Dynamics of the strong Wyrtki Jet events in the eastern Indian Ocean in 2014

The Wyrtki Jet plays an important role in the tropical Indian Ocean circulation, yet its variability near the eastern boundary was observed scarcely. In this study, a strong Wyrtki Jet event in the spring 2014 is analyzed using satellite-tracked surface drifter trajectories combined with the mooring data in the eastern Indian Ocean. The observed maximum zonal velocity reaches 1.2 m s-1, twice as much as the spring jet climatology. Following the strong spring jet, another eastward jet with velocity of 0.9 m s-1 is observed in July, when the seasonal zonal currents typically flow westward. The LICOM ocean general circulation model, which successfully simulates the observed zonal currents in the eastern Indian Ocean in 2014, is used to investigate the dynamics of the Wyrtki Jet event. Decomposed equatorial waves from the model simulation show that the 2014 event is dominated by wind-driven downwelling Kelvin waves and the reflected upwelling Rossby waves of earlier wind forcing in the central and eastern basin. Reconstructed zonal currents from the wave coefficients suggest that the second baroclinic-mode Rossby waves reflected from the Kelvin waves dominate the strong spring Wyrtki Jet, with higher-order baroclinic modes contributing more than 30% of the velocity amplitude. The eastward current in July 2014 is dominated by an intraseasonal wind-driven Kelvin wave. The disclosed dynamics highlight the important roles of the boundary reflections and higher-order modes in modulating the ocean circulation in the eastern Indian Ocean near the boundary.