Interdecadal pacific oscillation modulation of a South Atlantic precursor of following-winter ENSO
Abstract
The South Atlantic sea-surface-temperature (SST) dipole has been proposed as a precursor of following-winter El Niño–Southern Oscillation (ENSO), but its unstable relationship with ENSO has limited its predictive use. Here, using observations, reanalysis, and a large model ensemble, we show that this instability is modulated by the interdecadal pacific oscillation (IPO). During positive IPO phases, a positive austral-summer South Atlantic subtropical dipole (SASD) tends to precede El Niño, while its negative phase tends to precede La Niña one year later; during negative IPO phases, this relationship is weak. The associated pathway involves a Southern Hemisphere Rossby-wave teleconnection from the South Atlantic to the South Pacific, followed by a South Pacific wind–SST footprint, wind–evaporation–SST feedback, Walker-circulation adjustment, and a thermocline response in the equatorial Pacific. Leave-one-out cross-validation shows that the SASD provides one-year forecast skill beyond persistence mainly during positive IPO years, and model ensemble members largely reproduce the sign of this IPO modulation. These results identify the IPO as a slowly varying background condition under which the South Atlantic can become a useful ENSO precursor, offering a physically interpretable basis for long-lead ENSO prediction.