Collaborative Research: RAPID--Characterizing the Water Isotope Signature of an El Nino Event for Paleoclimate and Hydroclimate Studies

This project secures funds under the auspices of the Rapid Response Research (RAPID) program to take advantage of the strong-to-very strong El Niño/Southern Oscillation (ENSO) forecast for the water year 2023-2024 to establish a longitudinal array of precipitation and stream sampling stations to constrain the meteoric water delta oxygen-18, deuterium-excess, and delta oxygen-17 signal of a strong ENSO event. The researchers will sample precipitation and stream water monthly over the course of a year and analyze these samples for oxygen and hydrogen isotopes. They will also collect spot samples of small-stream waters across this same longitudinal transect during Fall 2023 and Spring 2024 and simulate the source and pathways of precipitation using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model while using spatial statistics to produce isoscapes of our stable isotope result across the southwestern United States (U.S.). This research is premised on the idea that substantial uncertainty remains regarding how hydroclimate (including precipitation, evapotranspiration and runoff) in the arid and semi-arid southwestern U.S. will respond to rising carbon dioxide and warmer temperatures considering that ENSO plays an outsize role in modifying wintertime southwestern U.S. hydroclimate. There is a large body of data from research that examines how ENSO will change in response to warming, utilizing both Global Climate Model (GCM) projections of future climate and paleoclimate records but these data are poorly constrained. Much of the paleoclimate evidence of southwestern U.S. hydroclimate and its links to the ENSO comes from proxy records of past meteoric water oxygen isotopes, with the typical assumption that lower proxy delta oxygen-18 corresponds with greater wintertime moisture and a shift towards ENSO conditions. Consequently, the ability to test the sensitivity of western U.S. hydroclimate to warming and its links with ENSO relies upon constraining the relationship between the ENSO and meteoric water delta oxygen-18 across the western U.S. This causal link, however, is not well-established for strong ENSO events that may dominate climate in the future. Furthermore, there is currently no data to understand how ENSO modifies delta oxygen-17 values, which is emerging as a promising analytical isotopic tool to constrain the role of moisture source dynamics and evaporative effects in carbonate and clay isotope archives of paleoclimate. The potential broader impacts include supporting two graduate students and providing a long-term monitoring dataset for improving reconstructions of the strength and frequency of ENSO events in the past which will help constrain the range of variability observed in the region necessary for future forecasts. The research fits well into the potentially transformative, high risk, and quick-response research imperative of the RAPID program. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.