Characteristics of Springtime Atmospheric Ridging Over the Pacific Northwest

The hydroclimatology of the Pacific Northwest is characterized by wet winters and dry summers; however, the end of the rainy season is subject to considerable year-to-year variability. Early onset of the dry season can introduce challenges for water resource managers. Historically, anomalously dry springs are the result of persistent atmospheric ridging, with notable examples in May 2015 and 2018. As climate change may affect the persistence, frequency, and intensity of ridges, it is important to understand potential changes in springtime ridging. Future changes will have implications for the length of the rainy season and consequently freshwater availability throughout the region. Therefore, we characterize the historical climatology of springtime atmospheric ridging through a focus on persistence, frequency, and magnitude. A ridge index is developed to identify anomalous springtime ridging events relevant to the Portland, Oregon region of the Pacific Northwest. Resulting trends and potential systematic changes in ridge characteristics are discussed, including several notable periods of ridging occurring over the most recent decade. Impacts associated with historical periods of frequent or persistent ridging are quantified over the region, demonstrating the relationships between ridge events and snowpack, precipitation, and temperature. The ridge index will be applied to a suite of state-of-the-art climate models to determine model fidelity in simulating historical ridging climatology. This will facilitate constraining of uncertainty in climate model projected ridge characteristics under simulations of future global warming. Results from this research will enhance our understanding of variability and projected change in springtime atmospheric ridging over the Pacific Northwest, which will inform water resource managers of potential water availability changes.