[A-CG33] Multi-scale ocean-atmosphere interaction in the tropics

[ACG33-02] Multi-year ENSO: impact, mechanisms, and predictability

★Invited Papers

*Tomoki Iwakiri^1,2, Masahiro Watanabe¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Meteorological Research Institute)

Keywords:ENSO

El Niño–Southern Oscillation (ENSO) events occasionally recur one after the other in the same polarity, called multi-year ENSO. Multi-year La Niña brings different climate impacts over Japan, and therefore understanding mechanisms and predictability are important. We found that multi-year El Niño and La Niña events are roughly symmetric. The composite multi-year ENSO reveals that anomalous ocean heat content (OHC) in the equatorial Pacific persists beyond the first peak, stimulating another event. This prolonged OHC anomaly is caused by meridional Ekman heat transport counteracting geostrophic recharge-discharge that otherwise acts to change the OHC anomaly. A meridionally wide pattern of sea surface temperature anomalies observed during multiyear ENSO is responsible for the Ekman heat transport. Furthermore, 100-member climate predictions were conducted for multi-year La Niña in 2020-2022. The ensemble-mean captures the second-year La Niña while the inter-member spread is explained by the position of the North Pacific high and negative North Pacific meridional mode. It contributes to the modulation of the apparent meridional scale of La Niña, and thus our proposed mechanisms are helpful in understanding ENSO persistence for the seasonal forecast.