ENSO's Shrinking 20th Century Footprint Revealed in a Half‐millennium Coral Core from the South Pacific Convergence Zone

Abstract: A 492-year-long, continuous δ18O time series from a massive Porites coral colony in Ta’u,
American Samoa, records contrasting responses to different types of El Niño-Southern Oscillation (ENSO)
through a mixed sea surface temperature 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 twentieth century, the Ta’u δ18O 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 sea surface temperature variability is not on
average correlated with ENSO. This movement appears to be related to overall shrinking of the ENSO
footprint over the twentieth 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.