5:15 PM - 7:15 PM
[ACG43-P09] Quasi-biennial Variability Influence on Indian Ocean Equatorial Undercurrent
Keywords:Indian Ocean, Equatorial Undercurrent, Interannual variability
Empirical orthogonal function (EOF) and complex EOF (CEOF) analysis were conducted on subsurface zonal velocity anomalies (SZVA) in the tropical Indian Ocean for the period of 1979-2018. The first two leading EOF modes account for 48.7% of the total variance, capturing the spatiotemporal patterns of SZVA associated with wind-forced wave dynamics across sub-annual to interdecadal timescales. EOF1 reveals a coherent SZVA pattern throughout the tropical Indian Ocean during the mature phase of the Indian Ocean Dipole (IOD), while EOF2 indicates an east-west tilt fluctuation along the equator during the IOD transition phase. Lead-lag correlations between the first two principal components (PC1 and PC2) correspond to approximately a quarter of the quasi-biennial (QB) band period. A composite life cycle of the anomalous events, derived from the first CEOF mode of the reconstructed SZVA, demonstrates an oscillation along the equator between the east-west tilt mode (EOF2) and basin-scale coherent mode (EOF1). The co-evolution of three-dimensional SZVA, wind stress, and D20 (and pressure) anomalies indicates a coupled ocean-atmosphere system driven by wind-forced waves, which governs the life cycle of IO-EUC variability on QB timescale. Consistent with the inherent-oscillation framework of the IOD, the transitions between positive and negative IOD stages, along with the propagations and reflections of wind-forced and boundary-generated waves, contribute to the QB component and phase transition mechanisms of IO-EUC variability. Higher-order baroclinic mode Rossby waves, which propagate relatively slowly, play a vital role in shaping the life cycle and phase transitions of the IO-EUC variability under the inherent IOD oscillation.