Indian Ocean Dipole Modes Associated with Different Types of ENSO Development

Abstract This study identifies several modes of coevolution of various types of El Niño–Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) by performing rotated season-reliant empirical orthogonal function (S-EOF) analysis with consideration of ENSO asymmetry. The first two modes reveal that early-onset ENSO is associated with subsequent strong IOD development, whereas late-onset ENSO forces an obscure IOD pattern with marginal SST anomalies in the western Indian Ocean. Further studies show that El Niño starting before early summer can more easily force an IOD event than that starting in late summer or fall, even when they are of equivalent magnitudes. This is because the atmospheric responses over the Indian Ocean to the eastern Pacific warming are in sharp contrast between early and late summer. Early-onset (late onset) El Niño can (cannot) cause favorable atmospheric circulation conditions over the Indian Ocean for inducing the western Indian Ocean warming, which facilitates the subsequent IOD development. In addition, the different propagations of ocean dynamic Rossby waves during the early- or late-onset types of ENSO are also accountable for the different IOD development. For the higher-order modes, the rotated S-EOF of “Niño only” cases shows a coevolution between a negative IOD mode and a date line Pacific El Niño, with warm sea surface temperature anomalies originating from the northern Pacific meridional mode.