MOISTURE SOURCES OF EL NINO RELATED STORMS AND THE ROLE OF ATMOSPHERIC RIVERS: A STABLE ISOTOPE PERSPECTIVE

Beginning in 2012, California experienced the most severe drought noted in the last 1200 years. Although southern California remains in extreme drought, the strong El Niño of 2015-2016 brought some relief to northern California. For example, winter storms delivered enough precipitation to put California’s largest reservoir, Shasta Lake, at greater than 100% of its historic average for the first time in three years. Despite the significant influence that the El Niño/Southern Oscillation (ENSO) can have on California precipitation, stable isotope signatures of precipitation during strong El Niño events in northern California remain poorly constrained. In order to investigate the potentially variable isotopic signature of El Niño storms in northern California, we placed automatic precipitation samplers at five sites along a N-S transect. These include two coastal sites (Santa Cruz, Bodega Bay), one upslope site (Cazadero), and two leeward sites (Santa Clara, Shasta Lake). At each location, we collected monthly integrated and hourly event-scale rain samples for stable isotope (δ²H, δ¹⁸O) analysis. Here we present data from two sites, Santa Cruz and Shasta Lake, that are adjacent to sites of ongoing cave monitoring and speleothem record development, and thus can inform our understanding of how ENSO is recorded in the paleo-record.

Three large atmospheric river (AR) storms hit northern California between February and April 2016. Precipitation from these ARs comprised 18% of total winter rainfall at both sites. Northern California was also hit by several other significant winter storms, which contributed 18% of winter precipitation in Shasta Lake and 45% in Santa Cruz. At Shasta Lake monthly-integrated δ²H and δ¹⁸O isotope data from March are the least negative of the winter, and HYSPLIT back-trajectory analysis suggests a subtropical origin for the March ARs. However, at Santa Cruz, March precipitation δ²H and δ¹⁸O are the most negative of the winter, suggesting factors besides vapor source may influence precipitation isotope values. Drip water δ²H and δ¹⁸O from both caves are similar to local precipitation, adding confidence to the idea that speleothems reliably record meteoric water signatures. This modern event-specific analysis should help improve speleothem-based reconstructions of El Niño in California.