The ocean currents of the tropical Pacific Ocean vary with El Niño‐Southern Oscillation (ENSO) cycles. A mooring time series obtained during 2014–2018 in the western Pacific reveal that interannual variability extends to Lower Equatorial Intermediate Current (LEIC). The LEIC velocity anomalies are significantly correlated with the Niño‐3.4 index at an 11‐month lag. Monthly velocity data from the global ocean physical reanalysis product and from a linear continuously stratified ocean model during 1993–2018 capture the 2015–2016 signal and are used to identify the underlying mechanism. During El Niño (La Niña) events, the direct wind forcing in the central tropical Pacific and the reflection of Kelvin waves in the eastern Pacific during the first autumn‐winter period contribute comparably to an eastward (westward) current anomaly in the western Pacific during the second autumn‐winter period. This process is achieved mainly through the generation of the second baroclinic mode and off‐equatorial westward‐propagating downwelling (upwelling) Rossby waves. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) is interconnected with the interannual variability of upper ocean circulation. Through analyzing a 4‐year mooring observation and a 26‐year reanalysis product at 0°, 142°E, we find that the interannual variability of the Lower Equatorial Intermediate Current (LEIC) lying between 600 and 820 m is also influenced by ENSO. The LEIC flows westward during boreal autumn‐winter and reverses to an eastward‐flowing current during spring‐summer. However, in the following year of El Niño events, the westward‐flowing LEIC disappears during autumn‐winter. With the aid of a linear continuously stratified ocean model, we demonstrate that the absence of the LEIC during this period is mainly related to pressure anomaly fields at intermediate depths caused by the off‐equatorial westward‐propagating first meridional and second baroclinic modes downwelling Rossby waves. These Rossby waves are generated by the anticyclonic wind stress curl associated with the easterly wind anomaly in the eastern Pacific and by the eastern boundary reflection of equatorial Kelvin waves initially triggered by the westerly wind anomaly in the central Pacific during the mature stage of El Niño events. The above two originations make comparable contributions to the interannual variability. Key Points: Lower equatorial intermediate zonal current anomaly is significantly correlated with Niño‐3.4 index at an 11‐month lagEquatorial intermediate interannual variability is related to second baroclinic mode Rossby waves mainly generated by ENSO mature stage windRossby waves from direct wind forcing and reflection of Kelvin waves contribute comparably to intermediate interannual variability [ABSTRACT FROM AUTHOR]