Topographic Rossby waves (TRWs) are sub-inertial waves with periods ranging from several days to several hundreds of days, which are induced by variations in cross-isobathic motion as fluid columns are stretched and compressed over sloping topography under the conservation of potential vorticity (Oey & Lee, 2002;Rhines, 1970). TRWs are bottom trapped and decay upward in the presence of stratification (Rhines, 1970). As early as the 1970s, TRWs with periods of 1-2 weeks and wavelengths of 100-200 km were observed in the northwest Atlantic, decaying upward with depth (Thompson, 1971;Thompson & Luyten, 1976).

The SCS western boundary current (WBC) is the major component of the SCSTF and largely explains the variability of the SCSTF (

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 Niño events. Simulations show that equatorial Kelvin and Rossby waves were excited by winds associated with El Niño 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.