Inherent quasi-biennial (QB) oscillation (1.5-2 years) of Indian Ocean dipole (IOD) accounts for a large part of the Indian Ocean Equatorial Undercurrent (IOEUC) variability, though it has not received much attention until now. In this study, we explore the QB variability in the IOEUC, its phase transitions, and connections to the IOD using long-term ocean reanalysis and model outputs with various statistical methods. The two leading EOF modes of the anomalous subsurface zonal currents capture the dominant QB variability in the IOEUC. A coherent temporal lead-lag period of 4-5 months between these two modes reveals a typical life cycle characterized by the basin-scale displacement during equilibrium phases and east-west tilting flows during disequilibrium phases. Consistent with the charge-recharge framework of the IOD, the switch between positive and negative IOD stages along with the propagations and reflections of wind-forced and boundary-generated waves provide the QB component and phase transition mechanism of the IOEUC variability. Higher-order baroclinic mode Rossby waves with relative slow propagating speeds play an important role in shaping the life cycle and phase transitions of the IOEUC variability under the inherent IOD oscillation. This analysis represents a pathway to improve our understanding of Indian Ocean current system in maintaining and redistributing tracers throughout the climate system.

Indian Ocean,,Equatorial Undercurrent,Indian Ocean Dipole,Quasi-biennial variability