Subtropical Countercurrent Research Articles

Revisit of the Occurrence of the Kuroshio Large Meander South of Japan

AbstractA unique characteristic by the Kuroshio off the southern coast of Japan is its bimodal path variations. In contrast to its straight path that follows the coastline, the Kuroshio takes a large meander (LM) path when its axis detours southward by as much as 300 km. Since 1950, eight Kuroshio LM events took place and their occurrences appeared random. By synthesizing available in situ/satellite observations and atmospheric reanalysis product, this study seeks to elucidate processes conducive for the LM occurrence. We find neither changes in the inflow Kuroshio transport from the East China Sea nor in the downstream Kuroshio Extension dynamic state are determinant factors. Instead, intense anticyclonic eddies with transport > 20 Sv (1 Sv ≡ 106 m3 s−1) emanated from the Subtropical Countercurrent (STCC) are found to play critical roles in interacting with Kuroshio path perturbations southeast of Kyushu that generate positive relative vorticities along the coast and lead the nascent path perturbation to form a LM. Occurrence of this intense cyclonic–anticyclonic eddy interaction is favored when surface wind forcing over the STCC is anticyclonic during the positive phasing of Pacific decadal oscillations (PDOs). Such wind forcing strengthens the meridional Ekman flux convergence and enhances eddy generation by the STCC, and seven of the past eight LM events are found to be preceded by 1–2 years by the persistent anticyclonic wind forcings over the STCC. Rather than a fully random phenomenon, we posit that the LM occurrence is regulated by regional wind forcing with a positive PDO imprint.

State Estimates and Forecasts of the Eddy Field in the Subtropical Countercurrent in the Northern Philippine Sea

AbstractA strongly nonlinear eddy field is present in and around the subtropical countercurrent in the northern Philippine Sea (NPS). A regional implementation of the Massachusetts Institute of Technology General Circulation Model–Estimating the Circulation and Climate of the Ocean four-dimensional variational assimilation (MITgcm-ECCO 4DVAR) system is found to be able to produce a series of 2-month-long dynamically consistent optimized state estimates between April 2010 and April 2011 for the eddy-rich NPS region. The assimilation provides a stringent dynamical test of the model, showing that a free run of the model forced using adjusted controls remains consistent with the observations for 2 months. The 4DVAR iterative optimization reduced the total cost function for the observations and controls by 40%–50% from the reference solution, initialized using the Hybrid Coordinate Ocean Model 1/12° global daily analysis, achieving residuals approximately equal to the assumed uncertainties for the assimilated observations. The state estimates are assessed by comparing with assimilated and withheld observations and also by comparing 1-month model forecasts with future data. The state estimates and forecasts were more skillful than model persistence and the reference solutions. Finally, the continuous state estimates were used to detect and track the eddies, analyze their structure, and quantify their vertically integrated meridional heat and salt transports.

Spatial distribution of polycyclic aromatic hydrocarbons in the Philippine Sea, Western Pacific and the impact factors analysis

Knowing the polycyclic aromatic hydrocarbons (PAHs) pollution properties in ocean is highly needed to protect the open sea. In July to August of 2020, twenty surface water samples were obtained from the East China Sea to the Philippine Sea. The ranges of Σ15PAHs concentrations were 3.188–5.29 ng L−1 in dissolved phase, 0.455–1.305 ng L−1 in particulate phase in the Philippine Sea. 3, 4-Ring PAHs were the most abundant PAHs. Their spatial differences in dissolved phase were mainly caused by human activities, the summer monsoon from the Philippine Islands and the Northern Equatorial Current, and the Kuroshio Current and Subtropical Countercurrent. The source analysis showed that PAHs in surface water in the Philippine Sea may come from coal combustion. It is the first time to analyze the occurrence and distribution of PAHs in the Philippine Sea.

Climatic Effects of Spring Mesoscale Oceanic Eddies in the North Pacific: A Regional Modeling Study

A high-resolution atmospheric model of the Weather Research and Forecast (WRF) is used to investigate the climatic effects of mesoscale oceanic eddies (OEs) in the North Pacific (NPac) in spring and the respective effects of OEs in the northern NPac associated with the Kuroshio Extension (KE) and of OEs in the southern NPac related to the subtropical countercurrent. Results show that mesoscale OEs in the NPac can strengthen the upper-level ridge (trough) in the central (eastern) subtropical NPac, together with markedly weakened (strengthened) westerly winds to its south. The mesoscale OEs in the whole NPac act to weaken the upper-level storm track and strengthen lower-level storm activities in the NPac. However, atmospheric responses to the northern and southern NPac OEs are more prominent. The northern NPac OEs can induce tropospheric barotropic responses with a tripole geopotential height (GPH) anomaly pattern to the north of 30° N, while the OEs in both the northern and southern NPac can enhance the upper-level ridge (trough) in the central (eastern) subtropical NPac. Additionally, the northern NPac OEs can shrink the lower-level subtropical high and weaken the easterly trade winds at the low latitudes, while the southern NPac OEs result in a southward shift of the lower-level subtropical high and an eastward shift of the upper-level westerly jet stream. The southern and northern NPac OEs have similar effects on the storm track, leading to an enhanced lower-level storm track over the KE via moistening the atmospheric boundary layer; and they can also exert significant remote influences on lower- and upper-level storm activities over the Northeast Pacific off the west coast of North America. When the intensities of OEs are doubled in the model, the spatial distribution of atmospheric responses is robust, with a larger and more significant magnitude. Additionally, although OEs are part of the mesoscale oceanic processes, the springtime OEs play an opposite role in mesoscale sea-surface temperature anomalies. These findings point to the potential of improving the forecasts of extratropical springtime storm systems and the projections of their responses to future climate change, by improving the representation of ocean eddy-atmosphere interaction in forecast and climate models.

Western Pacific Zooplankton Community along Latitudinal and Equatorial Transects in Autumn 2017 (Northern Hemisphere)

During the autumn of 2017, a study was conducted to assess the zooplankton community composition in three sections (two latitudinal, going from Japan to the equator, and one longitudinal on the equator) of the Western Pacific Ocean. A total of 384 species of zooplankton adults and 21 groups of zooplankton larvae were identified, with copepods being the predominant taxon. The common dominant species across the three sections were Acrocalanus gibber, Canthocalanus pauper, Oithona similis, Paracalanus aculeatus, and Oncaea venusta. Zooplankton abundance was the highest in the equator section, with a mean abundance of 258.94 ± 52.57 ind./m3. Comparatively, a low abundance was recovered from the Subtropical Countercurrent (STCC) region, while the highest abundance holding stations were located in the eastern equatorial and North Equatorial Countercurrent (NECC) regions. Pearson’s correlation, canonical correspondence analysis, and other methods were used to analyze the relationship between environmental factors and zooplankton. We found that the Shannon–Wiener diversity index and Pielou’s uniformity index were significantly correlated (p < 0.05) with concentrations of nitrite and chlorophyll a. The distribution of zooplankton was also limited by nutrients, chlorophyll a, and dissolved oxygen. In addition, we reveal differences in the abundance of species in the equatorial and latitudinal seas. We found that not only temperature and nutrient salinity, but also ocean currents and the movement of water masses, influence the distribution of zooplankton communities in the Western Pacific.

Influence of anomalous low-level circulation on the Kuroshio in the Luzon Strait during ENSO

Basin-scale wind curl anomalies in the North Pacific associated with El Niño increase westward Kuroshio transport in the Luzon Strait, whereas those associated with La Niña reduce it. However, the Kuroshio current in the Luzon Strait is further affected by mesoscale eddies and local wind stress. Numerical experiments are conducted to demonstrate that the anomalous low-level circulation over the Philippine sea during El Niño weakens the northeasterly monsoon and favors the development of mesoscale eddies at the same time. This is caused by the increased baroclinic instability in the Subtropical Countercurrent (STCC)/North Equatorial Current region. Our results are supported by the long-term observational data. Consequently, the Kuroshio pattern in the Luzon Strait during El Niño tends to be more unsteady. During La Niña, the Kuroshio loop is enhanced in response to strengthened northeasterly winds. Furthermore, vorticity stretching, caused by local wind stress and associated curl anomalies during La Niña, contributes to the formation of a cyclonic eddy off the southeastern Taiwan.

Submesoscale Currents in the Subtropical Upper Ocean Observed by Long-Term High-Resolution Mooring Arrays

AbstractAlthough observational efforts have been made to detect submesoscale currents (submesoscales) in regions with deep mixed layers and/or strong mesoscale kinetic energy (KE), there have been no long-term submesoscale observations in subtropical gyres, which are characterized by moderate values of both mixed layer depths and mesoscale KE. To explore submesoscale dynamics in this oceanic regime, two nested mesoscale- and submesoscale-resolving mooring arrays were deployed in the northwestern Pacific subtropical countercurrent region during 2017–19. Based on the 2 years of data, submesoscales featuring order one Rossby numbers, large vertical velocities (with magnitude of 10–50 m day−1) and vertical heat flux, and strong ageostrophic KE are revealed in the upper 150 m. Although most of the submesoscales are surface intensified, they are found to penetrate far beneath the mixed layer. They are most energetic during strong mesoscale strain periods in the winter–spring season but are generally weak in the summer–autumn season. Energetics analysis suggests that the submesoscales receive KE from potential energy release but lose a portion of it through inverse cascade. Because this KE sink is smaller than the source term, a forward cascade must occur to balance the submesoscale KE budget, for which symmetric instability may be a candidate mechanism. By synthesizing observations and theories, we argue that the submesoscales are generated through a combination of baroclinic instability in the upper mixed and transitional layers and mesoscale strain-induced frontogenesis, among which the former should play a more dominant role in their final generation stage.

Submesoscale Fronts and Their Dynamical Processes Associated with Symmetric Instability in the Northwest Pacific Subtropical Ocean

AbstractSubmesoscale density fronts and the associated processes of frontogenesis and symmetric instability (SI) are investigated in the northwest Pacific subtropical countercurrent (STCC) system by a high-resolution simulation and diagnostic analysis. Both satellite observations and realistic simulation show active surface fronts with a horizontal scale of ~20 km in the STCC upper ocean. Frontogenesis-induced buoyancy advection is detected to rapidly sharpen these density fronts. The direct straining effect of larger-scale geostrophic flows is a primary influence on the buoyancy-gradient frontogenetic tendency and frontal baroclinic potential vorticity (PV) enhancement. The enhanced lateral buoyancy gradients in conjunction with atmospheric forced surface buoyancy loss can produce a negative Ertel PV and trigger frontal SI in the STCC region. Up to 30% of the mixed layer (ML) inside a typical eddy has negative PV in the high-resolution simulation. As a result, the cross-front ageostrophic secondary circulations tend to restratify the surface boundary layer and induce a large vertical velocity reaching ~100 m day−1, substantially facilitating the vertical communication of the STCC system. At the same time, the SI is identified to be responsible for a forward cascade of geostrophic kinetic energy in the STCC region, despite the coexistence of ML eddies and SI in the deep winter ML. Therefore, these active density fronts and their SI-associated submesoscale processes play important roles in the enhanced vertical exchanges (e.g., heat, nutrients, and carbon) and energy transfer to smaller scales in the eddy-active STCC upper ocean, as well as triggering phytoplankton blooms at the periphery of eddies.

Intraseasonal variability of the Kuroshio east of Taiwan, China, observed by subsurface mooring during 2016–2017

The mean velocity structure and variability of the Kuroshio east of Taiwan, China, were investigated using mooring observations acquired at 23°N/122°E between January 2016 and May 2017. Power spectrum analysis reveals marked intraseasonal variability in 70-95 days in the meridional Kuroshio velocity in the upper 800 m. Hybrid Coordinate Ocean Model assimilation data adequately reproduce variations of the Kuroshio. Westward propagating mesoscale eddies that originated from the Subtropical Countercurrent play an important role in intraseasonal variability in the Kuroshio velocity. Cyclonic (anticyclonic) eddies are often associated with negative (positive) sea surface height anomaly, resulting in positive (negative) potential vorticity in the subsurface layer of the eddy center, which decreases (increases) isopycnal slope and then weakens (strengthens) the Kuroshio. When anticyclonic (cyclonic) eddies are active, the Kuroshio axis moves offshore (inshore). Corresponding to the Kuroshio intraseasonal variability (ISV), the variation of the intermediate water salinity also had a significant ISV induced by the mesoscale eddies.

Atmospheric Responses to Mesoscale Oceanic Eddies in the Winter and Summer North Pacific Subtropical Countercurrent Region

In the winter and summer North Pacific Subtropical Countercurrent region, the atmospheric responses to 20,000+ mesoscale oceanic eddies (MOEs) are examined using satellite and reanalysis data from 1999 to 2013. The composite results indicate that surface wind speed, cloud, and precipitation anomalies are positively correlated with sea surface temperature anomalies in both seasons. The surface wind speed anomalies and convective precipitation anomalies show dipolar structures centering on MOEs in winter and on unipolar structures in summer. In both seasons, the vertical mixing mechanism plays an obvious role in the atmospheric responses to MOEs. In addition, the distributions of sea level pressure anomalies in winter reflects the effects of the pressure adjustment mechanism. Due to the seasonal variations in the atmospheric background state and the MOEs, the sensitivities of surface wind speeds, clouds, and precipitation responses to MOEs in summer are over 30% higher than those in winter.

Interannual to Decadal Variations of Submesoscale Motions around the North Pacific Subtropical Countercurrent

The outputs from a submesoscale permitting hindcast simulation from 1990 to 2016 are used to investigate the interannual to decadal variations of submesoscale motions. The region we focus on is the subtropical Northwestern Pacific including the subtropical countercurrent. The submesoscale kinetic energy (KE) is characterized by strong interannual and decadal variability, displaying larger magnitudes in 1996, 2003, and 2015, and smaller magnitudes in 1999, 2009, 2010, and 2016. These variations are partially explained by those of the available potential energy (APE) release at submesoscale driven by mixed layer instability in winter. Indeed, this APE release depends on the mixed layer depth and horizontal buoyancy gradient, both of them modulated with the Pacific Decadal Oscillation (PDO). As a result of the inverse KE cascade, the submesoscale KE variability possibly leads to interannual to decadal variations of the mesoscale KE (eddy KE (EKE)). These results show that submesoscale motions are a possible pathway to explain the impact associated with the PDO on the decadal EKE variability. The winter APE release estimated from the Argo float observations varies synchronously with that in the simulation on the interannual time scales, which suggests the observation capability to diagnose the submesoscale KE variability.

Seasonal Reversal of the Near‐Surface Chlorophyll Response to the Presence of Mesoscale Eddies in the South Pacific Subtropical Countercurrent

AbstractMesoscale eddies have been shown to have significant effects on biogeochemical cycles, as observed in local levels of near‐surface chlorophyll. There are many regional differences in the expected response of near‐surface chlorophyll to the presence of eddies, caused by differences in the driving mechanisms. However, in many high eddy activity areas, previous studies found low correlation and an inconsistent chlorophyll anomaly response to the presence of eddies. One such area is in the South Pacific Subtropical Countercurrent. Using GlobColour ocean color data and Aviso altimetry data, an investigation of the area found that a seasonal reversal occurs in the character of the chlorophyll anomaly within eddies (reversal from positive to negative and vice versa). The cause of this reversal is inferred to be a seasonally changing limiting factor within the region. Argo float profiles colocated inside and outside of eddies are used to show the coincidence of chlorophyll anomalies with seasonally changing mixed layer depths and the ability of the eddies to access deep nutrient pools. Observations of other mechanisms, such as eddy stirring or eddy‐Ekman pumping, are found to be seasonally less important than the mixed layer depth change‐induced nutrient flux . Additionally, metrics are developed to globally identify oceanic regions in which such seasonal reversals in chlorophyll anomalies could occur.

Seasonality of the Kuroshio intensity east of Taiwan modulated by mesoscale eddies

Abstract The intensity of the Kuroshio Current east of Taiwan shows a clear seasonality, and the Kuroshio is affected by mesoscale eddies east of Taiwan. Sea surface geostrophic velocity and mesoscale eddies derived from satellite altimeter data and gridded Argo-only geostrophic velocity data were used to analyze the mesoscale eddy contribution to the Kuroshio intensity seasonality in the East Taiwan Channel. The low potential vorticity water stacked at the northern side of the subtropical countercurrent (STCC) and eddy generation favored the cyclonic eddy. Between August and January, the STCC near the western boundary area (125°E-135°E) showed two branches and a significant negative zonal velocity meridional gradient between 19°N and 21°N from October to December, which led to a higher occurrence of cyclonic eddies. These eddies reached the area east of Taiwan in 100 days and resulted in the dominance of cyclonic eddies between January and April and a weakened Kuroshio Current.

Intraseasonal-to-Interannual Variability of the Upper-Layer Zonal Currents in the Tropical Northwest Pacific Ocean

The upper-layer circulation in the Tropical Northwest Pacific Ocean (TNWP) is characterized by the alternating zonal currents, such as North Equatorial Current (NEC) and Subtropical Countercurrent (STCC). The variabilities of the upper-layer zonal currents in the TNWP, which were obtained directly from 75 kHz acoustic Doppler current profiler (ADCP) measurements and indirectly from geostrophic calculations, were investigated on intraseasonal to interannual timescales from 2006 to 2014. Surface geostrophic current derived from sea surface height anomaly (SSHA) approximated well to the in-situ measurements with ADCP that was slightly better than the absolute geostrophic current obtained from AVISO. The interannual signals related to the El Nino-Southern Oscillation (ENSO) were quite apparent, especially near the boundary (about 17.5°N–19°N) between NEC and STCC. We found that the eastward-flowing southern STCC develops during the decaying phase of ENSO and weakens during La Nina. Strong easterlies during strong La Nina (e.g., 2010 La Nina event) seem to influence NEC to expand further north to the southern STCC region and this coincides with the meridional migration of the Sverdrup streamfunction. The southern STCC region is profoundly influenced by the mesoscale/sub-mesoscale eddy activities, which can intensify the eastward flow of STCC and ultimately contribute to the intraseasonal signal. We found that this intraseasonal signal weakened when La Nina happened in 2010, which is presumably associated with the northward expansion of NEC. Possible factors related to the discrepancies between surface geostrophic current and sub-surface ADCP data are discussed.

Effect of larval swimming in the western North Pacific subtropical gyre on the recruitment success of the Japanese eel.

The possible effect of directional larval swimming on the recruitment success of the Japanese eel, Anguilla japonica, was examined with a three-dimensional particle-tracking ocean circulation model using horizontal northwestward swimming and diel vertical migration (DVM). Four separate experiments included virtual larvae (v-larvae) movement from the spawning area over 290 days (total migration) and 160 days (stage A), from the STCC eddy region in 70 days (stage B), and from the origin of the Kuroshio in 60 days (stage C) to evaluate the effect of directional swimming and DVM compared to simple drifting. Passive or random swimming were not the most effective strategies for larvae dispersing from the spawning area because most v-larvae remained south of 20°N without entering the Kuroshio. Northwestward swimming resulted in wider dispersion and a better chance of successful recruitment, with v-larvae becoming widely distributed in the STCC eddy zone, arriving at the east coast of the Philippines (stage A), escaping the STCC eddy area and reaching the Kuroshio (stage B), and crossing the Kuroshio into the East China Sea shelf (stage C). DVM slightly shortened the migration period due to faster shallow layer ocean currents during nighttime. The NEC transported non-swimming v-larvae westward to the Kuroshio and occasionally northward into the Subtropical Countercurrent (STCC) area where eddies transported v-larvae westward into the Kuroshio, but less than with swimming. Directional swimming increased recruitment success, northwestward swimming was more effective than other directions, and a slower swimming speed was still better than no/random swimming in sensitivity tests. The present study demonstrated a first view of the possibility that Japanese eel larvae might be able to use a strategy of single-direction swimming to increase arrival at their recruitment areas.

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.

Subtropical countercurrent variations in cooling climates induced by freshwater forcing over the subarctic North Atlantic

The subtropical countercurrent (STCC) could affect ocean eddy activity and the atmosphere above. How the STCC varied in a cooling scenario, such as the stadials during the last deglaciation (19–11 ka; ka = 1000 years ago), is still unclear. By 1-Sv (1 Sv = 106 m3 s− 1) freshwater hosing over the subpolar North Atlantic to suppress the Atlantic Meridional Overturning Circulation (AMOC), the STCC variations under pre-industrial (PI) and last glacial maximum (LGM, ~ 21 ka) conditions are analyzed. In both hosing experiments, the northern hemisphere cools. The upper ocean becomes less stratified, and the wintertime mixed layer deepens in the Kuroshio–Oyashio extension (KOE) region. As a result, more mode waters (~ 20%) are formed, and they become more vertically uniform. The enhanced mode waters induce a stronger vertical shear and accelerate the STCC based on thermal wind relationship. The strengthened STCC further induces warm ocean temperature and upward air-sea heat flux anomalies along it. While the STCC strengthens in both hosing experiments, the location variations are different. In the PI hosing experiment, the Bering Strait is open. Fresh water enters the North Pacific through the Bering Strait and increases the stratification in the downstream of the KOE region. The mixed layer front shifts westward and induces a robust westward shift of mode waters and the STCC. In the LGM hosing experiment, on the contrast, there is no obvious location shift because the Bering Strait is closed.

Decadal Variability of Eddy Kinetic Energy in the South Pacific Subtropical Countercurrent in an Ocean General Circulation Model

AbstractThe eddy kinetic energy (EKE) associated with the Subtropical Countercurrent (STCC) in the western subtropical South Pacific is known to exhibit substantial seasonal and decadal variability. Using an eddy-permitting ocean general circulation model, which is able to reproduce the observed, salient features of the seasonal cycles of shear, stratification, baroclinic production, and the associated EKE, the authors investigate the decadal changes of EKE. The authors show that the STCC region exhibits, uniquely among the subtropical gyres of the world’s oceans, significant, atmospherically forced, decadal EKE variability. The decadal variations are driven by changing vertical shear between the STCC in the upper 300 m and the South Equatorial Current below, predominantly caused by variations in STCC strength associated with a changing meridional density gradient. In the 1970s, an increased meridional density gradient results in EKE twice as large as in later decades in the model. Utilizing sensitivity experiments, decadal variations in the wind field are shown to be the essential driver. Local wind stress curl anomalies associated with the interdecadal Pacific oscillation (IPO) lead to upwelling and downwelling of the thermocline, inducing strengthening or weakening of the STCC and the associated EKE. Additionally, remote wind stress curl anomalies in the eastern subtropical South Pacific, which are not related to the IPO, generate density anomalies that propagate westward as Rossby waves and can account for up to 30%–40% of the density anomalies in the investigated region.

Zonal migration and transport variations of the Kuroshio east of Taiwan induced by eddy impingements

Variability of the Kuroshio east of Taiwan was observed at a cross-stream transect ~ 50km south of the PCM-1 line with an array of three moored ADCPs measuring for ~ 23 months, supplemented with eleven repeated shipboard surveys. Observations of the Kuroshio's velocity structure reveal the absence of an obvious regular seasonal signal, but significant variability at 70–200 day period for both maximum velocity axis migration and transport due to interactions with mesoscale eddies. Empirical orthogonal function (EOF) analysis shows the migration and transport modes explain 46% and 29% of the total variance, respectively, which is in contrast to the findings at the PCM-1 line where the transport mode explained more variance than did the migration mode. The Kuroshio transport in the upper 500m across a 150km section is 17.2Sv with a standard deviation of 5Sv. The estimated Kuroshio transport is 4.3Sv lower than that reported for the PCM-1 line, likely due to the interannual variations related to abundance of mesoscale eddies in the Subtropical Counter Current (STCC) region. Transport variability east of Taiwan is mostly caused by Kuroshio-eddy interactions. When single anticyclonic (cyclonic) eddies encounter the Kuroshio, they enhance (reduce) poleward transport, presumably by increasing (decreasing) the sea level anomaly (SLA) along the eastern flank of the Kuroshio (correlation = 0.82). When a pair of eddies impinges on the Kuroshio, the upstream confluence and diffluence caused by the dipole eddies increases and decreases the Kuroshio transport, respectively. Furthermore, the eastward (westward) currents that result from either the single eddy or the dipole eddy produce flow divergence (convergence) adjacent to the Kuroshio's eastern edge, favoring the offshore (onshore) migration of the Kuroshio axis.

Eddy properties in the Subtropical Countercurrent, Western Philippine Sea

An array of six oceanographic moorings with acoustic and environmental sensors was deployed in the central Philippine Sea from April 2010 to April 2011. The location spanned 18−23°N, 124 – 130°E in the Subtropical Countercurrent (STCC). The most prominent feature of the data set was a densely-packed eddy field with about equal numbers of cyclones and anticyclones moving westward at 6–12kmd−1. Eddies of either sign displaced the thermocline about ±50m and had surface velocities exceeding 110cms−1. While warm eddies were slightly larger than cold eddies, the distance to maximum radial velocity was similar for both at about 65km, close to the local Rossby radius of deformation. The steepness parameter U/c in the eddies ranged from 3 to 10, accompanied by relative vorticity of order 0.1–0.3f, suggesting nonlinear, quasigeostrophic features with trapped cores rather than linear waves. This was borne out by the water mass analysis which showed high salinity, high spice North Pacific Tropical Water (NPTW) being transported westward in the warm eddy cores. The total KE and APE in eddies of both signs was about 1×1015J with 85% of the APE and 74% of the KE located above 250m depth. This equipartitioning of energy suggests mature eddies near equilibrium, that had been evolving for some time as they propagated into the area from the east.