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Traditionally, the estimated volume transport of the North Equatorial Current/Undercurrent (NEC/NEUC) is based on geostrophic equations and/or model results; however, direct observational evidence has not been acquired. We focused on one-year mooring observation data collected along 130°E and calculated the NEC/NEUC volume transport and explore its variability. Results show that the mean NEC and NEUC volume transports calculated from the mean velocity structures in the upper 950 m are 39 Sv and 6 Sv, respectively. Analysis of daily mooring data indicated that the volume transport of the NEC is approximately 52 (±14) Sv and the volume transport of the NEUC is approximately 18 (±13) Sv. A significant 40-day variation existed for the volume transport of both the NEC and NEUC. Overall, the intraseasonal variability (ISV) of the NEC is vertically coherent with that of the NEUC. Observations indicated that the NEUC has three cores centered at approximately 8.5°N (∼500 m), 12.5°N (∼700 m), and 17.5°N (∼900 m), of which the middle core (12.5°N) is the strongest. The 40-day variability of the NEC and NEUC is related to the variability of local wind stress curl anomalies among various Madden-Julian Oscillation (MJO) phases. When local wind field generates a negative (positive) wind stress curl anomaly, a weaker NEC (NEUC) and stronger NEUC (NEC) would occur.

Seasonal modulation of subthermocline eddy kinetic energy (EKE) east of the Philippines and its associated dynamics are studied, using mooring measurements and outputs from an eddy-resolving ocean general circulation model for the period from 2000 to 2017. Significantly high EKE appears below the thermocline in the latitude band between 5° and 14°N east of the Philippines. Separated by 10°N, the EKE in the northern and southern parts of the region shows nearly opposite seasonal cycles, with its magnitude reaching a maximum in early spring and minimum in summer in the northern part and reaching a maximum in summer and minimum in winter in the southern part of the region. Further investigation indicates that both baroclinic and barotropic instabilities are essential in generating the subthermocline eddies, but the seasonal variation of subthermocline EKE is mainly caused by the seasonal modulation of barotropic instability. The seasonal modulation of barotropic instability in the northern and southern part of the region is associated with the seasonal evolution of North Equatorial Undercurrent and Halmahera Eddy, respectively.

As large-scale ocean circulation is a key regulator in the redistribution of oceanic energy, evaluating the multi-decadal trends in the western Pacific Ocean circulation under global warming is essential for not only understanding the basic physical processes but also predicting future climate change in the western Pacific. Employing the hydrological observations of World Ocean Atlas 2018 (WOA18) from 1955 to 2017, this study calculated the geostrophic currents, volume transport and multi-decadal trends for the North Equatorial Current (NEC), the North Equatorial Countercurrent (NECC), the Mindanao Current (MC), the Kuroshio Current (KC) in the origin and the New Guinea Coastal Undercurrent (NGCUC) within tropical western Pacific Ocean over multi-decades. Furthermore, this study examined the contributions of temperature and salinity variations. The results showed significant strengthening trends in NEC, MC and NGCUC over the past six decades, which is mainly contributed by temperature variations and consistent with the tendency in the dynamic height pattern. Zonal wind stress averaged over the western Pacific Ocean in the same latitude of each current represents the decadal variation and multi-decadal trends in corresponding ocean currents, indicating that the trade wind forcing plays an important role in the decadal trend in the tropical western Pacific circulation. Uncertainties in the observed hydrological data and trends in the currents over the tropical western Pacific are also discussed. Given that the WOA18 dataset covers most of the historical hydrological sampling data for the tropical western Pacific, this paper provides important observational information on the multi-decadal trend of the large-scale ocean circulation in the western Pacific.

The relationship of the tropical Indo-Pacific upper ocean heat content (HC) and the South China Sea (SCS) summer monsoon (SCSSM) onset during 1980-2020 was investigated. It was found that when HC anomalies in the preceding winter-spring are positive (negative) in the eastern Indian-western Pacific Ocean or negative (positive) in the southwestern Indian Ocean and eastern Pacific Ocean, SCSSM onset is usually early (late). Empirical orthogonal function (EOF) analysis suggested that the primary EOF modes of HC anomaly in both tropical Pacific and Indian Oceans are associated with El Nino-Southern Oscillation (ENSO). It is through modulating the westerly over the north Indian Ocean-SCS and the western north Pacific subtropical high (WNPSH) via the Walker circulation over the Indo-Pacific basin, monsoon circulation in the Indian Ocean that the ENSO related Indo-Pacific HC variation connects with the SCSSM onset. Both the second EOF modes for the tropical Pacific and Indian Ocean also show significant correlations with the SCSSM onset. The former related vertical motion over the SCS-far northwestern Pacific can influence the WNPSH and convection, thus occurrence of the SCSSM. The latter represents the intrinsic Indian Ocean HC variability; the air temperature variation associated with it can modulate the atmospheric circulation in the Indian Ocean-SCS through thermal wind balance, and thus the SCSSM onset by acting on the westerly over the SCS and the WNPSH. A western Pacific index and an Indian Ocean dipole index were defined to represent the leading HC modes in the tropical Pacific and Indian Ocean, respectively. A binary linear regression model was constructed using these two indices to improve SCSSM onset prediction.

Instability/stability in the North Equatorial Current (NEC) basin is studied based on data obtained from nine moorings deployed at 8.5°N, 10.5°N, 11.0°N, 12.5°N, 13.0°N, 15.0°N, 15.5°N, 17.5°N, and 18.0°N along 130.0°E during cruises in 2015–2017. In low latitudes, the Coriolis parameter and stratification ratio play important roles in NEC stability, whereas velocity shear and the layer depth ratio are important for NEC stability in high latitudes. Beneath the westward NEC, eastward zonal jets occur intermittently centered around 8.5°N, 12.5°N, and 17.5°N along 130.0°E. Similar to the NEC, the main body of these zonal jets also deepens with latitude. In the boundary layer comprising the bottom NEC and upper zonal jets, the growth rate of the NEC is attributed not only to velocity shear but also to zonal jet velocity based on the longwave assumption. Based on the shortwave assumption, the growth rate is proportional to zonal jet velocity but has no relationship with velocity shear. Climatologically, the growth rate in the boundary layer is not zero at 8.5°N, 12.5°N, and 13.0°N, where the velocity shear and zonal jets are larger than at other stations. The instability also occurs at the time node when the zonal jets are strong enough, although the mean zonal jets may disappear at this station.

We investigated the intraseasonal variability of equatorial Pacific subsurface temperature and its relationship with El Nino-Southern Oscillation (ENSO) using Self-Organizing Maps (SOM) analysis. Variation in intraseasonal subsurface temperature is mainly found along the thermocline. The SOM patterns concentrate in basin-wide seesaw or sandwich structures along an east-west axis. Both the seesaw and sandwich SOM patterns oscillate with periods of 55 to 90 days, with the sequence of them showing features of equatorial intraseasonal Kelvin wave, and have marked interannual variations in their occurrence frequencies. Further examination shows that the interannual variability of the SOM patterns is closely related to ENSO; and maxima in composite interannual variability of the SOM patterns are located in the central Pacific during CP El Nino and in the eastern Pacific during EP El Nino. These results imply that some of the ENSO forcing is manifested through changes in the occurrence frequency of intraseasonal patterns, in which the change of the intraseasonal Kelvin wave plays an important role.

Near-inertial oscillation is an important physical process transferring surface wind energy into deep ocean. We investigated the near-inertial kinetic energy (NIKE) variability using acoustic Doppler current profiler measurements from a mooring array deployed in the tropical western Pacific Ocean along 130°E at 8.5°N, 11°N, 12.6°N, 15°N, and 17.5°N from September 2015 to January 2018. Spatial features, decay timescales, and significant seasonal variability of the observed NIKE were described. At the mooring sites of 17.5°N, 15°N, and 12.6°N, the NIKE peaks occurred in boreal autumn and the NIKE troughs were observed in boreal spring. By contrast, the NIKE at 11°N and 8.5°N showed peaks in winter and troughs in summer. Tropical cyclones and strong wind events played an important role in the emergence of high-NIKE events and explained the seasonality and latitudinal characteristics of the observed NIKE.

Variability of oceanic salinity, an indicator of the global hydrological cycle, plays an important role in the basin-scale ocean circulation. In this study, interannual to decadal variability of salinity in the upper layer of the Indian Ocean is investigated using Argo observations since 2004 and data assimilating model outputs (1992–2015). The southeastern Indian Ocean shows the strongest interannual to decadal variability of upper-ocean salinity in the Indian Ocean. Westward propagation of salinity anomalies along isopycnal surfaces is detected in the southern Indian Ocean and attributed to zonal salinity advection anomalies associated with the Indonesian Throughflow and the South Equatorial Current. Composite and salinity budget analyses show that horizontal advection is a major contributor to the interannual to decadal salinity variability of the southern Indian Ocean, and the local air–sea freshwater flux plays a secondary role. The Pacific decadal oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modulate the salinity variability in the southeastern Indian Ocean, with low salinity anomalies occurring during the negative phases of the PDO and ENSO and high salinity anomalies during their positive phases. The Indonesian Throughflow plays an essential role in transmitting the PDO- and ENSO-related salinity signals into the Indian Ocean. A statistical model is proposed based on the PDO index, which successfully predicts the southeastern Indian Ocean salinity variability with a lead time of 10 months.

Herein, the spatial characteristics of the Equatorial Undercurrent (EUC) in the region between Mindanao and New Guinea and the origins of the EUC and the North Equatorial Countercurrent (NECC) were investigated using a combination of high-resolution Argo hydrographic data and numerical modeling. The Mindanao Current (MC) results in the formation of a strong EUC-depth eastward current that splits into two branches near the Halmahera Eddy west of 132 degrees E, with one branch directly flowing eastward without experiencing the Halmahera Eddy rotation effect, and the other branch flowing southwestward and then turning northeastward. The EUC and NECC were difficult to distinguish between 132 degrees E and 136 degrees E, since the former combined with the latter to form a continuous equator-tilted current. At similar to 137 degrees E, the EUC axis started turning toward the equator, and an isolated EUC core was observed at 143 degrees E and similar to 200-m depth. The Pacific EUC was shown to originate at similar to 130 degrees E, where it featured an isolated current core with a maximum velocity of 5-10cm/s at similar to 1 degrees N and 200- to 400-m depth. The EUC and NECC in the western tropical Pacific were found to have the same water sources, namely, the MC and the New Guinea Coastal Undercurrent, with the shallow parts (centered at similar to 100-m depth) of MC and New Guinea Coastal Undercurrent finally reaching the NECC and their deeper parts (centered at similar to 200-m depth) finally reaching the EUC.

The seasonal variability of the North Equatorial Current (NEC) is investigated with four moorings at 10.5°N, 13°N, 15.5°N, and 18°N along 130°E from September 2014 to September 2015 and five moorings at 8.5°N, 11°N, 12.5°N, 15°N and 17.5°N along 130°E collected during 2016 and 2017 cruises. Generally, the mooring data shows that the NEC is strongest during boreal spring and summer and weakest in autumn for the stations located at 8.5°N, 10.5°N, 11°N, 12.5°N, 13°N, 17.5°N and 18°N, but the seasonal phase is almost reversed at 15°N and 15.5°N. In the area east of 140°E and north of 10°N, as the seasonal cycle of the wind stress curl anomaly is almost identical at different latitudes, the Rossby waves play a dominant role in the spatial distribution of the seasonal NEC. As the latitude increases, the rate of decrease in the propagation speed of Rossby waves becomes slower, resulting that the delay of the seasonal sea surface height anomaly (SSHa) tends to be smaller. In the area east of 140°E and between 5°N and 10°N, the seasonal phase of SSHa is delayed more rapidly than that in the south of 5°N, which is the result of local wind and Rossby waves. In the area west of 140°E and from 7°N to 15°N, the seasonal phase of the wind stress curl anomaly advances sharply with increasing latitude, corresponding well to the meridional phase advance of the seasonal SSHa and surface NEC, suggesting that the local wind plays a dominant role.

Based on direct measurements of the Kuroshio current velocity at 18°N by an array of three moorings from January 2018 to February 2020, the intraseasonal variability (ISV) of the Kuroshio and possible dynamic mechanism are studied. The Kuroshio transport in the upper 350m between 122.7°E and 123.3°E is estimated to be 6.5 Sv ± 2.6 Sv. It is revealed for the first time that both the current velocity and volume transport the Kuroshio at 18°N have a significant 50-60-day ISV, which contributes to over 30% of the total variance. Further analysis indicates the ISV of the Kuroshio is caused by the westward propagating eddies with a wavelength of about 633 km and a propagation speed of about 13 cm/s. In addition, the transport mode (74.2%) of the Kuroshio at 18°N is dominant, rather than the migration mode (11.6%). That is different from the Kuroshio east of Taiwan. The findings of this study will highlight the important role of westward Rossby waves (eddies) with a finite wavelength in modulating the intraseasonal variability of the Kuroshio transport near its origin.

Seasonal and interannual variability of the New Guinea Coastal Current (NGCC) and New Guinea Coastal Undercurrent (NGCUC) were investigated with 3 years of mooring measurements off the northern coast of New Guinea and outputs from the Ocean General Circulation Model for the Earth Simulator during 1980-2018. Acoustic Doppler Current Profilers mounted on the two moorings captured variations of the currents in the upper 800 m off the New Guinea coast during 2015-2018. NGCC is a seasonally reversing current in the upper 100 m, which flows southeastward in boreal winter with maximum velocity of 63 cm/s near the surface, and flows northwestward in boreal summer with maximum velocity of -55 cm/s at 80 m. NGCUC flows northwestward all year round between 100 and 400 m, and its temporal mean velocity reaches -40 cm/s at 200 m. A seasonally reversing current named New Guinea Coastal Intermediate Current with speed of 10 cm/s is detected below the NGCUC, which is in phase with the NGCC on seasonal time scale. Seasonal variation of the NGCUC is also in phase with that of the NGCC, and it is strong in boreal summer and weak in winter. Such seasonal signal reaches down to the depth of 800 m. Both mooring measurements and model outputs indicate that NGCUC demonstrates significant interannual variations associated with El Nino-Southern Oscillation (ENSO), with its velocity core shoaling during El Nino and deepening during La Nina, but the net transport of NGCUC exhibits no significant relationship with ENSO. Plain Language Summary The currents north of the New Guinea coast transport water masses crossing the equator from the Southern Hemisphere to the Northern Hemisphere, and modulate the thermohaline structure in the equatorial Pacific, playing an important role in the El Nino-Southern Oscillation cycle and the interhemisphere water exchange. To better understand the structure and variability of these currents, we deployed two subsurface moorings and obtained the velocity time series measurements down to 800 m. A seasonally reversing current is detected below the traditionally known upper-ocean currents, and the monsoon wind forcing is noticed to reach the depth of 800 m, much deeper than the previously acknowledged 150 m. The velocity core of the New Guinea Coastal Undercurrent is revealed shoaling and deepening during El Nino and La Nina, respectively, but the transport of this current exhibits no close relationship with ENSO, which is probably due to the transport restriction through Vitiaz Strait. This research improved our understanding of the vertical structure and variability of the currents off the New Guinea coast.

The Pacific interior subtropical−tropical cells (STCs) and their relation to the two types of El Nino-Southern Oscillation (ENSO) are investigated by using GODAS reanalysis ocean data for the period of 1980–2017. The results show that the interior STC transport into the equatorial region across 9°S and 9°N has a close relationship with the eastern Pacific (EP) ENSO, while it is much weaker with the central Pacific (CP) ENSO. It is suggested that the effect of interior STCs on the tropical Pacific climate is reflected in its relation with the western Pacific thermocline depth or SSHA. During the EP El Nino, the anomalous interior STCs at 9°S and 9°N converge to the equatorial region from the lag months of − 25 to − 8, leading to an accumulation of heat content in the equatorial Pacific; from the lag months of − 8 to 10, they diverge poleward, inducing a discharge of equatorial heat content. The peak poleward interior STC anomaly first appears at 9°N at a zero-lag time, while that at 9°S is observed 4–5 months later. But there is also no appearance of a time lag between the interior STCs at 9°N and 9°S in recharging the period during the EP La Nina mature phase. However, during CP El Nino, only the conspicuous anomalous interior STC divergence appears during the mature and decay phases for the lag months of − 2 to 10, with being symmetric at 9°N and 9°S.

The spatiotemporal variability of equatorial Pacific upper ocean heat content (HC) and subsurface heat during two types of El Nino-Southern Oscillation (ENSO), namely eastern and central Pacific (EP and CP) types, is investigated using subsurface ocean heat budget analysis. Results show that HC tendencies during both types of ENSO are mainly controlled by oceanic heat advection beneath the mixed layer to the thermocline, and the role of net surface heat flux can be neglected. The most important three terms are the zonal and vertical advections of anomalous heat by climatological currents (QU 0 T′, QW 0 T′) and zonal advection of climatological heat by anomalous current (QU′T 0). The large contribution of QU 0 T′ extends from west to east along the equatorial Pacific. The considerable contribution of QU′T 0 is confined to the east of 160°W, and that of the QW 0 T′ is observed in the central Pacific between 180°E and 120°W. In particular, a major contribution of QW 0 T′ is also observed in the far eastern Pacific east of 100°W during EP ENSO. There is also a small contribution from meridional advection of climatological heat by anomalous current (QV′T 0). In contrast, the meridional advection of anomalous heat by climatological currents (QV 0 T′) and vertical advection of climatological heat by anomalous current (QW′T 0) are two damping factors in the HC tendency, with the former dominating. Differences in spatial distribution of the heat advection associated with the two types of ENSO are also presented. We define a warm water heat index (WWH) as integrated heat content above 26 kg m−3 potential density (26σ ɵ ) isopycnal depth within 130°E–80°W and 5°S–5°N. Further examination suggests that the recharge–discharge of WWH is involved in both types of El Nino, though with some differences. First, it takes about 42 (55) months for the evolution of a recharge–discharge cycle during an EP (CP) ENSO. Second, the EP El Nino event peaks during the discharge phase, 7–8 months after the recharge time. The CP El Nino peaks during the recharge phase, 4–5 months before the recharge time. The locations of HC anomalies in the El Nino mature phase relative to those at recharged time explain why the EP and CP El Nino peak in different stages of the recharge–discharge process.

The Western Pacific Warm Pool (WPWP) plays an important role in the global climate through modulating deep convections, ENSO, monsoon onsets, etc. Due to the vast spatial range and huge heat storage of the WPWP, near-real-time monitoring of its three-dimensional variations remains challenging. Based on Argo observations and three reanalysis data sets, we find that the Nino4 sea surface temperature (SST) index captures the interannual variability of the WPWP well. The Nino4 SST can explain approximately half of the variance of the WPWP heat content and almost all the variance of the east-west migration of the WPWP. An assessment of 31 CMIP5 models also reveals that models with larger interannual spectral powers and amplitudes of the Nino4 SST tend to simulate larger variations in the heat content and east-west migration of the WPWP. A surface heat budget analysis further shows that the Nino4 SST and WPWP are physically connected through basin-scale horizontal advections of mean temperatures by anomalous horizontal currents, which dominate the interannual variations of both the Nino4 SST and WPWP. Our results indicate that the Nino4 SST can efficiently estimate the interannual WPWP changes and a reliable predictor of the onset time of the South China Sea summer monsoon and Bay of Bengal summer monsoon, without the need to calculate the eastern boundary location and heat content of the WPWP. Moreover, a better simulation of the SST and horizontal currents in the Nino4 region can help to reduce model bias when reproducing the WPWP's interannual variabilities.

Decadal variations of the transport and bifurcation latitude of the North Equatorial Current (NEC) in the northwestern tropical Pacific Ocean over 1959–2011 are investigated using outputs of the Ocean Analysis/Reanalysis System 3 prepared by the European Centre for Medium-Range Weather Forecasts. The results indicate that the NEC transports at different longitudes have different decadal fluctuations, which are strongest around 139°E. The NEC bifurcation latitude (NBL) has its largest decadal variations around 150 m. Extremes of the decadal NEC transport and NBL before 1975 correspond to different circulation anomalies from those after 1975. The regression map against decadal NBL exhibits negative sea surface height (SSH) anomalies and a cyclonic gyre anomaly over the northwestern tropical Pacific Ocean, while that against the decadal NEC transport exhibits a dipole structure, with positive/negative SSH anomalies to the north/south of about 13°N. Furthermore, decadal variations of the NEC transport and NBL over the whole period have different correlations with Pacific Decadal Oscillation (PDO) and Tropical Pacific Decadal Variability (TPDV). Generally, the decadal NEC transport shows higher correlations with PDO than with TPDV, while the NBL has higher correlations with TPDV than with PDO. The high correlation of decadal NEC transport with PDO mainly comes from that of its northern branch with PDO, while its southern branch shows higher correlation with TPDV.