NOAA/WDS Paleoclimatology - Ta'u, American Samoa Coral d18O Data from 1520-2011 CE

A 492-year long, continuous d18O time series from a massive Porites coral colony in Ta'u, American Samoa, records contrasting responses to different types of ENSO (El Niño-Southern Oscillation) through a mixed sea surface temperature (SST) and salinity signal. Currently, conventional El Niño (La Niña) events generate cold and salty (warm and fresh) anomalies at Ta'u, while Modoki El Niño (La Niña) events warm (cool) the waters at Ta'u. Over the course of the 20th century, the Ta'u d18O record underwent a polarity shift in its response to conventional ENSO: a warm and fresh (cool and salty) response to El Niño (La Niña) was replaced by the opposite pattern. We interpret this as evidence for the movement of the Eastern Pacific ENSO null zone, the narrow band of the surface ocean where SST variability is not on average correlated with ENSO. This movement appears to be related to overall shrinking of the ENSO footprint over the 20th century. We infer no such trend in the Modoki footprint. The five century long Ta'u record shows dramatic, century-scale changes in ENSO-band variability. Comparisons with other ENSO reconstructions lead to conflicting interpretations: the Ta'u coral may have recorded changes in the strength of ENSO or in its spatial footprint. Changes in the spatial footprint manifest as a changing sensitivity to ENSO at any given location, presenting challenges to established methods of ENSO reconstruction.