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This dissertation explores the temporal evolution and drivers of seasonal precipitation across the Western United States, with the purpose of enabling improved water management decisions through the application of novel research. This goal is accomplished by 1) identifying the recurrence interval and climatological context of a California seasonal precipitation phenomenon named ‘Miracle March’, 2) exploring the conditional predictability of spring precipitation in Northern California and costal Oregon based on the phase of the Pacific Decadal Oscillation (PDO) and the anomaly of the preceding winter’s precipitation, and 3) developing a 625-year tree-ring reconstruction of wet season precipitation in the Transverse Ranges of Southern California, above the heavily populated Los Angeles metropolitan area (LA).
The term ‘Miracle March’ entered the California water lexicon in 1991. Since then, it has frequently been used as a basis for hoping that the month of March, which is at the tail end of the wet season, might be able to recover a winter precipitation deficit and alleviate drought conditions. However, there has been a lingering question about how well founded this “hope” is in scientific reality. Through a climatological analysis of the instrumental record, it was demonstrated that the 1991 ‘Miracle March’ was unprecedented and should not be considered as a likely reality when making water management decisions during a dry winter. In addition, the storm type that initiated the transition to wetter conditions was an extreme late season Atmospheric River that made landfall on February 27th, which, had it happened just 1-2 days earlier, would have greatly reduced the March anomaly, and the emergence of the ‘Miracle March’ concept.
Chapter 2 extends work on this critical spring precipitation season by exploring the degree to which winter precipitation anomalies (October-February) presage spring precipitation anomalies (March-May) in the Western United States. The potential for this inter-seasonal variability to be influenced by broader interannual climate variability is also explored. A moving window correlation analysis reveals the emergence of regions with extended periods where winter-to-spring seasonal precipitation persistence (WSPP) is more common, and intervening periods where the seasonal precipitation anomalies tend to be in opposite phases. Notably, Northern California and coastal Oregon exhibit a large statistically significant coherence between WSPP and phases of the PDO. The potential for conditionally forecasting spring precipitation based on PDO phases and preceding winter precipitation conditions based on this research, could aid in water resource governance just prior to entering the dry summer season.
The final chapter shifts from the present to the past 625 years to investigate a wider envelope of wet season, October-April, precipitation variability inherent in the climate system. A collection of 13 tree-ring chronologies, based on hundreds of trees, and regional precipitation totals were used to reconstruct wet season precipitation totals from 1400-2024 in the Transverse Ranges above LA. The tree-ring chronologies are highly correlated with instrumental wet season precipitation totals (r = 0.882, 1952-2014) and atmospheric river precipitation (r = 0.823, 1952-2014). Compared to pre-instrumental period from 1400-1800, the 19th and 20th century had the wettest years on record while simultaneously experiencing an amplification in year-to-year wet season variability. An increase in hydroclimate whiplash, and the associated risks such as water shortages, wildfires and flooding, underscores the need for novel management practices to maintain resiliency despite greater water extremes.
The findings presented here improve our understanding of past and present variability in seasonal precipitation in the Western United states, with a focus on information that is applicable to water management. The research frameworks in this dissertation are expandable to other hydrologically important and sensitive regions of the world, such as other Mediterranean climate regions, and the results discussed could be further explored to help improve the skill of seasonal precipitation forecasts, which are often lauded as highly impactful for water management purposes.