A Bayesian model for predicting local El Nino events using tree ring widths and cellulose δ18O

The oxygen stable isotopic composition (delta O-18) of cellulose recorded in annual tree rings reflects the climate and precipitation history experienced during tree growth and development. Here, we show proxy evidence of El Nino events over the past 30 years using juniper tree rings from southern California, United States. The relationship between tree ring delta O-18 in alpha cellulose and annual ring width was negative during most years, reflecting amount-driven fractionation during precipitation. During El Nino years, the relationship between delta O-18 and ring width was positive with the largest ring widths correlated to the heaviest delta O-18. Warmer sea surface temperatures during vapor formation and the strengthening of vapor transport from the eastern Pacific Ocean inland is the most likely mechanism driving heavier delta O-18 in precipitation during El Nino years. Based on this varying relationship between tree ring width and climate-dependent delta O-18 values, we created a model to estimate the probability that a given annual tree ring was formed during an El Nino or non-El Nino year. The methods used in this analysis differ from standard dendrochronological technique because we explicitly account for the varying relationship between climate and tree ring characteristic during an El Nino or non-El Nino year. Moreover, our approach accommodates uncertainty in model parameters and predictions better than traditional classification methods. The application of this model to prehistory tree samples or samples of unknown age may allow for El Nino detection and subsequent determination of changes in El Nino frequency.