Asymmetric decadal changes in the relationship between warm water volume and ENSO around 2000

The warm water volume (WWV), representing oceanic heat content, is the most widely used oceanic predictor of El Niño–Southern Oscillation (ENSO). However, the decadal variation in the relationship between WWV and ENSO remain unclear. Here we investigate the decadal changes in WWV’s predictive skill on ENSO from two aspects: the positive-negative ENSO event asymmetry and the Western-Eastern Pacific asymmetry. In this study, it’s found that an asymmetric decadal shift occurs in the predictive ability of WWV on El Niño and La Niña. Before 2000, WWV can predict ENSO beyond one-year lead, with the predictability reaching maximum at 8 months lead. This lead relation between WWV and ENSO decreased after 2000, mainly for the dramatically weakened La Niña predictability with the lead time of maximum predictability changing from 8 months to 3 months, while the El Niño predictability remained robust beyond 8 months lead. Such decreased predictive ability of WWV on La Niña is linked to the increased multi-year La Niña after 2000. We also identified the distinct roles and changes of WWV in the Western and Eastern Pacific Oceans. Revealed by information flow, after 2000 the stabilizing influences of Western Pacific WWV on ENSO increased and the key region expanded southward (from 4°S–3°N to 10°S–3°N), while the destabilizing influences of Eastern Pacific WWV on ENSO weakened and its scope narrowed (from 4°S–3°N to 2°S–2°N). Specifically in the Western Pacific after 2000, not only is the equatorial WWV important, but the off-equator (6°S–10°S) WWV in southern hemisphere is also significant. These results further extend our understanding on the prediction of ENSO by WWV.