Collaborative Research: Stable isotope constraints on the hydrologic expression of the El Nino/Southern Oscillation in the tropical Pacific Ocean

About This Grant

Variability in winds and sea surface temperature in the tropical Pacific produces the cycle of El Niño and La Niña events. These events produce both powerful storms and droughts, but their cycling is irregular and difficult to predict. The isotopic composition of oxygen preserved in fossil corals is one of the best tools scientists have for understanding how El Niño and La Niña have changed in the past. The composition is influenced by temperature but also by the salinity of seawater. Their respective influences must be separated to understand the magnitude of past El Niño and La Niña events and their impacts on the global water cycle. The proposed research combines analysis of rain and seawater, climate models, and fossil coral data to address this scientific question. The researchers will create a detailed map, or “isoscape,” showing how oxygen isotope values in seawater and rainfall vary across the modern tropical Pacific during El Niño and La Niña events. This map will be coupled with simulations from an ocean model. As a result of this work, scientists will have a more comprehensive understanding of how the intensity of past El Niño and La Niña events are recorded in coral oxygen isotope values. The proposed research will advance understanding of how El Niño-Southern Oscillation (ENSO)-related hydrologic anomalies may be inferred from oxygen isotope (delta-18O) values in tropical Pacific seawater. The researchers will utilize samples from long-running seawater and precipitation collection sites across the tropical Pacific and create a new isotope-enabled ocean reanalysis product for the Pacific basin at high spatial resolution. This combination will allow, for the first time, a direct assessment of the simultaneous isotopic anomalies associated with ENSO phases across multiple sites. The results will enable researchers to quantify the contributions of ocean circulation, atmospheric moisture balance, and precipitation delta-18O to seawater delta-18O values during different phases of ENSO. Results will also quantify seawater delta-18O and temperature influences on coral delta-18O values and reveal whether coral records of seawater delta-18O indicate stronger ENSO-related hydroclimate variability in recent decades, which is critical to inform planning for the impacts of ENSO events. The temporal continuity of the dataset, capturing ENSO phases across the basin, will enhance the community’s ability to interpret paleoclimate information of past tropical Pacific climate change. The project will support training for a graduate student, a postdoctoral scientist, and high school students. A new educational module on stable isotopes and the water cycle will be developed for 6th grade students. This module will involve hands-on learning and stable isotope analysis of local water samples, with assessment based on pre- and post-tests to measure educational outcomes and student understanding. 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.