Stable isotope signatures of northern California precipitation during an El Niño winter: implications for speleothem paleoclimate reconstructions

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 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. At each location, we collected monthly integrated and hourly event-scale rain samples for stable isotope (δ2Hp, δ18Op) analysis beginning in December 2015. 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 in February and March 2016. Precipitation from these ARs comprised 18% of total winter rainfall at both sites. At Shasta Lake monthly-integrated δ2Hp and δ18Op from March are the least negative of the winter, and HYSPLIT back-trajectory analysis suggests a subtropical origin for the March ARs. The most negative monthly integrated δ2Hp and δ18Op at Shasta Lake occur in December, and HYSPLIT back-trajectories suggest Central-North Pacific storm origins. However, monthly δ18Op at Shasta Lake also shows a positive correlation with rain day surface temperature. At Santa Cruz, March δ2Hp and δ18Op are the most negative of the winter, suggesting factors besides vapor source may also influence precipitation isotope values, but there is no clear relationship between monthly δ18Op and surface temperature or rainfall amount. Drip water δ2H and δ18O from both caves are similar to local precipitation but changes are lagged relative to changes in monthly δ2Hp and δ18Op. This modern analysis, which will continue through non-El Niño winters, will help improve speleothem-based ENSO reconstructions in California.