Luzon Strait Transport Research Articles

Uncovering Interdecadal Pacific Oscillation’s Dominance in Shaping Low-Frequency Sea Level Variability in the South China Sea

The low-frequency sea level variability in the South China Sea (SCS) is examined using high-resolution regional ocean model simulations that span the last six decades. The analysis reveals interdecadal oscillations with a periodicity of 12–13 years as the dominant mode of sea level variability in the SCS. The fluctuations in the Luzon Strait transport (LST) are identified as primary drivers of interannual to interdecadal sea level variability, rather than atmospheric forcing within the SCS. Fourier spectrum analysis is employed to investigate the association between SCS sea level variability and the Interdecadal Pacific Oscillation (IPO), using principal components of SCS sea surface height anomalies, wind stress curl, wind stress components, net short wave flux, as well as the LST and various climate indices. The variations in the SCS sea level are driven by the IPO, which modifies the LST and ocean heat content, impacting the steric sea level.

Coherent Interannual–Decadal Potential Temperature Variability in the Tropical–North Pacific Ocean and Deep South China Sea

AbstractClimate variability over the Tropical and North Pacific Ocean (TPO and NPO, respectively) modulates marginal sea variability. The South China Sea (SCS), the largest marginal sea in the western NPO, is an outstanding example of a region that responds quickly to climate change. However, there is considerable uncertainty regarding the response of the deep SCS to large‐scale climate variability. Multivariate empirical orthogonal function analysis revealed three prominent modes of interconnected temperature anomaly fluctuations within the TPO and NPO. These coherent modes highlight the interactive dynamics among climate variations and reveal their modulation mechanisms for previously less explored potential temperature variabilities in the deep SCS. On the atmospheric bridge, external forces modify the upper‐layer Luzon Strait Transport (LST) by adjusting the Ekman transport and Kuroshio intrusion. For the oceanic pathway, climate variations disturb the deep‐layer LST by adjusting the barotropic flows in the upper layer.

Impact of Mass Redistribution on Regional Sea Level Changes Over the South China Sea Shelves

AbstractThis study investigates long‐term sea level changes in the South China Sea (SCS) using a validated high‐resolution regional ocean model simulation for the Maritime Continent. The contributions of ocean mass redistribution and steric sea level are examined to understand the sea level variations. The ocean bottom pressure (OBP) serves as an indicator of sea level variations linked to alterations in ocean mass flux. The OBP accounts for over 80% of the total sea level change over the shelves, while the steric sea level emerges as the dominant factor, contributing over 50% to the sea level change in the deep SCS. Luzon Strait transport shows a weakening trend in the last six decades, resulting in higher heat accumulation and larger steric expansion in the deep SCS. The ocean mass redistribution acts as a mechanism to balance the contrasting steric induced sea level changes over the deep SCS and shallow continental shelves.

Regime Shifts in the Damage Caused by Tropical Cyclones in the Guangdong–Hong Kong–Macao Greater Bay Area of China

Tropical cyclones (TCs) pose a significant threat in terms of wind-induced damage and storm risk to the Guangdong–Hong Kong–Macao Greater Bay Area (GBA) of China. In this research, annual power dissipation index (PDI) and storm surge and wave destructive potential (SDP) index from June to November were used to estimate the damage caused by the TC events in the buffer zone of the GBA. The regime shifts in 1993 and 2013 were identified through the Bayesian changepoint detection in six TC datasets. The TC-induced damage during 1994–2012 (the low-damage period) was weaker than that in 1977–1993 and 2013–2020 (the high-damage periods). The intensity and size of stronger TCs are the dominant factors responsible for the interdecadal changes in the TC damage. This may be explained by the influences of sea surface temperature (SST), surface latent heat flux and mid-level relative humidity. During high-damage periods, TCs can extract more energy from the ocean, leading to increased TC intensity and larger size. Conversely, during low-damage periods, TCs undergo a decrease in energy intake, resulting in reduced TC power and smaller size. The variations in the SST are relative to the Luzon Strait transport. In addition, the reduction in TC translation speed is unfavorable for the development of TCs in low-damage periods. Further research suggested that mid-level steering flow affects the TC movement velocity. The results offer valuable insights into the variations in the TC-induced damage in the GBA.

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.

Novel Insights Into the Zonal Flow and Transport in the Luzon Strait Based on Long‐Term Mooring Observations

AbstractThe zonal flow and volume transport in the Luzon Strait play a crucial role in modulating the circulation, heat and material balance, and biogeochemical processes in the South China Sea (SCS), but the quantitative values remain unclear due to lack of long‐term direct observations. Here, this knowledge gap is bridged by analyzing 4‐year velocity measurements from a mooring array along 119.9°E. Based on the novel data, the mean value of the upper‐layer Luzon Strait transport (i.e., LST_up) is estimated at −4.54 ± 1.69 Sv. Seasonally, the westward LST_up attains its peak and trough in January and June with values of −6.80 ± 1.46 Sv and −2.59 ± 0.76 Sv, respectively. At the interannual time scale, the LST_up was strongest in 2016–2017 but weakest in 2017–2018. Further analysis suggested that local winds and the combination of local winds and upstream Kuroshio transport are likely the dominant modulation mechanisms for its seasonal and interannual variations, respectively. In the middle layer, a quasi‐steady cyclonic flow structure is identified and the volume transport is therefore trivial. We further found that seasonal variation of the middle‐layer transport is dominated by the eastward flow of this cyclonic structure. Corresponding to the gravest‐mode response in the Luzon Strait, seasonal‐to‐interannual variations of this middle‐layer eastward flow are nearly in‐phase with the upper‐layer westward flow. Overall, the above observational results provide a benchmark for the flow and transport in the Luzon Strait which can help understand the dynamics of the SCS circulation and validate and improve regional numerical simulations.

A monthly resolved coral δ13C and δ18O record of changes in the Kuroshio Current into the South China Sea via the Luzon Strait

The Kuroshio Current enters the South China Sea (SCS) via the Luzon Strait and exerts significant effects on circulation and water mass properties of this marine basin. However, because of a lack of continuous oceanographic observations, multi-decadal variations in Luzon Strait Transport (LST) remain uncertain. In this study, monthly resolved coral δ13C and δ18O data from Xiaoliuqiu Island, located offshore of southwestern Taiwan and along the Kuroshio Current intrusion route, are used to reconstruct LST from 1977 to 2003 and investigate interannual and decadal variability. On an interannual timescale, greater LST leads to stronger upwelling, which brings deep low-temperature and high-salinity sub-surface waters containing 13C-depleted dissolved inorganic carbon to the surface; this is reflected in lower coral δ13C and higher coral δ18O values. The interannual variability in the upper (0–745 m) and surface (0–1 m) layers of LST during 1977–2003 is also reconstructed using the coral δ18O record. Coral δ18O data indicate that LST has weakened since the 1990s. The upper layer of LST has decreased by nearly 25%, and the surface layer of LST has decreased by 60% over the 27 years of observation. Our results show that coral δ13C and δ18O data can be used to reconstruct interannual variations in LST.

Summer Anticyclonic Eddies Carrying Kuroshio Waters Observed by a Large CPIES Array West of the Luzon Strait

Abstract The Kuroshio intrusion into the South China Sea (SCS) in summer is weak and has rarely been reported by in situ observations. Here, we describe a new form of Kuroshio water intrusion that is strongest during the summer, the North Luzon Warm Eddy (NLWE), which is an anticyclonic eddy originating north of Luzon Island. From early July to mid-September 2018, two NLWEs moving northwestward were captured by a mooring array consisting of 27 current- and pressure-recording inverted echo sounders (CPIESs). The three-dimensional CPIES estimates reveal that the NLWEs carried large amounts of saline Kuroshio waters (S > 34.7 psu) in the subsurface, which was also evidenced by Argo float profiles. The Kuroshio intrusion was confined to waters shallower than the 14.8°C isotherm. Historical data for NLWEs suggest that they occur mostly during the summer but are absent between November and March, which is attributed to seasonal wind stress curl (WSC). However, because the seasonal signal of WSC during summer is small, intraseasonal WSC is directly responsible for the generation of NLWEs. Significance Statement This paper describes a new type of Kuroshio water intrusion into the South China Sea (SCS)—the North Luzon Warm Eddy (NLWE), which is an anticyclonic eddy generated north of Luzon Island. The eddy mostly occurs during summer when the Kuroshio intrusion is commonly considered the weakest. From observations of a large CPIES array, we provide a cradle-to-grave picture of the NLWE. NLWEs are estimated to contribute almost half of the westward Luzon Strait transport during the summer and, as such, play an important role in the seasonal stratification and circulation in the northeastern SCS.

How Much Heat and Salt Are Transported Into the South China Sea by Mesoscale Eddies?

AbstractEddy‐induced transport through the Luzon Strait critically affects the physical and biological properties of the South China Sea (SCS). The eddy‐induced volume, heat and salt transport are comprehensively evaluated based on the South China Sea Physical Oceanic Dataset (SCSPOD) observations. To avoid the bias involved in extreme eddy events, 76 shedding anticyclonic eddies, 46 shedding cyclonic eddies, 29 locally formed anticyclonic eddies and 40 locally formed cyclonic eddies from 1993 to 2018 are taken into account and merged into composites. The total annual intrusions of volume, heat and salt contributed by all four types of eddies are 0.77 Sv, 8.78 × 10−4 PW and 7.88 × 104 kg·s−1, respectively. The statistical analysis shows that annual eddy‐induced intrusion accounts for 11.3%–38.9% of the upper‐layer Luzon Strait transport, which highlights the non‐negligible role of eddies in water exchange through the Luzon Strait.

The Role of the Mindoro-Sibutu Pathway on the South China Sea Multi–layer Circulation

AbstractThe role of the Mindoro Strait-Sibutu passage pathway in influencing the Luzon Strait inflow to the South China Sea (SCS) and the SCS multi-layer circulation is investigated with a high-resolution (0.1° × 0.1°) regional ocean model. Significant changes are evident in the SCS upper layer circulation (250-900 m) by closing the Mindoro-Sibutu pathway in sensitivity experiments, as Luzon Strait transport is reduced by 75%, from −4.4 Sv to −1.2 Sv. Because of the vertical coupling between the upper and middle layers, closing the Mindoro-Sibutu pathway also weakens clockwise circulation in the middle layer (900–2150 m), but there is no significant change in the deep layer (below 2150 m). The Mindoro-Sibutu pathway is an important branch of the SCS throughflow into the Indonesian Seas. It is also the gateway for oceanic waves propagating clockwise around the Philippines Archipelago from the western Pacific Ocean into the eastern SCS, projecting El Niño-Southern Oscillation sea level signals to the SCS, impacting its interannual variations and multi-layer circulation. The results provide insights into the dynamics of how upstream and downstream passage throughflows are coupled to affect the general circulation in marginal seas.

Kuroshio intrusion in the Luzon Strait in an eddy-resolving ocean model and air-sea coupled model

The Kuroshio intrusion in a quasi-global eddy-resolving model (LICOMH) and a fully air-sea coupled model (LICOMHC) was evaluated against observations. We found that the Kuroshio intrusion was exaggerated in the former, while biases were significantly attenuated in the latter. Luzon Strait transport (LST) in winter was reduced from −8.8×106 m3/s in LICOMH to −6.0×106 m3/s in LICOMHC. Further analysis showed that different LST values could be explained by different large-scale and local surface wind stresses and the eddies east to the Luzon Strait as well. The relatively stronger cyclonic eddies in LICOMH northeast of the Luzon Island led to weak Kuroshio transport and strong intrusion through the Luzon Strait. The summed transport of all three factors was approximately 2.0×106 m3/s, which was comparable with the difference in LST between the two experiments. The EKE budget showed that strong EKE transport and the baroclinic transformation term led to strong cyclonic eddies east of the Kuroshio in LICOMH, while surface winds contributed little to the differences in the eddies.

Intraseasonal Variability of Cross-Slope Flow in the Northern South China Sea

AbstractCross-slope flow plays an important role in the exchange of material, heat, and momentum between the continental shelf and the open sea. In the northern South China Sea (SCS), long-period observations confirm that there is significant cross-slope flow. The variability of this flow is dominated by the intraseasonal component (i.e., the 10–90-day period band) that contributes 74.6% of the total standard deviation. The 10–90-day bandpassed cross-slope flow exhibits almost the same direction vertically in the observed layers, and its first empirical orthogonal function, whose direction is also not changed, contributes 86.7% to its total variance. The strong 10–90-day bandpassed cross-slope flow is phase locked to the boreal winter half year. The intraseasonal variability of cross-slope flow is mainly associated with mesoscale eddies west to the Luzon Strait. The contrasting baroclinic instability growth rates, strong in winter and weak in summer, result in a seasonal cycle of mesoscale eddy kinetic energy, that is, vigorous in winter and weak in summer, which explains the winter phase lock. The interannual variability of baroclinic instability growth rate is mainly determined by the vertical shear of velocity. The strongest vertical shear of velocity from 2014 to 2016 occurred in the winter of 2016/17 and induced the most rapid baroclinic instability growth rate and consequently the largest mesoscale eddy kinetic energy, which resulted in the strongest intraseasonal variability of cross-slope flow. The vertical shear of velocity in the northern SCS is mainly determined by the Luzon Strait transport.

Can Tropical Pacific Winds Enhance the Footprint of the Interdecadal Pacific Oscillation on the Upper-Ocean Heat Content in the South China Sea?

AbstractIn this study, an enhanced footprint of the interdecadal Pacific oscillation (IPO) on the upper-ocean heat content (OHC) in the South China Sea (SCS) since the 1990s is revealed. The negative OHC–IPO correlation is significant (r= −0.71) during 1990–2010 [period 2 (P2)], whereas it is statistically insignificant during 1960–80 [period 1 (P1)]. Analyses show that the scope of the equatorial Pacific wind anomalies is wider during P2 compared with that during P1 due to a larger east–west SST gradient and enhanced tropical warming in the Indian Ocean. When the IPO is negative during P2, a wider scope of the wind stress anomalies associated with the IPO could lead to 1) the southward migration of the North Equatorial Current bifurcation latitude (NECBL) by affecting the wind stress curl over the key region where it is near the climatological NECBL and 2) an increase in the interbasin pressure gradient (sea surface height difference) between the western Pacific and the SCS; these two processes strengthen the Kuroshio and weaken the Luzon Strait transport (LST) or SCS throughflow into the SCS. Also, 3) the equatorial Pacific wind anomalies are wide enough to directly weaken the LST in the SCS through the “island rule.” These three pathways finally change the oceanic gyre in the SCS and increase the OHC. Our results suggest that the scope of the tropical wind stress is the crucial factor when we consider the relationship between the upper ocean thermal conditions in the SCS and the Pacific variability.

Decadal Variation of the Kuroshio Intrusion Into the South China Sea During 1992–2016

AbstractDecadal variation of the Kuroshio intrusion into the South China Sea (KIS) during 1992–2016 is investigated using high‐resolution Hybrid Coordinate Ocean Model reanalysis data. The results indicate that both the yearly occurrence frequency of the Kuroshio intrusion and Luzon Strait transport show significant decadal variations and were generally high in approximately 1993, 2003–2004, and 2015–2016 and low in 1998–1999 and 2009–2010. The KIS is closely connected to the decadal oscillation of the meridional sea surface height anomaly (SSHA) gradient west of the Luzon Strait (LS). Further analyses reveal that large‐scale wind forcing in the western tropical Pacific Ocean is a major factor regulating decadal variation of the KIS, while local wind forcing has little impact. Cyclonic wind stress curl anomalies (WSCAs) over the western tropical Pacific Ocean induce negative SSHAs that propagate westward to the eastern coast of the Philippines via upwelling Rossby waves, weakening the Kuroshio transport and consequently enhancing the KIS. In addition, signals of negative SSHAs can propagate clockwise along the Philippines as coastal Kelvin waves and hence strengthen the northward SSHA gradient west of the LS, eventually leading to a stronger KIS. The situation is reversed when anticyclonic WSCAs prevail in the western tropical Pacific Ocean. The Philippines‐Taiwan Oscillation is linked to decadal WSCAs over the western tropical Pacific Ocean and hence regulates the decadal KIS.

Insight into Western Pacific Circulation from South China Sea Coral Skeletal Radiocarbon

ABSTRACTThe trajectory of the Kuroshio, the western boundary current in the north Pacific, influences regional climate. It intrudes into the South China Sea (SCS) through the Luzon Strait, resulting in the exchange of water, nutrients, heat, and salt between the Pacific and SCS. It has been reported that the trajectory of the Kuroshio has varied with decadal climate changes. However, there has been no report of an observation-based estimate of the variation in the Luzon Strait transport. Here, a 50-year, high-resolution coral skeletal radiocarbon (Δ14C) dataset from 1946 to 1994 is reported from Currimao, northwest of Luzon Island. Δ14C has been used as a sensitive tracer of seawater, and our data indicates a significant increase in Δ14C from 1946 to 1994 related to atmospheric nuclear bomb testing, with more rapid increase in the SCS than in the Pacific. The unusual, rapid Δ14C increase in the 1950s found in our SCS corals together with seasonal variation in Δ14C will helps constrain physical oceanographic models for the western Pacific, including the SCS.

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.

Significant salinity increase in subsurface waters of the South China Sea during 2016–2017

The South China Sea (SCS) is the largest semi-enclosed marginal sea in the North Pacific. Salinity changes in the SCS play an important role in regional and global ocean circulation and the hydrological cycle. However, there are few studies on salinity changes over the SCS due to lack of high-quality and long-term observations. In the past decade, the deployment of floats from the Argo program in the SCS and their accumulated temperature and salinity profiles have made it possible for us to examine salinity changes over the entire basin. In this study, salinity changes were investigated with Argo and underwater glider temperature and salinity observations and gridded temperature-salinity objective analyses (UK Met Office Hadley Centre EN4.2.1 objective analysis and China Argo Real-time Data Center BOA_Argo). The results indicated that the subsurface water in the entire SCS became significantly saltier during 2016–2017. The most significant salinity increase was found during 2016 in the northeastern SCS. The subsurface water in the northeastern SCS exhibited a salinity maximum above 35, which was recorded by three Argo floats during 2015–2016. Such high salinity water was rarely observed and reported prior to the Argo era. Average salinity of 2016–2017 along the 25.5σθ−23.5σθ isopycnal surfaces in the whole SCS is 0.014−0.130 higher than the climatology. Increases in subsurface salinity started from the northeastern SCS and extended southwestward gradually. Moreover, the subsurface salinity changes, especially in the northern SCS, exhibited a semiannual lead behind the subsurface Luzon Strait transport. Further analysis indicated that the predominance of advection, driven by subsurface Luzon Strait transport, led to salinification along the western boundary of the SCS. In other parts of the SCS, negative wind stress curl trends tended to preserve the high salinity characteristics of the subsurface water.

A revisit of the interannual variation of the South China Sea upper layer circulation in summer: correlation between the eastward jet and northward branch

The interannual variation of the South China Sea upper layer circulation in summer is revisited based on analysis of current derived from altimetry data, Acoustic Doppler Current Profilers moorings, and numerical simulations. Results show not only the interannual variation of the eastward jet (eastward branch), but also its anti-correlation with the northward branch. On interannual time scale, when the eastward branch is enhanced, the northward branch is weakened, and vice versa. Their variations are largely related to the change of the South China Sea summer monsoon (SCSSM), and are strongly influenced by the Luzon strait Transport (LST). Composite analysis reveals a stronger SCSSM and LST into the SCS in the developing phase of El Nino would lead to an eastward branch dominant circulation pattern, whereas a weaker SCSSM and reduced LST into the SCS in the decaying phase of El Nino favors a northward branch dominant circulation pattern. The distinct composite patterns appear in El Nino and Southern Oscillation cycles, rather than episodic event or multiyear El Nino or La Nina. Contribution of the transport of major straits in the SCS to the interannual variation of the SCS summer circulation is quantitatively evaluated for the first time, and the results show that the change of the planetary vorticity flux through three major straits (Luzon strait contributes most) is as equally important as the vorticity input change from local wind stress curl.

A New Assessment of Mesoscale Eddies in the South China Sea: Surface Features, Three‐Dimensional Structures, and Thermohaline Transports

AbstractBy analyzing 22 years (1993–2015) of daily eddy data, statistics of surface eddy properties were refreshed in the South China Sea. More than 7,000 of historical Argo profiles were collocated into eddy‐centered coordinates to reveal the composite mean three‐dimensional structure of eddies. The results indicate that eddies of both polarities have long conical shape, with a maximum (minimum) density anomaly of 0.55 kg/m3 (−0.51 kg/m3) at 60 m (90 m) in the composite cyclonic (anticyclonic) eddy. Temperature and salinity anomalies also peak at eddy cores, with values of −1.5 °C and 0.15 psu in the cyclonic eddy and 1.4 °C and −0.16 psu in the anticyclonic eddy. The temperature and density anomalies extend vertically to 400–500 m, while the salinity anomalies are apparent only in the upper 150 m. The temperature anomalies contribute about 90% of the density anomalies. Mixed layer depths in cyclonic eddies are on average 15 m shallower than those in anticyclonic eddies. The rotation of the composite cyclonic (anticyclonic) eddy generates meridional heat transport of 1.4 × 1012 W (−3.1 × 1012 W) and salt transport of −4.0 × 104 kg/s (5.6 × 104 kg/s). More than 90% of the heat and salt transports are concentrated in the upper 300 and 100 m, respectively. Compared to the meridional transports, the westward propagation of eddies results in zonal heat and salt transports on the same orders of magnitudes. The westward propagation of eddies also generates a basin‐scale westward water transport of 1.4 Sv, equivalent to about 30% of the annual‐mean Luzon Strait transport.

Salinification in the South China Sea Since Late 2012: A Reversal of the Freshening Since the 1990s

AbstractSalinification has occurred in the South China Sea from late 2012 to the present, as shown by satellite Aquarius/Soil Moisture Active Passive data and Argo float data. This salinification follows a 20 year freshening trend that started in 1993. The salinification signal is strongest near the surface and extends downward under the seasonal thermocline to a depth of 150 m. The salinification occurs when the phase of the Pacific Decadal Oscillation switches from negative to positive. Diagnosis of the salinity budget suggests that an increasing net surface freshwater loss and the horizontal salt advection through the Luzon Strait driven by the South China Sea throughflow contributed to this ongoing salinification. In particular, a decrease in precipitation and enhanced Luzon Strait transport dominated the current intense salinification. Of particular interest is whether this salinification will continue until it reaches the previous maximum recorded in 1992.