Sea level anomalies in the southeastern tropical Indian Ocean as a potential predictor of La Nina beyond one-year lead

Most climate forecast agencies failed to make successful predictions of the strong 2020/2021 La Nina event before May 2020. The western equatorial Pacific warm water volume (WWV) before the 2020 spring failed to predict this La Nina event because of the near neutral state of the equatorial Pacific Ocean in the year before. A strong Indian Ocean Dipole (IOD) event took place in the fall of 2019, which is used as a precursor for the La Nina prediction in this study. We used observational data to construct the precursory relationship between negative sea level anomalies (SLA) in the southeastern tropical Indian Ocean (SETIO) in boreal fall and negative Nino 3.4 sea surface temperature anomalies index one year later. The application of the above relation to the prediction of the 2020/2021 La Nina was a great success. The dynamics behind are the Indo-Pacific "oceanic channel" connection via the Indian Ocean Kelvin wave propagation through the Indonesian seas, with the atmospheric bridge playing a secondary role. The high predictability of La Nina across the spring barrier if a positive IOD should occur in the previous year suggests that the negative SETIO SLA in fall is a much better and longer predictor for this type of La Nina prediction than the WWV. In comparison, positive SETIO SLA lead either El Nino or La Nina by one year, suggesting uncertainty of El Nino predictions.