A review on the South China Sea western boundary current

The advances in understanding the South China Sea （SCS） western boundary current （SCSwbc） have been reviewed since the works of Dale （1956） and Wyrtki （1961） in the middle of the 20th century. The features of the pattern of SCSwbc and the oceanic phenomena associated with it are focused on. The current is driven mainly by monsoon over the SCS and partially by winds over the tropical Pacific governed by the island rule. The SCSwbc exhibits strong seasonal variation in its direction and patterns. In winter, the current is strong and flows southwestward along the South China shelf and slope from the east of Dongsha Islands to the northern central Vietnamese coast, then turns to the south along the central and southern Vietnamese coast, and finally partially exits the SCS through the Karimata Strait. In summer and early fall, the SCSwbc can be divided into three segments based on their characteristics. The southern segment is stable, flowing northward from the Karimata Strait up to about 11 N, where it separates from the coast forming an eastward offshore current. The separation of the current from Vietnamese coast induces some striking features, such as upwelling and cold sea-surface temperature. The middle segment off the central Vietnamese coast may have a bimodal behavior： northward coastal current and meandering current in early summer （June–July）, and cyclonic gyre in later summer and early fall （August–September）. The northern segment is featured by the summer SCS Warm Current on the South China shelf and a southwestward subsurface current along the continental slope.

Impact of the South China Sea Throughflow on the Pacific Low-Latitude Western Boundary Current:A Numerical Study for Seasonal and Interannual Time Scales

Prior studies have revealed that,as a part of the Pacific tropical gyre,the South China Sea throughflow（SCSTF） is strongly influenced by the Pacific low-latitude western boundary current（LLWBC）.In this study,ocean general circulation model（OGCM） experiments with and without connection to the South China Sea（SCS） were performed to investigate the impact of the SCSTF on the Pacific LLWBC.These model experiments show that if the SCS is blocked,seasonal variability of the Kuroshio and Mindanao Current becomes stronger,and the meridional migration of the North Equatorial Current（NEC） bifurcation latitude is enhanced.Both in seasonal and interannual time scales,stronger Luzon Strait transport（LST） induces a stronger Kuroshio transport combined with a southward shift of the NEC bifurcation,which is unfavorable for a further increase of the LST;a weaker LST induces a weaker Kuroshio transport and a northward shifting NEC bifurcation,which is also unfavorable for the continuous decrease of the LST.

The winter western boundary current of the South China Sea:physical structure and volume transport in December 1998

The unique survey in December 1998 mapped the entire western boundary area of the South China Sea（SCS）,which reveals the three-dimensional structure and huge volume transport of the swift and narrow winter western boundary current of the SCS（SCSwwbc） in full scale. The current is found to flow all the way from the shelf edge off Hong Kong to the Sunda Shelf with a width around 100 km and a vertical scale of about 400 m. It appears to be the strongest off the Indo-China Peninsula, where its volume transport reached over 20×10~6 m^3/s. The current is weaker upstream in the northern SCS to the west of Hong Kong. A Kuroshio loop or detached eddy intruded through the Luzon Strait is observed farther east where the SCSwwbc no more exists. The results suggest that during the survey the SCSwwbc was fed primarily by the interior recirculation of the SCS rather than by the＂branching＂ of the Kuroshio from the Luzon Strait as indicated by surface drifters, which is likely a near-surface phenomenon and only contributes a minor part to the total transport of the SCSwwbc. Several topics related to the SCSwwbc are also discussed.

Net-phytoplankton communities in the Western Boundary Currents and their environmental correlations

This study investigated net-phytoplankton biomass, species composition, the phytoplankton abundance horizontal distribution, and the correlations between net-phytoplankton communities and mesoscale structure that were derived from the net samples taken from the Western Boundary Currents during summer, 2014. A total of 199 phytoplankton species belonging to 61 genera in four phyla were identified. The dominant species included C limacodium frauenfeldianum, Thalassiothrix longissima, Rhizosolenia styliformis var. styliformis, Pyrocystis noctiluca, Ceratium trichoceros, and Trichodesmium thiebautii. Four phytoplankton communities were divided by cluster analysis and the clusters were mainly associated with the North Equatorial Counter Current(NECC), the North Equatorial Current(NEC), the Subtropical Counter Current(STCC), and the Luzon Current(LC), respectively. The lowest phytoplankton cell abundance and the highest T richodesmium filament abundance were recorded in the STCC region. The principal component analysis showed that T. thiebautii preferred warm and nutrient poor water. There was also an increase in phytoplankton abundance and biomass near 5°N in the NECC region, where they benefit from upwellings and eddies.

Reversal process of the South China Sea western boundary current in autumn 2011

Using merged sea level anomaly and absolute geostrophic velocity products from satellite altimetry and Argos drifter data, we analyzed the reversal process of the South China Sea （SCS） westem boundary current （SCSwbc） from a summer to winter pattern in 2011 and important oceanic phenomena during this process. Results show that the outbreak time of the northeast monsoon over the southern SCS lagged that over the northern SCS by about 1 month. During the SCS monsoon reversal period, the SCSwbc reversed rapidly into the winter pattern at the Guangdong continental slope in late September. Subsequently, the southward Vietnam coastal boundary current strengthened. However, the northward Natuna Current maintained a summer state until mid-October. Thus, the balance between the southward and northward currents was lost when they met, their junction moved gradually southward. However, a loop current formed southeast of Vietnam because the main stream of the Vietnam Offshore Current （VOC） remained near its original latitude, Meanwhile, the VOC and associated dipole circulation system strengthened. After mid- October, the northward Natuna Current began to weaken, the loop current finally shed, becoming a cool ring. The VOC and its associated dipole sub-basin circulation system also weakened gradually until it disappeared.

Multi-decadal trends in the tropical Pacific western boundary currents retrieved from historical hydrological observations

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 multidecadal 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.

NONLINEAR DYNAMICAL ANALYSIS OF BIFURCATION AND CONFLUENCE OF THE PACIFIC WESTERN BOUNDARY CURRENTS

In this paper, we analyze the bifurcation and the confluence of the Pacific western boundary currents by an analytical approach. Applying the conservation law, the geostrophic balance relation and the Bernoulli integral to a reduced gravity model, we get a quantitative relation for the outflow and the inflow, and establish the related formulae for the width and the veering angle of offshore currents under the inflow condition. Furthermore, a comparison between the volume transport based on the observation data and the analytical value for the Pacific western boundary currents is presented, which validates the theoretical analysis.

On the Instabilities and Transitions of the Western Boundary Current

We study the stability and dynamic transitions of the western boundary currents in a rectangular closed basin.By reducing the infinite dynamical system to a finite dimensional one via center manifold reduction,we derive a non-dimensional transition number that determines the types of dynamical transition.We show by careful numerical evaluation of the transition number that both continuous transitions(supercritical Hopf bifurcation)and catastrophic transitions(subcritical Hopf bifurcation)can happen at the critical Reynolds number,depending on the aspect ratio and stratification.The regions separating the continuous and catastrophic transitions are delineated on the parameter plane.

The western South China Sea currents from measurements by Argo profiling floats during October to December 2007

The three dimensional structure of the western boundary current east of the Vietnam coast was determined from measurements by Argo profiling floats which deployed near the east of the Vietnam Coast in October 2007. The trajectories of the Argo floats provided robust evidence that there does exist southward flowing current along the Vietnam coast. The southward current begins at about 15°N, 111°E, flowing along the 1 000 m isobath and extending to 5°N south. The estimated surface and parking depth velocities obtained from the floats suggest that this southward current can extend to 1 000 m depth. The mean surface velocity of the western boundary current is about 49 cm/s, with the maximum speed exceeding 100 cm/s occurring at 11.6°N, 109.5°E in the direction of 245°. The mean parking depth （1 000 m） velocity is 12-16 cm/s with the maximum speed of 36 cm/s occurring at 12.1°N, 109.7°E in the direction of 239°. The water mass analysis suggests that the Kuroshio surface water and NPIW are not apparent in the western boundary current although this time was the favored season for the Kuroshio intrusion into Luzon Strait. The mean geostrophic currents suggest that the Kuroshio intrusion into Luzon Strait during October to December 2007 is very weak, as most of the intruding water was carried eastward at 14°N near the western boundary and little flowed southward along the western boundary, providing an explanation for the absence of Kuroshio surface water and NPIW in the water mass analysis. There is a strong cyclonic circulation in the SSCS during this time, which induces a strong mixing in the western boundary companied by a homogeneous salinity layer between 300 600 m in the salinity vertical distribution. No reversal undercurrent occurred at the intermediate depth along the western boundary east of the Vietnam coast during October to December 200%

Circulation and Heat Flux along the Western Boundary of the North Pacific

On the basis of the conductivity temperature depth(CTD)observation data off the coast of the Philippines(7.5°–18°N,130°E–the east coast of the Philippines)in the fall of 2005,the water mass distribution,geostrophic flow field,and heat budget are examined.Four water masses are present:the North Pacific Tropical Surface Water,the North Pacific Sub-surface Water,the North Pacific Intermediate Water,and the Antarctic Intermediate Water(AAIW).The previous three corresponded with the North Equatorial Current(NEC),the Kuroshio Current(KC),and the Mindanao Current(MC),respectively.AAIW is the source of the Mindanao Undercurrent.The mass transport of NEC,KC,and MC is 58.7,15,and 27.95Sv,respectively(relative to 1500db).NEC can be balanced by the transport across the whole transect 18°N(31.81 Sv)and 7.5°N(26.11 Sv)but not simply by KC and MC.Direct calculation is used to study the heat flux.In sum,1.45PW heat is transported outwards the observed region,which is much more than that released from the ocean to the air at the surface(0.05PW).The net heat lost decreased the water temperature by 0.75℃each month on average,and the trend agreed well with the SST change.Vertically,the heat transported by the currents is mainly completed in the upper 500 m.

Circulation in the South China Sea is in a state of forced oscillation:Results from a simple reduced gravity model with a closed boundary

The South China Sea(SCS)is a narrow semi-enclosed basin,ranging from 4°–6°N to 21°–22°N meridionally.It is forced by a strong annual cycle of monsoon-related wind stress.The Coriolis parameter f increases at least three times from the southern basin to the northern basin.As a result,the basin-cross time for the first baroclinic Rossby wave in the southern part of the basin is about 10-times faster than that in the northern part,which plays the most vitally important role in setting the circulation.At the northernmost edge of SCS,the first baroclinic Rossby wave takes slightly less than 1 year to move across the basin,however,it takes only 1–2 months in the southernmost part.Therefore,circulation properties for a station in the model ocean are not solely determined by the forcing at that time instance only;instead,they depend on the information over the past months.The combination of a strong annual cycle of wind forcing and large difference of basin-cross time for the first baroclinic Rossby wave leads to a strong seasonal cycle of the circulation in the SCS,hence,the circulation is dominated by the forced oscillations,rather than the quasi-steady state discussed in many textbooks.The circulation in the SCS is explored in detail by using a simple reduced gravity model forced by seasonally varying zonal wind stress.In particular,for a given time snap the western boundary current in the SCS cannot play the role of balancing mass transport across each latitude nor balancing mechanical energy and vorticity in the whole basin.In a departure from the steady wind-driven circulation discussed in many existing textbooks,the circulation in the SCS is characterized by the imbalance of mechanical energy and vorticity for the whole basin at any part of the seasonal cycle.In particular,the western boundary current in the SCS cannot balance the mass,mechanical energy,and vorticity in the seasonal cycle of the basin.Consequently,the circulation near the western boundary cannot be interpreted in terms of the wind stress and thermohaline forcing at the same time.Instead,circulation properties near the western boundary should be interpreted in terms of the contributions due to the delayed wind stress and the eastern boundary layer thickness.In fact,there is a clear annual cycle of net imbalance of mechanical energy and vorticity source/sink.Results from such a simple model may have important implications for our understanding of the complicated phenomena in the SCS,either from in-situ observations or numerical simulations.

Interannual variability of eddy kinetic energy in the South China Sea related to two types of winter circulation events

Interannual variations of the eddy kinetic energy(EKE)related to two types of winter circulation events(“O”and“U”)were investigated based on the outputs of the Ocean General Circulation Model(OGCM)for the Earth Simulator(OFES)and the corresponding energetic analyses.Results show that the EKE is strong and extends eastward to offshore the Vietnam coast about 2°,associated with the weaker South China Sea western boundary current(SCSwbc)in“O”type years,while the EKE is weak and high value that can be attained is narrowed along the coast,associated with the stronger SCSwbc in“U”type years.The energy budget shows that the wind stress and barotropic/baroclinic instability are important factors to regulate the EKE in“U”and“O”years.For“U”years,under a strong winter monsoon forcing,the SCSwbc strengthen,the directly wind work and barotropic conversion from the mean kinetic energy(MKE)to EKE are weak,thus the EKE decrease corresponding to the baroclinic conversion from the kinetic energy to potential energy.However,the situation is reversed in“O”years.Under the influence of El Niño events,wind stress forces can weaken SCSwbc and enhance EKE in pattern“O”,whereas La Niña events have relatively weaker influences.The barotropic conversion rate in“O”type is nearly eight times of the“U”type.The pressure work and advection term are the main sources to greatly suppress EKE in the SCSwbc region.

An extended variable-grid global ocean circulation model and its preliminary results of the equatorial Pacific circulation

To investigate the interaction between the tropical Pacific and China seas a variable-grid global ocean circulation model with fine grid covering the area from 20°S to 50°N and from 99° to 150°E is developed. Numerical computation of the annually cyclic circulation fields is performed. The results of the annual mean zonal currents and deep to abyssal western boundary currents in the equatorial Pacific Ocean are reported. The North Equatorial Current,the North Equatorial Countercurrent, the South Equatorial Current and the Equatorial Undercurrent are fairly well simulated. The model well reproduces the northward flowing abyssal western boundary current.From the model results a lower deep western boundary current east of the Bismarck-Solomon-New Hebrides Island chain at depths around 2 000 m has been found. The model results also show that the currents in the equatorial Pacific Ocean have multi-layer structures both in zonal currents and western boundary currents, indicating that the global ocean overturning thermohaline circulation appears of multi-layer pattern.

Wave Effect on the Ocean Circulations Through Mass Transport and Wave-Induced Pumping

The wave Coriolis-Stokes-Force-modified ocean momentum equations are reviewed in this paper and the wave Stokes transport is pointed out to be part of the ocean circulations. Using the European Centre for Medium-Range Weather Forecasts 40-year reanalysis data(ERA-40 data) and the Simple Ocean Data Assimilation(SODA) version 2.2.4 data, the magnitude of this transport is compared with that of wind-driven Sverdrup transport and a 5-to-10-precent contribution by the wave Stokes transport is found. Both transports are stronger in boreal winter than in summers. The wave effect can be either contribution or cancellation in different seasons. Examination with Kuroshio transport verifies similar seasonal variations. The clarification of the efficient wave boundary condition helps to understand the role of waves in mass transport. It acts as surface wind stress and can be functional down to the bottom of the ageostrophic layer. The pumping velocities resulting from wave-induced stress are zonally distributed and are significant in relatively high latitudes. Further work will focus on the model performance of the wave-stress-changed-boundary and the role of swells in the eastern part of the oceans.

On the Importance of High-Resolution in Large-Scale Ocean Models

Eddying global ocean models are now routinely used for ocean prediction,and the value-added of a better representation of the observed ocean variability and western boundary currents at that resolution is currently being evaluated in climate models.This overview article begins with a brief summary of the impact on ocean model biases of resolving eddies in several global ocean-sea ice numerical simulations.Then,a series of North and Equatorial Atlantic configurations are used to show that an increase of the horizontal resolution from eddy-resolving to submesoscale-enabled together with the inclusion of high-resolution bathymetry and tides significantly improve the models’abilities to represent the observed ocean variability and western boundary currents.However,the computational cost of these simulations is extremely large,and for these simulations to become routine,close collaborations with computer scientists are essential to ensure that numerical codes can take full advantage of the latest computing architecture.

Regime shift of the dominant factor for halocline depth in the Canada Basin during 1990–2008

The World Ocean Database（WOD） is used to evaluate the halocline depth simulated by an ice-ocean coupled model in the Canada Basin during 1990–2008. Statistical results show that the simulated halocline is reliable.Comparing of the September sea ice extent between simulation and SSM/I dataset, a consistent interannual variability is found between them. Moreover, both the simulated and observed September sea ice extent show staircase declines in 2000–2008 compared to 1990–1999. That supports that the abrupt variations of the ocean surface stress curl anomaly in 2000–2008 are caused by rapid sea ice melting and also in favor of the realistic existence of the simulated variations. Responses to these changes can be found in the upper ocean circulation and the intermediate current variations in these two phases as well. The analysis shows that seasonal variations of the halocline are regulated by the seasonal variations of the Ekman pumping. On interannual time scale, the variations of the halocline have an inverse relationship with the ocean surface stress curl anomaly after 2000,while this relationship no longer applies in the 1990 s. It is pointed out that the regime shift in the Canada Basin can be derived to illustrate this phenomenon. Specifically, the halocline variations are dominated by advection in the 1990 s and Ekman pumping in the 2000 s respectively. Furthermore, the regime shift is caused by changing Transpolar Drift pathway and Ekman pumping area due to spatial deformation of the center Beaufort high（BH）relative to climatology.

A modification to the Munk wind-driven ocean circulation theory

In order to fulfill the no-slip condition at the western and eastern boundaries of the ocean basin, introduced ＂effective wind stress＂, which has much larger spatial variations towards the boundaries than in the ocean interior. The effective wind stress can thus be decomposed into spatially slow-varying and fast varying components. Careful scale analysis on the classical Munk winddriven ocean circulation theory, which consists of the interior Sverdrup flow and the western boundary current but of no eastern boundary current, shows that the wind stress curl appearing in the Sverdrup equation must have negligible spatial variations. In the present model the spatially slow-varying component of the wind stress appears in the Sverdrup equation, and the spatially fastvarying component becomes the forcing term of the boundary equations. As a result, in addition to the classical Munk solution the present model has an extra term at the western boundary which （Northern Hemisphere） increases the northward transport as well as the southward return transport, and has a term at the eastern boundary corresponding to the eastern boundary current.