South China Sea Western Boundary Research Articles

Drifter-Observed Reversal of the South China Sea Western Boundary Current From Summer to Autumn

Abstract This study analyzes the formation, reversal, and variation of the South China Sea Western Boundary Current (SCSWBC) by nine trajectories of Beidousatellite-tracked drifters, combined with reanalysis current data and satellite remote sensing wind data. It is indicated that: (1) the maximum current speeds of Drifters 1490016, 1490026, 1488526, 862259, and 927745 exceed 1.5 m/s, with the speed of Drifter 862259 even reaching 2.3 m/s in the strong current region of the SCSWBC; (2) the SCSWBC in autumn can be sourced from either the northeastward current in the central South China Sea in summer or the westward current in the northeastern South China Sea in September; (3) the monsoon wind transition is attributed for the formation, disappearance, and reversal of the SCSWBC from summer to autumn; and (4) the interannual variation of the SCSWBC mainly depends on the transition time of the monsoon wind.

Phase Shift of the Winter South China Sea Western Boundary Current Over the Past Two Decades and Its Drivers

AbstractThe winter South China Sea (SCS) western boundary current (WBC) has presented a significant weak–strong–weak phase shift in the last two decades, as revealed by observations and model products. Its phase shift has mainly been attributed to the joint effect of wind stress curl over the SCS and Luzon Strait transport (LST). Changes in the wind stress curl and planetary vorticity input through the Luzon Strait are the two major sources maintaining the winter SCS WBC state in different phases. The LST is closely related to the Kuroshio strength and thus the Pacific North Equatorial Current (NEC) bifurcation. Therefore, the winter SCS WBC phase shift is attributed to phase shifts of the winter monsoon and the NEC bifurcation, which are mostly related to those of the Indian Ocean Dipole Mode and Atlantic Multidecadal Oscillation, respectively. Understanding the SCS circulation low‐frequency variability is essential to forecast its thermodynamic state and regional climate effect.

Winter–Summer Transition in the Southern South China Sea Western Boundary Current

Abstract The winter–summer transition in the southern South China Sea (SCS) western boundary current (WBC) is studied. Two categories have been identified. In case 1, the southern SCS WBC transition in the lower layer (below the thermocline) lags that in the upper layer (above the thermocline). In case 2, there is no transition lag at full depth. In both categories, the geostrophic balance dominates the transition. In case 1, the upper layer geostrophic balance is dominated by the sea surface height pressure gradient (SSHPG) and Coriolis forcing during southern SCS WBC transition. Therefore, there is no transition lag with depth in the upper layer. Below the thermocline layer, the competition between the SSHPG and the density pressure gradient (DPG) determines the transition. During the transition, the amplitudes of the SSHPG and DPG are basically equivalent. The SSHPG needs time to develop sufficiently larger than the DPG. Therefore, the transition in the deeper layer significantly lags that in the shallower layer. The reversal of the SSHPG is mainly attributed to the change in the basin-scale wind stress curl over the southern SCS. The change in the DPG is mainly associated with the cooling of the water along the western continental slope, which is induced by upwelling. In case 2, there is no cooling along the western continental slope, and then the amplitude of the DPG is always far smaller than that of the SSHPG. Responding to the change in the SSHPG, the southern SCS WBC transition behaves consistently at full depth. Significance Statement We have a comprehensive understanding of the South China Sea (SCS) circulation patterns in winter and summer. However, their seasonal transitions remain unclear, and a better understanding of them is potentially helpful for improving ocean circulation modeling and prediction. This paper focuses on the winter–summer transition in the SCS western boundary current (WBC). Above the thermocline (∼100 m), the transition behaves consistently in the vertical direction and is controlled by the conversion of the sea surface height–induced pressure gradient. Below the thermocline, the transition in the deeper layer of the WBC significantly lags that in the shallower layer of the WBC, which is associated with the competition between the SSH-induced pressure gradient and the density-induced pressure gradient at the sea surface.

Variation of Internal Solitary Wave Propagation Induced by the Typical Oceanic Circulation Patterns in the Northern South China Sea Deep Basin

AbstractLarge‐scale circulations are quite typical processes in the northern South China Sea (SCS) deep basin, yet their impacts on the variation of internal solitary waves (ISWs) remains poorly understood. We, here, focus on impacts of the upstream SCS western boundary current (SCSwbc) and three typical circulation patterns associated with different Kuroshio intruding paths on ISW propagation in this region. We show that the ISW modulated by the upstream SCSwbc gets a speedup, which is comparable to that induced by Coriolis effect, while the ISW amplitude shows an obvious reduction. Statistically, there is approximately one‐third time of 23 years (1993–2015) when circulation‐induced changes of wave speed exceed Coriolis‐induced changes. Specifically, the looping circulation pattern has the highest impact on significant speedup and amplitude reduction among the three patterns, while the leaping pattern has the lowest impact. These differences in wave characteristics caused by different circulation patterns result from the wave scattering, focusing and Doppler effects.

Characterizing meso- to submesoscale features in the South China Sea

In this study the mesoscale and submesoscale features in the South China Sea (SCS) are characterized based on a submesoscale-permitting numerical simulation (MITgcm llc4320) with nominal horizontal grid spacing of 1/48° (~2.2 km in the SCS). The simulation includes tidal forcing, which is particularly crucial in the SCS where tides play a key role in many aspects of regional dynamics. Evaluation against observations demonstrates that the simulation is capable of reproducing a wide spectrum of dynamical features in the SCS. The simulation is then used to characterize meso- to submesoscale features in the SCS and the key findings are summarized as follows: (i) the SCS is rich in submesoscale features (e.g., vortices, filaments), which undergo significant seasonal variations in many regions; (ii) there is a clear dominance of positive surface relative vorticity (ζ) in most regions of the SCS presumably due to the centrifugal instability, but large negative ζ also emerges in areas with strong flow-topography interactions; (iii) the vertical motions in the upper SCS are dominated by tidal modulations with strong vertical velocities up to 0.01 m s−1, about an order of magnitude larger than those due to the quasi-balanced sub-inertial motions; and (iv) both the wintertime and summertime horizontal wavenumber spectra of kinetic energy scale roughly as k−2 (with k being the horizontal wavenumber) for wavelengths below 100 km. In addition, four key regions are highlighted with distinctive characteristics of submesoscale turbulence, including the northern SCS, Luzon Strait, the SCS western boundary current region in winter, and east of Vietnam in summer. A number of model deficiencies are noted, along with suggestions for improving model accuracy in order to better characterize the local submesoscale features as well as the underlying dynamics.

Interannual variability of South China Sea winter circulation: response to Luzon Strait transport and El Niño wind

The El Nino wind and Luzon Strait transport are important factors modulating the interannual variability of the southern and northern South China Sea (SCS) winter circulation, respectively. The joint effect of El Nino wind and westward Luzon Strait transport drives a dipolar gyre with an anticyclonic (cyclonic) circulation anomaly in the southern (northern) SCS, which enhances the cross-basin current that connects the SCS western boundary current with the eastern boundary of the SCS around the Mindoro Strait, and then effectively modulates the interannual variability of Mindoro Strait transport. An extreme El Nino and large Luzon Strait intrusion occurred in winter 2015/16, and mooring observations and remote sensing data confirmed the presence of an intense dipolar gyre. This resulted in an extreme eastward cross-basin current and outward SCS Mindoro Strait transport anomaly in 2015/16.

Buoyancy Effect on the Winter South China Sea Western Boundary Current

AbstractThe month‐to‐month variation of the winter South China Sea (SCS) western boundary current (WBC) along the western slope is examined using drifter observations, satellite altimetry data, and an ocean reanalysis. The most surprising phenomenon is that the WBC velocity at the sea surface reaches the maxima in November–December, which cannot be explained by wind forcing and Kuroshio intrusion alone. Analysis results demonstrate that buoyancy effect should be considered to explain the month‐to‐month variation besides wind‐Kuroshio effects. In winter, cold‐and‐salty advection by the WBC from the north decreases/reverses the zonal density gradient in the seasonal pycnocline induced by wind forcing and Kuroshio intrusion and therefore weakens wind‐Kuroshio‐induced WBC. Buoyancy effect on the winter SCS WBC is opposite to wind‐Kuroshio effects. In addition, buoyancy effect reaches the maximum in January, which is concurrent with wind‐Kuroshio effects. As a result of their competition, the zonal density gradient in the seasonal pycnocline is maximum in November–December, resulting in the maximum surface velocity along the western slope occurring in November‐December. This study demonstrates the importance of buoyancy forcing to the winter SCS WBC.

Geological and oceanographic controls on seabed fluid escape structures in the northern Zhongjiannan Basin, South China Sea

Mega-pockmarks, which can reach a few kilometers in width and a few hundred meters in depth, and associated fluid escape features in the Northern Zhongjiannan Basin, South China Sea, have received much attention in the recent years, due to their large variety of morphologies and sizes. In this work, an integrated interpretation of detailed multibeam bathymetry, conventional multi-channel seismic reflection and seismic oceanography sections covering the northern Zhongjiannan Basin, South China Sea, are used to image the seafloor morphology, the subsurface geology and the water column, in order to better understand the geological and oceanographic controls on the formation and evolution of the widespread fluid escape features in this region. Regional uplift and volcanism took place in this area near the Palaeogene–Neogene boundary and, during the Neogene–Quaternary post-rift thermal subsidence period, differential subsidence and diapirism deformed the overlying kilometer-scale successions. Mainly after the Miocene, focused flow of fluids originated at depth occurred along gas pipes, polygonal faults, tapered reflectors and faults into the shallow sediments, resulting in intense fluid blowouts and complex fluid escape structures on the seafloor. Our results show that elongated pockmarks, pockmark gullies and gullies in the northern slope are generally controlled by underlying buried channels and gullies. However, pockmark gullies and gullies in the southern slope are controlled by gravity sliding/slumping, along with crescent pockmarks developing parallel to water depth contours. To varying degrees, intense fluid escape activities associated with mud diapirism and gas escape in the study area resulted in mud extrusion and the formation of mud volcanoes, complex seabed morphologies and irregular pockmarks. Mega-pockmarks, collapse structures and blind valleys have formed at the apexes of tectonic uplifts related to volcanism and above basement highs, which suggests that these processes played a role in creating pathways for fluid escape. Bottom currents associated with the South China Sea Western Boundary Current, imaged here for the first time with the seismic oceanographic method, mainly flow to the south, and erode the seafloor, making pockmark walls and slopes of gullies and channels in the north steeper than those in the south. Some fluid escape structures, such as the mega-pockmarks and pockmark gullies, are buried by sediment transported by the bottom currents. Turbidity currents flowing downslope along the long pockmark gullies also erode the seafloor and make the NW slopes of the mud volcanoes less steep than those in the SE. Eddies imaged by the seismic sections may also play a role in transporting sediment to the deep sea in this area. Some of the reported fluid escape structures are still active, as indicated by the fluid pathways that reach the seafloor and plumes in the water column imaged by the seismic sections.

Extreme subsurface warm events in the South China Sea during 1998/99 and 2006/07: observations and mechanisms

Conductivity–temperature–depth observations, objectively analyzed data, and model output are used to investigate the variability of subsurface temperature in the South China Sea (SCS) during 1948–2010. Two extreme subsurface warm events are identified during 1998/99 and 2006/07, with no corresponding extreme surface warming except in 1998. Mixed-layer heat budget analysis reveals that the lack of significant heat input from surface net heat flux or from current advection is responsible for the absence of extreme surface warming during 1999, and 2006/07. The surface net heat flux alone cannot explain the first phases of subsurface warming during 1998/99 and 2006/07. Warm advection from the southern SCS in 1998/99 and from the Kuroshio intrusion in 2006/07, induced by anomalous ocean currents, is likely the major contributor to warming of the subsurface water. During the second phase of warming, the surface net heat flux plays a damping role to cool the subsurface water, and the warm advection induced by anomalous SCS western boundary currents from the southern SCS leads to extremely warm subsurface water anomalies. The results show the importance of the Pacific western boundary currents, especially the Kuroshio, in maintaining extreme subsurface warm events in the SCS.

Weakest winter South China Sea western boundary current caused by the 2015–2016 El Niño event

AbstractDuring the winter of 2015–2016, the strongest El Niño event of the twenty‐first century occurred. At the same time, volume transport (VT) time series of the South China Sea western boundary current (SCSWBC) exhibited a minimum value of 3.7 Sv (1 Sv = 1 × 106 m3 s−1) toward the southwest, indicating the weakest strength ever recorded in boreal winter (from November to February). The South China Sea (SCS) cyclonic gyre, inferred from the satellite‐derived surface absolute geostrophic current, was significantly reduced. It was considered that the weakened wind stress curl (negative anomaly) over the SCS resulting from an anticyclone over the Philippine Sea played an essential role. The anticyclone arose from a Rossby‐wave response to a negative sea surface temperature anomaly in the northwest Pacific. This idea is further supported by composite analysis, which shows that during El Niño (La Niña) winter, negative (positive) wind stress curl anomalies prevail in the Philippine Sea and the SCS; thus, the wind stress curl over the SCS is reduced (strengthened), leading to a weaker (stronger) SCS cyclonic gyre and SCSWBC. The mean VT of SCSWBC is 4.7 Sv (5.6 Sv), which is smaller (larger) than 5.2 Sv in normal years. This study provides robust observational evidence from long‐term in situ volume transport monitoring that El Niño can have a significant impact on the SCSWBC through an atmosphere‐bridged teleconnection.

Features and variability of the South China Sea western boundary current from 1992 to 2011

Different from the traditional definition of the South China Sea western boundary current (SCSWBC), in this paper, only the southwestward and southward currents along the northern and western slopes in the SCS, which are closely associated with the basin-wide wind stress curl, are defined as the SCSWBC, while the flows on the southwestern shelf driven directly by the local wind stress are regarded as part of the shelf circulation. Using a new reanalysis dataset of the SCS in conjunction with the in situ and remote sensing data, the main features and variability of the SCSWBC from 1992 to 2011 were studied. Dictated by the prevailing monsoonal winds and in- and outflows, the SCSWBC in winter extended the full length of the western slope and reached its maximum intensity off the southeast coast of Vietnam, while in summer the main body of the SCSWBC was limited to the northern half of the western slope and merged with the northward coastal current to form the Vietnam Offshore Current (VOC) at about 12° N. Moreover, the respective seasonal patterns of the SCSWBC showed pronounced interannual variations in its structure, including the axis, the width, and the maximum depth. The strength of the SCSWBC, with the transport of −11.8 ± 3.5 Sv in winter and −3.0 ± 1.6 Sv in summer off the central coast of Vietnam, also varied significantly from year to year. It was demonstrated that the monsoonal forcing over the SCS, the interannual variability of which was closely associated with El Nino events, played an important role in modulating the interannual variability of the SCSWBC, whereas the influence from the upper-layer Luzon Strait transport was secondary.

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) western 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.

Adaptive observation in the South China Sea using CNOP approach based on a 3‐D ocean circulation model and its adjoint model

AbstractThis study investigates the effect of adaptive (or targeted) observation on improving the midrange (30 days) forecast skill of ocean state of the South China Sea (SCS). A region associated with the South China Sea Western Boundary Current (SCSWBC) is chosen as the “target” of the adaptive observation. The Conditional Nonlinear Optimal Perturbation (CNOP) approach is applied to a three‐dimensional ocean model and its adjoint model for determining the sensitive region. Results show that the initial errors in the sensitive region determined by the CNOP approach have significant impacts on the forecast of ocean state in the target region; thus, reducing these initial errors through adaptive observation can lead to a better 30 day prediction of ocean state in the target region. Our results suggest that implementing adaptive observation is an effective and cost‐saving way to improve an ocean model's forecast skill over the SCS.

Sequence stratigraphic framework of a mixed turbidite-contourite depositional system along the NW slope of the South China Sea

Little is known about the interaction between the deep Pacific Ocean dynamics and the evolving Chinese continental margin at the time of Indonesian Gateway restriction during the late Cenozoic. A sequence stratigraphic approach, which includes characteristic alongslope variations, has been applied to the sedimentary sequences in the northwestern South China Sea (SCS) based on high-resolution multichannel seismic and bathymetric data collected during Cruise R/V SONNE 221 in May 2012. In the present study, a seismostratigraphic analysis of the Pearl River Mouth Basin sedimentary sequence was performed in the southernmost rifted block, the Yitong Ansha. In correlation with the regional lithostratigraphy, this indicates that syn-rift strata deposited from Paleogene to middle Miocene times are heavily deformed by volcanic intrusions emerging from the transitional continental crust. Two satellite basins formed and a leeward regressive-transgressive carbonate shelf ramp developed on the NE flanks of the volcanic intrusions from the middle to the late Miocene. The unconformity related to reef-drowning, interpreted as a third-order sequence boundary (1–10 Ma), is overlain by a lowstand systems tract, which developed when a downslope prograding channel-levee system merged with an upslope migrating wavy sheet. The associated mixed turbidite-contourite depositional system has persisted to the present. The interaction between turbidite and hemipelagite depositional systems, which were overprinted by the SCS Western Boundary Current regime, resulted in the formation of the Yitong Ansha Drift, a giant, elongated and detached, southward-migrating contourite drift. During deposition of the transgressive systems tract, mass transport deposits were funneled through a non-migrating debris-flow channel with levees, which subsequently filled when highstand systems tract deposition began during the early Pleistocene. Contemporaneously, an intensification of bottom current activity resulted in sediment scouring and incision of a contourite moat. Subsidence displaced the margin from subaerial exposure to upper intermediate water downward to lower intermediate water, thereby reducing the upwelling effect associated with trapped flow perturbations around the summits of the volcanic intrusions and which may have changed the flow regime at the bases of their slopes. Therefore, future studies should verify whether the intensification of bottom currents is a result of a mutual feedback between tectonics and oceanography, taking subsidence into different water masses into account, or merely a global oceanographic signal.

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 11A degrees 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.

A general circulation model study of the dynamics of the upper ocean circulation of the South China Sea

The temporal‐spatial structure and the formation mechanisms of the South China Sea (SCS) circulation are studied using the Princeton Ocean Model (POM). The model well reproduces the observed sea surface height (SSH) annual cycle and some current system such as the SCS Western Boundary Current (SCSWBC) system and the Kuroshio Loop Current (KLC). Four sensitivity experiments are carried out to reveal the dynamic mechanisms of the SCS circulation. The results show that most of the seasonal variability of the SCS is controlled predominantly by wind forcing. The Kuroshio affects the mean SSH significantly but contribute little to the variability. The SCSWBC system consists of the SCS Warm Current (SCSWC) and the Vietnam Coastal Current (VCC). In winter the SCSWBC splits into two branches separated at ∼14°–18°N. The northern branch is the SCSWC flowing northward and the southern branch is the VCC flowing southward. In summer, however, the SCSWBC flows unidirectionally northward from the Karimata Strait to the Taiwan Strait. The temporal variation of the SCSWBC, especially the VCC, is determined by the wind forcing over the interior SCS. Buoyancy forcing can strengthen (weaken) the summer (winter) SCSWBC. The KLC is crucial to the SCS circulation north of 18°N. It ultimately determines the appearance of the SCSWC in winter.