In a deep channel at the Pacific Yap-Mariana Junction, intensified variability of flow and isotherm displacements over 3,000-4,600 m is revealed by two separate observations: the recent one during 2014-2016 and the two 20 years ago during 1996-1998. Observed velocity and isotherm displacements can reach 45 cm s −1 and~600 m at 4,200 m, respectively. Such intensified variability is demonstrated to arise from topographic Rossby waves (TRWs) since vertical profiles of observed currents reasonably conform to TRW features of hyperbolic intensification with depth and highly vertical coherence in phase. Mercator Ocean model outputs reproduce observed TRWs and are used to discuss their energy sources. Both the energetic surface eddy moving across rough topography and mixed barotropic-baroclinic instability of the background flow appear to be the candidates to generate TRWs.Plain Language Summary Traditionally, the deep sea has been viewed as quiet, with weak horizontal and vertical motions; however, results from three subsurface moorings in a deep channel at the Pacific Yap-Mariana Junction show that the fluctuations of deep current speed and isotherm displacement intensify significantly with increasing depth. The observed velocity and isotherm displacements can reach 45 cm s −1 and~600 m at 4,200 m, comparable to that in the upper ocean. We interpret these deep fluctuations as topographic Rossby waves (TRWs) since vertical distributions of observed velocities obey TRW features of hyperbolic intensification with depth and highly vertical coherence in phase. With the aid of Mercator Ocean model outputs, the TRWs are deemed to arise from the energetic surface eddy moving across the rough topography and barotropic and baroclinic instabilities of the background flow.

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.

Oceanic intraseasonal variability (ISV) plays a vital role in the initiation and development of El Niño events. Mooring observations reveal contrasting characteristics of upper-ocean zonal current ISV during different El Niño events. Compared to the 1997-1998 event, ISV of the 2015-2016 event was significantly weaker (by around 30-50%) over the equatorial Pacific basin, and the variability maximum was shifted to the east (~160°E). These differences can be largely explained by wind forcing features. ISV of surface winds in 2015 is overall weaker than in 1997. Intraseasonal westerly wind bursts in 1997 occurred near the western boundary and originated from the Indian Ocean, whereas in 2015, perturbations were mainly propagated from the northeast Pacific and achieved the peak power at~160°E of the equator. These changes are likely associated with the decadal warming of the northeast subtropical Pacific and weakening of the Indian Ocean intraseasonal oscillation.Plain Language Summary Sea surface warmings of strong El Niño events are usually triggered by intraseasonal oscillations prior to the events. Particularly, intraseasonal eastward ocean currents generated by westerly wind bursts carry the warm water of the western Pacific warm pool to the central Pacific and contribute to the surface warming. In this paper, we compare the intraseasonal zonal current variations during two strong El Niño events, namely, the 1997-1998 and 2015-2016 events. Although the two El Niño events were of comparable strengths, intraseasonal variations in 1997 were stronger than those in 2015 by 3050%. These differences can be largely explained by the contrasting features of wind forcing. Intraseasonal oscillations were originated from the Indian Ocean in 1997, whereas those in 2015 were mainly from the northeast subtropical Pacific. The varying origin is likely associated with the warming of the northeast subtropical Pacific and weakened Indian Ocean intraseasonal oscillation during recent years.