Kai Yang, Wenju Cai, Gang Huang, Benjamin Ng, Guojian Wang

Published in Geophysical Research Letters, MAR 2021

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.

Plain Language Summary

The strong pIOD event in 2019, featuring strong cooling in the east and mild warming in the west Indian Ocean, induced devastating climate extremes over the populous Indian Ocean-rim countries. Its impact was amplified by the preceding CP El Niño, which has been suggested to play an important role in the generation of the 2019 strong pIOD. This study investigated the relationship of a strong pIOD with a preceding CP El Niño based on observations and model outputs. The results show that a CP El Niño in the preceding December-January-February and MAM has a preconditioning effect for development of a following strong pIOD in JJA, by generating easterly anomalies and shoaling the thermocline over the equatorial EIO in late May. However, the easterly anomalies also generate off-equatorial downwelling Rossby waves which reflect at the western boundary as downwelling Kelvin waves. The downwelling Kelvin waves may weaken the preconditioning effect, or even trigger negative IODs. In addition, JJA stochastic westerlies are detrimental to initial EIO cooling. Thus, whether a preceding CP El Niño leads to a strong pIOD is not deterministic, and is to some extent random, although strong pIOD appears to be more likely when preconditioned by a CP El Niño.

Yang, K., Cai, W., Huang, G., Ng, B., & Wang, G. (2021). Is preconditioning effect on strong positive Indian Ocean Dipole by a preceding Central Pacific El Niño deterministic? Geophysical Research Letters, 48, e2020GL092223. https://doi.org/10.1029/2020GL092223