitation variability starting circa 1800 CE are also evaluated.