IOD-driven Quasi-biennial Variability Influence on Indian Ocean Equatorial Undercurrent

Inherent quasi-biennial (QB) variability (with a period of 1.5–2 years) of the Indian Ocean Dipole (IOD) significantly influences the interannual variability of the Indian Ocean Equatorial Undercurrent (IO-EUC), yet it has received limited attention until now. This study investigates the QB variability of the IO-EUC, focusing on its phase transitions and dynamical connections to the IOD, using long-term ocean reanalysis and linear ocean model outputs with various statistical methods. Two leading empirical orthogonal function (EOF) modes of anomalous subsurface zonal currents capture the dominant QB variability of the IO-EUC. A coherent temporal lead-lag relationship of 4–5 months between the principal components of these leading modes indicates a typical life cycle characterized by a basin-wide, same-sign displacement along the equator during equilibrium phases, and an east-west tilting subsurface mode during disequilibrium phases. Consistent with the inherent-oscillation framework of the IOD, the switch between positive and negative IOD stages, along with the propagations and reflections of wind-forced and boundary-reflected equatorial waves, contributes to the QB component and phase transition mechanisms of the IO-EUC. Importantly, higher-order baroclinic mode Rossby waves with relatively slower phase speeds are found to play a crucial role in shaping the life cycle and phase transitions of IO-EUC under the inherent IOD oscillation. These findings enhance our understanding on the new scenario of the Indian Ocean current system in maintaining and redistributing tracers throughout the climate system.