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.

This study reveals the seasonal variability of the lower and upper deep branches of the Pacific Meridional Overturning Circulation (L‐PMOC and U‐PMOC) in the Yap‐Mariana Junction (YMJ) channel, a major gateway for deep flow into the western Pacific. On the western side of the YMJ channel, mooring observations in 2017 and in 1997 show the seasonal phase of the L‐PMOC at depths of 3,800–4,400 m: strong northward flow with speed exceeding 20 cm s⁻¹ and lasting from December to next May and weak flow during the following 6 months. On the eastern side of the channel, mooring observations during 2014–2017 show two southward deep flows with broadly seasonal phases, one being the return flow of L‐PMOC below ~4,000 m and with the same phase of L‐PMOC but reduced magnitude. The second, shallower, southward deep flow corresponds to the U‐PMOC observed within 3,000–3,800 m and with opposite phase of L‐PMOC, that is, strong (weak) southward flow appearing during June–November (December–May). Seasonal variations of the L‐PMOC and U‐PMOC are accompanied by the seasonal intrusions of the Lower and Upper Circumpolar Waters (LCPW and UCPW) in lower and upper deep layers, which change the isopycnal structure and the deep currents in a way consistent with geostrophic balance.

The Philippine Sea is located within the pathway of the propagating Madden–Julian oscillation (MJO) system and is also the destination for the North Pacific mesoscale oceanic eddies and waves. As such, the Philippine Sea is characterized by striking intra‐seasonal variability (20–90 days; ISV) that plays a key role in bridging weather and climate. Spatial patterns and temporal features of intra‐seasonal oceanic variability along the Philippine coast are investigated using mooring observations and outputs from an eddy‐resolving general circulation model. The amplitude of ISV determined from sea level anomaly (SLA) is found to decrease from the northern and coastal Philippine Sea to the south and interior Pacific Ocean. In contrast, eddy kinetic energy (EKE) features a northward decreasing gradient with a maximum at about 6^°N. The meridional distribution of SLA and EKE spectra indicates that the ISV period increases with latitude and is symmetrical about the equator. Mesoscale eddies in the upper layer are tracked to explore the statistical distribution. Westward propagating mesoscale eddies and intraseasonal Rossby waves related to dynamic instabilities are an important source of the oceanic ISV in the Philippine Sea. Clear coastal propagation of ISVs related to coastal Kelvin waves is detected the south of about 14^°N. Composite analysis shows that the MJO is another important forcing shaping the spatial features of Philippine ISV intensity and contributes close to half of the observed total ISV.

Meridional ocean current in the northwestern Pacific was documented by seven subsurface moorings deployed at 142°E during August 2014-October 2015. A sandwich structure of the tropical meridional overturning circulation (TMOC) was revealed between 0–6°N that consists of a surface northward flow (0–80 m), a thermocline southward flow (80–260 m; 22.6–26.5 σ _θ ), and a subthermocline northward flow (260–500 m; 26.5–26.9 σ _θ ). Based on mooring data, along with satellite and reanalysis data, prominent seasonal-to-interannual variations were observed in all three layers, and the equatorial zonal winds were found to be a dominant cause of the variations. The TMOC is generally stronger in boreal winter and weaker in summer. During 2014–2015, the TMOC was greatly weakened by westerly wind anomalies associated with the El Niño condition. Further analysis suggests that the TMOC can affect equatorial surface temperature in the western Pacific through anomalous upwelling/downwelling and likely plays a vital role in the El Niño-Southern Oscillation (ENSO).

The lower deep branch of the Pacific Meridional Overturning Circulation (L‐PMOC) is a crucial element of the ocean's climate and biogeochemical systems through carrying the Lower Circumpolar Water (LCPW). For the first time, the pathway and volume transport of L‐PMOC from the Yap‐Mariana Junction (YMJ) to the Northern Philippine Basin (NPB) are revealed by a six‐mooring array measurement over 2019–2021. The L‐PMOC seasonally intrudes into the western Pacific at the YMJ. Then, it is directed into the West Mariana Basin (WMB) through YMJ‐Northern Channel with 1.41 ± 1.43/0.26 (mean ± standard deviation/total rms error) Sv, and further into the NPB through Kyushu‐Palau Ridge (KPR) Channel with 0.75 ± 0.53/0.18 Sv. Their difference 0.65 ± 1.35/0.28 Sv is the net lateral flux of LCPW into the WMB over the 2.5 yr. Analyses of a data‐assimilative ocean model solution suggest that the L‐PMOC transports through the deep channels are consistent with deep pressure gradients forced mainly by upper ocean processes.