Northwestern Tropical Pacific Ocean Research Articles

Evolution of 3-D chlorophyll in the northwestern Pacific Ocean using a Gaussian-activation deep neural network model

Insufficient studies in characterizing vertical structure of Chlorophyll-a (Chl-a) in the ocean critically limit better understanding about marine ecosystem based on global climate change. In this study, we developed a Gaussian-activation deep neural network (Gaussian-DNN) model to assess vertical Chl-a structure in the upper ocean at high spatial resolution. Our Gaussian-DNN model used the input variables including satellite data of sea surface Chl-a and in-situ vertical physics profiles (temperature and salinity) in the northwestern Pacific Ocean (NWPO). After validation test based on two independent datasets of BGC-Argo and ship measurement, we applied the Gaussian-DNN model to reconstruct temporal evolution of 3-D Chl-a structure in the NWPO. Our modelling results successfully explain over 80% of the Chl-a vertical profiles in the NWPO at a horizontal resolution of 1° × 1° and 1 m vertical resolution within upper 300 meters during 2004 to 2022. Moreover, according to our modelling results, the Subsurface Chlorophyll Maxima (SCMs) and total Chl-a within 0-300 m depths were extracted and presented seasonal variability overlapping longer-time trends of spatial discrepancies all over the NWPO. In addition, our sensitivity testing suggested that sea-water temperatures predominantly control 3-D structures of the Chl-a in the tropical NWPO, while salinity played a key role in the temperate gyre of the NWPO. Here, our development of the Gaussian-DNN model may also be applied to craft long term, 3-D Chl-a products in the global ocean.

Microplastics in the tropical Northwestern Pacific Ocean and the Indonesian seas

The abundance of microplastics fragments with sizes larger than 0.30–0.35 mm is measured during two research cruises in the tropical Northwestern Pacific Ocean and inside the Indonesian seas. The fragments are confirmed to be primarily nylon, polyester, and polyethylene etc. microplastics in the western Pacific by Fourier transform infrared spectroscopy (FT-IR), whereas dominated by polyethylene in the Indonesian seas based on Raman spectrum analyses. The abundance of the microplastics is found to be very low (<0.018 particles m−3) in the surface waters of the North Equatorial Current and the Kuroshio, suggesting no westward extension of the Great Pacific Garbage Belt and lack of microplastics accumulation due to mass convergence in the western boundary current. The abundances of the microplastics in the Makassar Strait of the Indonesian seas are found to be higher by an order of magnitude than those in the Northwestern Pacific, believed to be due to river run-offs carrying microplastics from nearby lands. The largest microplastics abundance is found in the northeastern Maluku Sea, where concurrent current measurements suggest a branch of the Indonesian Throughflow bringing accumulated microplastics from the eastern Indonesian seas into the convergent area of multiple strong currents. The study underlines the importance of ocean circulation in the microplastics distributions and transportation, and suggests the importance of high-resolution spatial mapping in the budget analysis of global microplastics distribution.

Characteristics of subsurface mesoscale eddies in the northwestern tropical Pacific Ocean from an eddy-resolving model

Subsurface eddies (SSEs) are common features of the ocean interior. They are particularly abundant in oceanic basins and the vicinity of major intermediate water outflows. They are responsible for subsurface transport of mass, heat, and salt. Analysis of high-resolution general circulation model data has revealed the existence of subsurface anticyclonic eddies (SSAEs) and subsurface cyclonic eddies (SSCEs) in the northwestern tropical Pacific Ocean. SSEs are abundant east of the Philippines (0°-22°N, 120°E-140°E) and in latitude bands between 9°N-17°N east of 140°E. The composite structure of SSEs was investigated. SSEs had a core at about 400-m water depth and their maximum meridional velocity exceeded 10 cm/s. They exhibited two cores with different salinity polarities in the surface and subsurface. Additionally, spatial distributions of heat transport induced by SSEs in the northwestern tropical Pacific were presented for the first time. A net equatorward heat flux toward a temperature up-gradient was observed. The analysis of eddy-mean flow interactions revealed that the circulation is baroclinically and barotropically unstable at different depths and to differing degrees. The energy conversions suggest that both barotropic and baroclinic instabilities are responsible for SSE generation east of the Philippines, whereas baroclinic instability caused by a horizontal density gradient and vertical eddy heat flux are important between 9°N and 17°N east of 140°E. Meridional movement of the north equatorial current and the north equatorial undercurrent can contribute to SSE generation in our study region.

Impact of Assimilation of Moored Velocity Data on Low‐Frequency Current Estimation in Northwestern Tropical Pacific

AbstractThis study investigates the impact of assimilation of Northwestern Tropical Pacific Ocean (NWTPO) moored currents observation system on low‐frequency (over a period of more than 90 days) current estimation. A low‐frequency scale‐selective ensemble optimal interpolation (EnOI) scheme is designed to assimilate the horizontally sparse moored currents into an eddy‐permitting ocean model. In this assimilation scheme, low‐frequency current signals are selected by removing intraseasonal signals in both observation and ensemble data, and a horizontal spatial filter is implemented to match the model forecast to the low‐frequency ensemble and observation. The assimilation of currents efficiently resolves simulated low‐frequency current displacements, especially for the Equatorial Undercurrent (EUC), the North Equatorial Countercurrent (NECC), and the North Equatorial Current (NEC). More realistic intensities of the NWTPO low‐frequency currents are also reproduced. With respect to the moored velocity profiles, the root‐mean‐square error (RMSE) of the simulated low‐frequency zonal velocity above a 500‐m depth is reduced from 10.9 to 5.8 cm/s using this low‐frequency EnOI scheme. The low‐frequency variation of NWTPO currents is reproduced by correcting the structures of different baroclinic modes. The increments of zonal velocities mainly correspond to the first baroclinic mode for the NECC and NEC, while for the EUC, they correspond to the leading three and higher‐order baroclinic modes. The improvements of the simulated stratification and pressure gradient remark the role of the assimilation of moored currents in adjusting available potential energy.

Simulation of monsoon intra‐seasonal oscillations in Geophysical Fluid Dynamics Laboratory models from Atmospheric Model Intercomparison Project integrations of Coupled Model Intercomparison Project phase 5

AbstractThe simulation of the intra‐seasonal variability (ISV), especially the fidelity of monsoon intra‐seasonal oscillations (MISO) over the South Asian summer monsoon (SASM), has been evaluated in Atmospheric Model Intercomparison Project run of three Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric general circulation models (AGCM), participating in Coupled Model Intercomparison Project phase 5. Two of the models, namely, “GFDL‐HIRAM‐C180” and “GFDL‐HIRAM‐C360” are global high‐resolution atmospheric models (HIRAMs), which are same but at different horizontal resolutions. Third model “GFDL‐CM3” is at moderate horizontal resolution, whose atmospheric component differs from HIRAMs. Results have led to the conclusion that simulation of ISV over SASM remains poor in GFDL AGCMs at higher horizontal resolution, except northwards propagation of 30–60‐day mode of MISO by HIRAMs. Our diagnostics revealed resemblance in key characteristics of ISV simulated in two HIRAMs, except spectral peak at lower frequencies (30–60 day), which is captured only by “GFDL‐HIRAM‐C180” over equatorial Indian Ocean. Based on these results, it is conjectured that although high horizontal resolution is crucial for AGCMs to reproduce observed northwards propagation of 30–60‐day mode of MISO, the simulation of nature of ISV over SASM is not influenced by the difference of horizontal resolution in two HIRAMs. Analysis implicates that a reasonable representation of meridional migration of horizontal moisture advection, the meridional gradient of low‐level specific humidity over Indian longitudes and precipitation–SST relationship over north Indian Ocean, the northwestern tropical Pacific Ocean and tropical Indian Ocean assures the northwards propagation of 30–60‐day mode of MISO in HIRAMs.

At-sea movements of wedge-tailed shearwaters during and outside the breeding season from four colonies in New Caledonia

The wedge-tailed shearwater (WTS) population of New Caledonia is one of the largest in the world, yet its biology and foraging ecology are poorly known. We studied WTS from 4 colonies in New Caledonia. We examined foraging behaviour and habitats using GPS receivers and light sensors during and outside the breeding season, respectively, and compared our findings with those from other WTS populations worldwide. During breeding, New Caledonian WTS alternated short foraging trips close to the colony over the lagoon, or off the reef edge, with longer trips over distant, deep waters. Whereas neighboring colonies overlapped at sea, especially during short trips, there was a clear separation of foraging zones between the pairs of colonies located in the southern versus northwestern parts of New Caledonia. Although WTS actively foraged and commuted to foraging zones during the day, they mainly returned to the colony or rested at night, indicating that they feed mainly during the day. Active foraging did not take place in more productive areas, suggesting that it may instead be related to the presence of sub-surface predators. Outside the breeding season, birds from 3 colonies had similar trans-equatorial migratory behaviour. All left New Caledonia at the same time of the year with a fast, northeasterly movement and wintered over deep waters in the same sector of the northwestern tropical Pacific Ocean. At overwintering sites, they spent most of their non-foraging time presumably sitting on the water, especially at night, making a slow westward movement before returning to New Caledonia. WTS from New Caledonia forage over warm, oligotrophic deep waters throughout their life cycle, and the species appears to have a flexible foraging strategy adapted to the various environmental conditions encountered across its wide tropical range.

Decadal variations of the North Pacific Tropical Water at 137°E

The decadal variations of the North Pacific Tropical Water (NPTW) at 137°E in the western North Pacific Ocean are investigated based on the repeated hydrographic observations along with two global gridded ocean products. The results indicate that the maximum salinity of NPTW experiences significant quasi-decadal variations, having maxima around 1979, 1987, 1995, 2004, and 2012, while minima around 1974, 1983, 1991, 1999, and 2008 during the period of interest. The NPTW area also shows similar quasi-decadal variation, expanding/shrinking as its maximum salinity increases/decreases at the 137°E section. These variations are induced mainly by changes in the mixed layer salinity in the source region and large-scale circulation in the northwestern tropical Pacific Ocean, both of which are related to the Pacific Decadal Oscillation. The underlying processes at work are further confirmed through conducting the subsurface salinity budget analysis. Besides, short-term processes are also at work through nonlinear interactions, especially after 2000.

Subsurface structure and variability of the zonal currents in the Northwestern Tropical Pacific Ocean

Seven subsurface moorings were deployed between 0°N and 7.5°N at 142°E in the western Pacific during August 2014–October 2015, obtaining unprecedented high-quality measurements of the North Equatorial Countercurrent (NECC) between 60 and 200 m and the underlying Northern Subsurface Countercurrent (NSCC) and North Equatorial Subsurface Current (NESC) between 300 and 600 m. During the observation period, the eastward-flowing NSCC showed a mean velocity of 5–15 cm s−1 between 2°N and 4°N, while the NESC manifested as intermittent westward currents of < 5 cm s−1 between 4°N and 6°N. Temporal variabilities in the NSCC and NESC are documented for the first time. Both the NSCC and NESC were strengthened when the NECC shifted to the south in boreal fall. The 2015–2016 El Niño induced significant strengthening of the NECC and weakening of the NESC. Further analysis combining ocean reanalysis data suggests that seasonal variations are likely coherent between the upper and lower (300–600 m) layers and associated with the meridional migration of the NECC. Seasonal variations of the three currents are likely first-mode baroclinic Rossby wave responses to basin-wide wind forcing. By contrast, interannual variations of the NSCC and NESC have more complicated vertical structure. El Niño events generally cause the strengthening and southward migration of the NECC and the weakening of the NESC. Ocean reanalysis products however show difficulties in realistically representing the observed subsurface structure and variability of the three currents, remarking the necessity of enhancing ocean in-situ observation systems.

Role of dust fluxes in stimulating Ethmodiscus rex giant diatom blooms in the northwestern tropical Pacific during the Last Glacial Maximum

The strontium (Sr) and neodymium (Nd) isotope compositions and clay-mineral assemblages of the lithogenic fraction of sediments in the southern Mariana Trench together with major- and trace-elements concentrations of bulk sediments have been determined to trace the sediment provenance and investigate the relationship between Asian dust input and blooms of the giant diatom Ethmodiscus rex (E. rex). Enrichment of barium (Ba) in relative to upper continental crust (UCC) and low average Rb/K ratios in all study cores point to both hydrothermal and volcaniclastic inputs to the sediments. Both the Sr-Nd isotope compositions and the clay-mineral assemblages of the lithogenic fraction reflect a two-component mixing system consisting of Mariana arc volcaniclastics and Asian eolian dust. Decreases in smectite content and kaolinite/chlorite ratio and increases in illite content and illite/kaolinite ratio just preceding the formation of laminated diatom mats (LDMs) suggest changes in the source of the eolian dust from eastern Asian deserts (EADs) to central Asian deserts (CADs) at the onset of the Last Glacial Maximum (LGM) which stimulated the blooming of E. rex giant diatom. These observations suggest a potential causal linkage among atmospheric circulation patterns, the sources of Asian eolian dust, and marine productivity. We postulate that intensified eolian dust input sourced from CADs may have played a greater role in promoting biological productivity in the oligotrophic northwestern tropical Pacific Ocean during the LGM than previously realized.

Targeted observation analysis of a Northwestern Tropical Pacific Ocean mooring array using an ensemble-based method

An important supplement for ocean observing systems, the Northwestern Tropical Pacific Ocean (NWTPO) mooring array including 15 moorings equipped with Acoustic Doppler Current Profilers (ADCP) devices was developed by the Chinese Academy of Sciences and deployed in 2013. This study assessed the performance of this mooring array in monitoring the intra-seasonal and low-frequency (above 91 days) variability of oceanic currents by conducting targeted observation analyses using an ensemble-based method. Key regions for monitoring intra-seasonal variability of the NWTPO circulation are the equator, Indonesian throughflow (ITF), headstream of the North Equatorial Countercurrent (NECC), and Subtropical Countercurrent (STCC). For monitoring intra-seasonal variability, the range of each mooring is confined to a local scale. Therefore, NWTPO moorings cannot adequately resolve intra-seasonal variability in areas of the ITF, the headstream of the NECC, and STCC due to location constraints of the moorings. For monitoring low-frequency variability of NWTPO circulation, the key regions are the Western Boundary Current (WBC), NECC, and the Equatorial Undercurrent (EUC). NWTPO moorings performed relatively well in monitoring the low-frequency variability, as indicated by the strong background correlations between each of the currents. The NWTPO mooring array plays an important role in monitoring the location and intensity of background currents. Because moorings are costly and require a high-density distribution for optimal performance, understanding the multi-timescale dynamical nature of the NWTPO current system is critical for the deploying future moorings in this region.

Model-based assessment of a Northwestern Tropical Pacific moored array to monitor intraseasonal variability

The Northwestern Tropical Pacific Ocean (NWTPO) moorings observing system, including 15 moorings, was established in 2013 to provide velocity profile data. Observing system simulation experiments (OSSEs) were carried out to assess the ability of the observation system to monitor intraseasonal variability in a pilot study, where ideal “mooring-observed” velocity was assimilated using Ensemble Optimal Interpolation (EnOI) based on the Regional Oceanic Modeling System (ROMS). Because errors between the control and “nature” runs have a mesoscale structure, a random ensemble derived from 20–90-day bandpass-filtered nine-year model outputs is proved to be more appropriate for the NWTPO mooring array assimilation than a random ensemble derived from a 30-day running mean. The simulation of the intraseasonal currents in the North Equatorial Current (NEC), North Equatorial Countercurrent (NECC), and Equatorial Undercurrent (EUC) areas can be improved by assimilating velocity profiles using a 20–90-day bandpass-filtered ensemble. The root mean square errors (RMSEs) of the intraseasonal zonal (U) and meridional velocity (V) above 500 m depth within the study area (between 0°N–18°N and 122°E–147°E) were reduced by 15.4% and 16.9%, respectively. Improvements in the downstream area of the NEC moorings transect were optimum where the RMSEs of the intraseasonal velocities above 500 m were reduced by more than 30%. Assimilating velocity profiles can have a positive impact on the simulation and forecast of thermohaline structure and sea level anomalies in the ocean.

Decadal variations of the transport and bifurcation of the Pacific North Equatorial Current

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.

Community composition of picoeukaryotes in the South China Sea during winter

Picoeukaryotes, the smallest protists, are highly diverse and abundant in the ocean. However, little information is available about their community composition in the tropical northwestern Pacific Ocean. This study collected surface and deep chlorophyll maximum (DCM) waters from the South China Sea (SCS) to study the picoeukaryotic composition by constructing clone libraries of the 18S rRNA gene. The libraries were dominated by the heterotrophic organisms, alveolates and Rhizaria, which accounted for 46% and 16% of total clones, respectively. MALV-I was the most abundant group in alveolates, and Rhizaria appears to be a key organism in the SCS, particularly within DCM layers. These results indicate that parasitism is significant in the oligotrophic and tropical SCS. Apart from core-dinoflagellates, chlorophytes, haptophytes, cryptophytes and pelagophytes were other important contributors to primary production in pico-sized fraction based on quantitative and qualitative data.

The reduction in the biomass of cyanobacterial N2 fixer and the biological pump in the Northwestern Pacific Ocean

The comparison of sediment trap data with physical and biogeochemical variables in the surface water column of the Tropical Northwestern Pacific Ocean (TNWPO) indicated that the magnitude of the springtime biological pump has reduced with time due to a corresponding decrease in the biomass of cyanobacterial N2 fixer. The decrease in the biomass of N2 fixer likely resulted from a reduction in phosphate concentrations in response to surface water warming and consequent shoaling of the mixed layer depth during the study period (2009−2014). The same reduction in biological pump was also observed during summer. However, the cause of the summer reduction remains uncertain and is worth assessing in future studies. Our findings have major implications for predicting future trends of the biological pump in the TNWPO, where significant warming has occurred.

Causes for intraseasonal sea surface salinity variability in the western tropical Pacific Ocean and its seasonality

AbstractPronounced intraseasonal variability (ISV; 20–90 day) of sea surface salinity (SSS) with a standard deviation of 0.12–0.20 psu is detected in the western tropical Pacific Ocean (PO) from measurements of Aquarius/SAC‐D satellite. These variations are not spatially uniform but show distinct regional features. The Hybrid Coordinate Ocean Model (HYCOM) well simulated the observed SSS variations, and a suite of parallel experiments were performed to understand the underlying physical processes. Surface forcing by atmospheric intraseasonal oscillations which are dominated by the Madden‐Julian oscillation (MJO) is largely responsible for producing the SSS ISV, while ocean internal variability plays a secondary role. Impact of atmospheric forcing is primarily through precipitation and wind stress‐driven oceanic processes. Their relative importance shows spatial variations. They have approximately equal importance in the western equatorial PO west of 155°E and the southwestern tropical PO. Wind stress effect dominates SSS ISV in the equatorial PO east of 155°E, while precipitation effect is larger in the northwestern tropical PO. In comparison, the effect of evaporation induced by wind speed change is smaller. The SSS ISV also shows evident seasonality in some areas, particularly in the far western equatorial basin and southwestern tropical PO. During boreal summer (winter), SSS ISV is enhanced (weakened) in the northwestern PO and weakened (enhanced) in the southwestern PO. Comparing with the strength of atmospheric forcing, seasonal variation of the ocean state, especially the mixed layer depth, is generally more important in causing such seasonality.

Large-Scale Atmospheric and Oceanic Conditions for Extensive and Persistent Icing Events in China

AbstractBased on the observational datasets of rime and glaze from 743 stations in China and the atmospheric circulation data from the NCEP–NCAR reanalysis during 1954–2009, large-scale atmospheric and oceanic conditions for extensive and persistent rime and glaze events were examined with a composite analysis. Results show that rime events mostly occur in northern China while glaze events are mainly observed in southern China. The icing events are accompanied by low temperature and high humidity but not necessarily by above-normal precipitation. The Asian low, blocking highs, strong moisture transport, and an inversion layer related to major abnormal circulation systems contribute to the occurrence and persistence of icing events in China. The Ural blocking high plays a major role in the glaze events, and the Okhotsk blocking high is closely related to the rime events. For glaze events, extratropical circulation anomalies and the southward outbreak of cold air play a dominant role. In contrast, the strong northward transport of warm and moist airflows plays a leading role and the blocking high and the southward outbreak of extratropical cold air take a supporting role for rime events. There is nearly an equal chance for occurrences of rime events under La Niña and El Niño backgrounds. However, glaze events more likely occur under the background of La Niña. Additionally, the sea surface temperatures from the tropical Indian Ocean to the tropical northwestern Pacific Ocean also contribute to the occurrence and maintenance of icing events in China.

Decadal variations of Pacific North Equatorial Current bifurcation from multiple ocean products

AbstractIn this study, we examine the decadal variations of the Pacific North Equatorial Current (NEC) bifurcation latitude (NBL) averaged over upper 100 m and underlying dynamics over the past six decades using 11 ocean products, including seven kinds of ocean reanalyzes based on ocean data assimilation systems, two kinds of numerical simulations without assimilating observations and two kinds of objective analyzes based on in situ observations only. During the period of 1954–2007, the multiproduct mean of decadal NBL anomalies shows maxima around 1965/1966, 1980/1981, 1995/1996, and 2003/2004, and minima around 1958, 1971/1972, 1986/1987, and 2000/2001, respectively. The NBL decadal variations are related to the first Empirical Orthogonal Function mode of decadal anomalies of sea surface height (SSH) in the northwestern tropical Pacific Ocean, which shows spatially coherent variation over the whole region and explains most of the total variance. Further regression and composite analyzes indicate that northerly/southerly NBL corresponds to negative/positive SSH anomalies and cyclonic/anticyclonic gyre anomalies in the northwestern tropical Pacific Ocean. These decadal circulation variations and thus the decadal NBL variations are governed mostly by the first two vertical modes and attribute the most to the first baroclinic mode. The NBL decadal variation is highly positively correlated with the tropical Pacific decadal variability (TPDV) around the zero time lag. With a lead of about half the decadal cycle the NBL displays closer but negative relationship to TPDV in four ocean products, possibly manifesting the dynamical role of the circulation in the northwestern tropical Pacific in the phase‐shifting of TPDV.

Geostrophic Circulation in the Tropical North Pacific Ocean Based on Argo Profiles

Abstract Absolute geostrophic currents in the North Pacific Ocean are calculated from the newly gridded Argo profiling float data using the P-vector method for the period of 2004–11. The zonal geostrophic currents based on the Argo profile data are found to be stronger than those based on the traditional World Ocean Atlas 2009 (WOA09) data. A westward mean geostrophic flow underneath the North Equatorial Countercurrent is identified using the Argo data, which is evidenced by sporadic direct current measurements and geostrophic calculations in history. This current originates east of the date line and transports more than 4 × 106 m3 s−1 of water westward in the subsurface northwestern tropical Pacific Ocean. The authors name this current the North Equatorial Subsurface Current. The transport in the geostrophic currents is compared with the Sverdrup theory and found to differ significantly in several locations. Analyses have shown that errors of wind stress estimation cannot account for all of the differences. The largest differences are found in the area immediately north and south of the bifurcation latitude of the North Equatorial Current west of the date line and in the recirculation area of the Kuroshio and its extension, where nonlinear activities are vigorous. It is, therefore, suggested that the linear dynamics of the Sverdrup theory is deficient in explaining the geostrophic transport of the tropical northwestern Pacific Ocean.

Revisit the interannual variability of the North Equatorial Current transport with ECMWF ORA‐S3

AbstractThe interannual variability of the North Equatorial Current (NEC) transport in the tropical northwestern Pacific Ocean is investigated with the output from ECMWF Ocean Analysis/Reanalysis System 3 (ORA‐S3). The results show that the amplitude and root mean square (RMS) of interannual NEC transport anomalies increase from about 3.0–4.0 Sv and 2.0 Sv at 170°E to above 5.0 and 3.4 Sv at 135°E, respectively. The NEC transport variation agrees well with the variation of the sea surface height (SSH) anomaly difference between the southern and northern boundaries of the NEC region. Further analysis near the Philippine coast suggests that their good agreement mainly comes from the agreement of the NEC transport and SSH variations south of the gyre boundary. Around the bifurcation point off the Philippine coast, the southern branch of the NEC transport is highly related to El Niño–Southern Oscillation (ENSO) events. During El Niño/La Niña years, westerly/easterly wind anomalies and positive/negative wind stress curl anomalies develop in the tropical northwestern Pacific Ocean south of 20°N before the mature phase. The wind forcing center moves eastward with time and reaches the easternmost position around 170°E several months before the mature phase. This wind forcing generates upwelling/downwelling Rossby waves, which propagate westward to result in negative/positive SSH anomalies, hence inducing a cyclonic/anticyclonic gyre anomaly, which is responsible for the increase/decrease of the NEC transport. The northern branch of the NEC transport near the Philippine coast has no significant simultaneous relation with ENSO events.

Typhoon Rammasun-Induced Near-Inertial Oscillations Observed in the Tropical Northwestern Pacific Ocean

Wind-induced near-inertial oscillations (NIOs) have been known to propagate their energy downward and equatorward, yet few observations have confirmed this in tropical regions. Using measurements from a moored ADCP in the tropical northwestern Pacific, we report an energetic NIO event associated with Typhoon Rammasun in May 2008, when an anti-cyclonic warm eddy existed around the mooring site. Our analyses reveal that the anti-cyclonic eddy traps the NIO energy at two layers around 120 and 210 m where the buoyancy frequency show high values. The NIO energy continuously decays at layers below its maximum at 210 m, and disappears at depths below the thermocline. During their propagation from 137 to 649 stretchedmeter depths (equivalent to 100-430 m), NIOs shift their frequencies from 0.92f to 1.05f probably due to the effective f, which changes its magnitude from smaller to larger than local inertial frequency f in the anti-cyclonic eddy. In addition, their vertical energy propagation becomes faster from 0.17 to 0.64 mm s^-1. Decomposition of downward and upward NIO energy propagation shows that the typhoon-induced NIOs remain 29% of their energy in the upper layer, and transfer 71% to the subsurface layers. Our results suggest that typhoon-induced NIOs interacting with meso-scale eddies can play an important role in providing the energy source available for ocean mixing in the tropical regions.