Is Preconditioning Effect On Strong Positive Indian Ocean Dipole by a Preceding Central Pacific El Niño Deterministic?

Abstract

The 2018 Central Pacific (CP) El Niño preceded the 2019 strong positive Indian Ocean Dipole (pIOD). The robustness of a CP El Niño preconditioning a strong pIOD has not been investigated. Here, we show that March-April-May (MAM) easterly anomalies over the equatorial eastern Indian Ocean (EIO) induced by a preceding CP El Niño drive a thermocline shallowing conducive to an early equatorial EIO cooling. However, the same winds also generate off-equatorial downwelling Rossby waves in the southern EIO, which reflect as downwelling Kelvin waves, able to weaken the initial anomalies. Furthermore, zonal winds in June-July-August (JJA) can either be beneficial or unfavorable for the developing pIOD. Only when the equatorial easterlies, EIO cooling, and thermocline shallowing are sufficiently amplified by JJA does a strong pIOD occur. Despite this, a multi-century model simulation suggests that development of majority of strong pIOD events is facilitated by a preceding CP El Niño.