Intensification and Dynamics of the Westward Equatorial Undercurrent During the Summers of 1998 and 2016 in the Indian Ocean

Using mooring observations and reanalysis, we show that anomalously strong westward Equatorial Undercurrent (wEUC) developed in June-July in 2016 and 1998 in the Indian Ocean, which coincided with extreme Indian Ocean Dipole (IOD) and El Nino events. Simulations show that equatorial Kelvin and Rossby waves were excited by winds associated with El Nino and positive IOD events during 2015 and 1997, and their negative phases during 2016 and 1998. The constructive relationship between the delayed-time contributions of eastern-boundary-reflected-waves that excited by the easterlies in 2015 and 1997 and the direct contributions of wind-forced-waves that excited by the westerlies in 2016 and 1998 resulted in the intensified wEUC. Slow intermediate-order baroclinic-modes, rather than fast low-order baroclinic-modes, dominated the strong wEUC. The eastern-boundary-reflected-waves dominated in 1997-1998 and directly wind-forced-waves dominated in 2015-2016. Our results emphasize the importance of constructive interactions of the directly-wind-forced and boundary-reflected waves in driving the interannual variability of Indian Ocean wEUC.