Southwest Of Taiwan Research Articles

The Periodic Cyclonic Eddy in Southwestern Taiwan and Its Interannual Variation Related to Large‐Scale Climate Variations

AbstractPeriodic eddies are a type of eddy that occur almost annually in fixed timeframes with similar patterns and trajectories. Nearly every year from April to June, under the combined effect of the barotropic instability of the mean flow and wind work, a cyclonic eddy (the Taiwan Cyclonic Eddy, TCE) forms in the southwest of Taiwan, then propagates westward, and finally dissipates near the Dongsha Islands. TCE exerts a significant impact on the Kuroshio intrusion into the South China Sea (SCS) and water exchange. Based on multi‐year in situ and satellite observations, this study reveals the thermohaline structure and evolutionary process of the TCE. The evolutions of the three‐dimensional structures of temperature, salinity, and geostrophic velocity of the TCE are analyzed based on reconstructed data. The TCE shows important interannual variations associated with El Niño–Southern Oscillation (ENSO), and the relationship between ENSO and the TCE is modulated by the Pacific Decadal Oscillation (PDO). In the negative phase of the PDO, the intensity of the TCE is significantly correlated with the Niño‐3.4 index. In contrast, in the positive phase, the ENSO–TCE relationship becomes weak and non‐significant. Further investigations indicate that these differences are related to the establishment of the low‐latitude Pacific–East Asian Teleconnection, influencing local wind stress curl in the region. This offers a new perspective on understanding the interannual variation of periodic mesoscale eddies in the SCS.

Region-dependent eddy kinetic energy budget in the northeastern South China Sea revealed by submesoscale-permitting simulations

Mesoscale eddies are active in the northeastern South China Sea (NESCS) and play an important role in the oceanic energy balance therein. In this study, the budgets of mesoscale eddy kinetic energy (EKE) in the NESCS were explored by simultaneously considering the interactions with both large-scale and submesoscale processes based on high-resolution simulations. We found that the regions southwest of Taiwan (SWT) and Luzon Strait (LS) are two energy-cascading hotspots, but they present quite different EKE budgets. Specifically, in the SWT, which has strong EKE, the baroclinic conversion associated with the release of available potential energy is the dominant EKE source, while the barotropic conversion between large-scale and mesoscale processes (BT LM ) and the wind stress work are the main EKE sinks. In addition, submesoscale processes are found to transfer energy reversely to mesoscale eddies, but at lower magnitudes. For the LS, which shows complicated island topography and energetic large-scale and submesoscale activities, the BT LM is the dominant EKE source, while the transfer of energy to submesoscale processes is an important EKE sink. This means that the kinetic energies in these two regions display opposing cascading directions. The cascade direction is inverse, from submesoscale to large-scale, in the SWT and forward, from large-scale to submesoscale, in the LS. These results expand our understanding of the EKE balance and energy cascade in the NESCS among multiscale dynamic processes and offer beneficial implications for improving parameterizations in ocean circulation models. • Budgets of EKE for the NESCS were analyzed across multiscale dynamic processes. • The SWT and LS regions are energy-cascading hotspots. • The EKE budgets differ for the SWT and LS regions. • The kinetic energy shows inverse and forward cascades in the SWT and LS regions.

A Model of Plate Bending at the Transition Zone From Subduction to Collision in Northernmost Manila Trench

AbstractIt is not rare to observe subduction zones transferring into collision zones. However, it is poorly understood the lateral variation in flexural deformation. Constrained by newly published seismic reflection data, we investigate the mechanical behavior of lithosphere from oceanic subduction to collision at the northern Manila trench using the 3‐D flexural modeling. Results show that both plate rigidity and the loadings increase from the Manila subduction zone to the Taiwan orogenic belt. In order to fit all observations, an upward loading was required at the transition zone from subduction to collision, which may be related to the variation of buoyancy due to density contrast of plate from south to north. The resultant maximum bending stress is consistent with the concentrated seismic activity at the southwest of Taiwan, suggesting that huge loading from the orogeny and lateral density change of subducting plate play important roles in bending and seismicity.

Submesoscale Coherent Vortices Observed in the Northeastern South China Sea

AbstractAlthough submesoscale coherent vortices (SCVs) have been observed in different parts of the world's oceans, most of them were captured by accident by a limited number of hydrographic profiles. Here, using concurrent velocity and temperature/salinity measurements from a submesoscale‐resolving mooring array (2 km), two oppositely rotating SCVs are for the first time reported in detail in the northeastern South China Sea (NESCS). For the anticyclonic (cyclonic) SCV, its core lays at 210 m (180 m) with a mean Rossby number of −0.76 (0.65); its maximum swirl velocity, radius, vertical scale, and Burger number are estimated to be 0.43 m s−1, 31 ± 5 km, 235 m, and 1.5 (0.23 m s−1, 16 ± 3 km, 100 m, and 2.5), respectively. Corresponding to the SCVs' submesoscale nature, their momentum is governed by gradient‐wind rather than geostrophic balance. Consequently, kinetic energy of the anticyclonic (cyclonic) SCV would be on average underestimated (overestimated) by 47% (68%) if the traditional geostrophic relation was used to diagnose the velocity. Further analysis shows that thermohaline properties within the anticyclonic and cyclonic SCVs are close to the Kuroshio water and the local NESCS water, respectively. By combing water mass tracing and high‐resolution simulations, we suggested that the anticyclonic (cyclonic) SCV was generated in the southern Luzon Strait (southwest of Taiwan) through current‐topography interaction. We further proposed that the observed SCVs here should be a common phenomenon in the NESCS, which may provide a novel route for the tracer exchange between the NESCS and the western Pacific.

On the Genesis of the South China Sea Mesoscale Eddies

The climatology of the mesoscale eddies in the upper layer of the South China Sea (SCS) is investigated for an understanding of its genesis using the outputs from a 1/12.5° ocean reanalysis. Employed is a recently developed multiscale energetics formalism on the basis of a multiscale window transform (MWT) and the theory of canonical transfer. Three scale windows, namely, background flow, mesoscale eddy and synoptic eddy, are differentiated, and fields on different scales are reconstructed henceforth. Diagnosis of the mesoscale eddy energy budget reveals that barotropic and baroclinic instabilities, wind work, advection and pressure work are essential ingredients of the eddy energy sources and sinks in the SCS, but their contributions vary from region to region. In the southwestern part of the SCS, the regional mesoscale eddy energy is mainly generated by barotropic instability, while in the northeastern SCS, baroclinic instability and the wind working directly on the eddies are the two dominant eddy generation processes. The eddies southwest of Taiwan are damped by outward energy transport via advection, while the decay of those southeast of Vietnam is due to pressure work. The three-scale framework also reveals that the interaction between the mesoscale eddies and higher-frequency synoptic eddies mainly serves as a sink for the mesoscale eddy energy in the SCS, except for the northeastern SCS, where significant inverse cascade of kinetic energy is found.

Coupled three-dimensional modelling of groundwater-surface water interactions for management of seawater intrusion in Pingtung Plain, Taiwan

Study regionA coupled framework, linking subsurface flow and surface hydrodynamics, is developed and applied to a real-world case study of Pingtung coastal aquifer in southwest of Taiwan, in East Asia. Study focusFEFLOW is adopted to develop a 3-D variable density and transient groundwater model of the Pingtung shallow aquifer lying 250 m below mean sea level (MSL). This model is coupled with a 1-D river network model, comprised of the main river and its two tributaries, using MIKE 11 through the IFM MIKE 11 coupling interface. The model is capable of analysing the relationship between rainfall, surface water and groundwater recharge lag time. Also, the analysis of potential river inundation and maximum river discharge enable the model to choose the best location to apply artificial recharge as a management scenario to mitigate the effect of seawater intrusion (SWI). To the authors’ knowledge, the developed coupled model is the first detailed integrated framework analysing the interaction of surface and subsurface water, with the capability to contribute to the restoration, rehabilitation, and management of the river network. New hydrological insights for the regionThe rainfall ratio in the wet season to dry season is significant in this plain comparing with the rest of Taiwan. Also, southern Taiwan experiences the largest sea and river interaction, while Kaoping River playing as a pathway role for inland lead of seawater intrusion.

Risk Assessment of Coastal Flooding under Different Inundation Situations in Southwest of Taiwan (Tainan City)

The Pacific island countries are particularly vulnerable to the effects of global warming including more frequent and intense natural disasters. Seawater inundation, one of the most serious disasters, could damage human property and life. Regional sea level rise, highest astronomic tide, vertical land motions, and extreme sea level could result in episodic, recurrent, or permanent coastal inundation. Therefore, assessing potential flooding areas is a critical task for coastal management plans. In this study, a simulation of the static flooding situation in the southwest coast of Taiwan (Tainan city) at the end of this century was conducted by using a combination of the Taiwan Digital Elevation Model (DEM), regional sea level changes reconstructed by tide gauge and altimetry data, vertical land deformation derived from leveling and GPS data, and ocean tide models. In addition, the extreme sea level situation, which typically results from high water on a spring tide and a storm surge, was also evaluated by the joint probability method using tide gauge records. To analyze the possible static flood risk and avoid overestimation of inundation areas, a region-based image segmentation method was employed in the estimated future topographic data to generate the flood risk map. In addition, an extreme sea level situation, which typically results from high water on a spring tide and a storm surge, was also evaluated by the joint probability method using tide gauge records. Results showed that the range of inundation depth around the Tainan area is 0–8 m with a mean value of 4 m. In addition, most of the inundation areas are agricultural land use (60% of total inundation area of Tainan), and two important international wetlands, 88.5% of Zengwun Estuary Wetlands and 99.5% of Sihcao Wetlands (the important Black-faced Spoonbills Refuge) will disappear under the combined situation. The risk assessment of flooding areas is potentially useful for coastal ocean and land management to develop appropriate adaptation policies for preventing disasters resulting from global climate change.

Statistical characteristics of mesoscale eddies on the continental slope in the northern South China Sea

The continental slope in the northern South China Sea (SCS) is rich in mesoscale eddies which play an important role in transport and retention of nutrients and biota. In this study, we investigate the statistical properties of eddy distributions and propagation in a period of 24 years between 1993 and 2016 by using the altimeter data. A total of 147 eddies are found in the continental slope region (CSR), including 70 cyclonic eddies (CEs) and 77 anticyclonic eddies (ACEs). For those eddies that appear in the CSR, the surrounding areas of Dongsha Islands (DS) and southwest of Taiwan (SWT) are considered as the primary sources, where eddies generated contribute more than 60% of the total. According to the spatial distribution of eddy relative vorticity, eddies are weakening as propagating westward. Although both CEs and ACEs roughly propagate along the slope isobaths, there are discrepancies between CEs and ACEs. The ACEs move slightly faster in the zonal direction, while the CEs tend to cross the isobaths with large bottom depth change. The ACEs generally move further into the basin areas after leaving the CSR while CEs remain around the CSR. The eddy propagation on the continental slope is likely to be associated with mean flow at a certain degree because the eddy trajectories have notable seasonal signals that are consistent with the seasonal cycle of geostrophic current. The results indicate that the eddy translation speed is statistically consistent with geostrophic velocity in both magnitude and direction.

Observations of marine snow and fecal pellets in a sediment trap mooring in the northern South China Sea

Sediment traps are an important tool for studying the source, composition and sedimentation processes of sinking particulate matter in the ocean. An in situ observational mooring (TJ-A-1) is located in the northern South China Sea (20.05°N, 117.42°E) at a water depth of 2 100 m and equipped with two sediment traps deployed at 500 m and 1 950 m. Samples were collected at 18-day intervals, and 20 samples were obtained at both depths from May 2014 to May 2015. Large amounts of fecal matter and marine snow were collected in the lower trap. The fluxes of marine snow and fecal pellets exhibited a fluctuating decrease between May 2014 and early August 2014 and then stabilized at a relatively low level. Scanning electron microscopy observations revealed that the main components of the marine snow and fecal pellets were diatoms, coccolithophores, radiolarians, and other debris, all of which are planktons mostly produced in photic zone. Used in conjunction with the particle collection range estimates from the lower trap and data on ocean surface chlorophyll, these marine snow and fecal pellets were related to the lateral transport of deep water and not vertical migrations from overlying water column. Moreover, the source area might be southwest of Taiwan.

Spatiotemporal Characteristics and Generation Mechanisms of Submesoscale Currents in the Northeastern South China Sea Revealed by Numerical Simulations

AbstractAlthough both geostrophic‐balanced mesoscale eddies and unbalanced small‐scale processes have been well studied in the northeastern South China Sea (NE‐SCS), less attention has been devoted to the submesoscales in between (i.e., O(1–10 km)), which is recognized as an important conduit connecting the balanced and unbalanced motions. Based on the output from a 1/30° OGCM simulation, spatiotemporal characteristics and generation mechanisms of submesoscales in the NE‐SCS are investigated in this study. Through examining the submesoscale relative vorticity and vertical velocity, the regions southwest of Taiwan (ST) and Luzon Strait (LS) are identified as two hot spots of submesoscales in the NE‐SCS. Seasonally, the submesoscales in region ST are much stronger in winter than summer, while those in LS do not show a significant seasonality. Statistical analysis suggests that the strength of submesoscales in regions ST and LS is highly correlated with the product of mixed‐layer depth and mesoscale strain rate (MSR) and with the MSR itself, respectively. By conducting theoretical scaling and energetics analysis, the authors find that the mixed‐layer instability whose strength is determined by the combination of mixed‐layer depth and MSR and the barotropic instability associated with current‐islands interactions are the dominant generation mechanisms of submesoscales in the above two regions, respectively. Further examinations of the submesoscale energy budget indicate that, to keep a balanced state, the generated submesoscales have to be dissipated by a forward energy cascade, highlighting the important role of submesoscales in the energy balance of the NE‐SCS circulation.

Observation system simulation experiments using an ensemble-based method in the northeastern South China Sea

An ensemble-based method for the observation system simulation experiment (OSSE) is employed to design optimal observation stations and assess the present observation stations in the northeastern South China Sea (SCS). We employed the 20-year (1992–2012) sea surface height (SSH) data to design an array to monitor the intraseasonal to interannual variability. The results show that the most key region was found located at the northwest of Luzon Island (LI) where the energetic Luzon cyclonic gyre (LCG) occurs; other key regions include the edge of the LCG, the northwest of the Luzon Strait (LS), and the southwest of Taiwan, China. By contrast, we found that the present observation stations might oversample at the northwest of the LS and undersample at the northwest of LI. In addition, the optimal stations perform better in a larger area than the present stations. In vertical direction, the key layer is located within the upper 200-m depth, of which the surface and subsurface layers are most valuable to the observing system.

A Census of the 1993–2016 Complex Mesoscale Eddy Processes in the South China Sea

Mesoscale eddy process with at least one splitting and/or merging event can be defined as either a complex process or a simple process. Investigation of the difference between these two categories could provide new insights into how different factors, such as the seabed topography, Kuroshio intrusion, and winds, affect the origin, migration, and decay of the mesoscale eddies. This study compared the characteristics of the complex against the simple eddy processes in the South China Sea (SCS) from 1993 to 2016. We comprehensively analyzed the eddy processes with regards to their characteristic points, trajectories, and networks. The simple and complex processes share many similarities but do show significantly different behaviors. Both the simple and complex processes mainly start from the eastern SCS. However, the complex processes mainly vanish in the western SCS whereas the simple processes disappear almost everywhere across the SCS. The complex processes last longer and migrate more than the simple processes. Lastly, the complex processes mainly move westward within the community. The complex processes can be further categorized into complex anticyclonic and cyclonic eddy processes. Spatially, the splitting and merging events mainly occur in the southwest of Taiwan, northwest of the Luzon Island, and the southeast of Vietnam. Temporally, the merging and splitting events mainly occur in the fall. The interaction among the communities reveals the different migration patterns of the complex anticyclonic and cyclonic eddy processes in the SCS.

Phytoplankton dynamics driven by vertical nutrient fluxes during the spring inter-monsoon period in the northeastern South China Sea

Abstract. A field survey from the coastal ocean zones to the offshore pelagic zones of the northeastern South China Sea (nSCS) was conducted during the inter-monsoon period of May 2014 when the region was characterized by prevailing low-nutrient conditions. Comprehensive field measurements were made for not only hydrographic and biogeochemical properties but also phytoplankton growth and microzooplankton grazing rates. We also performed estimations of the vertical turbulent diffusivity and diffusive nutrient fluxes using a Thorpe-scale method and the upwelling nutrient fluxes by Ekman pumping using satellite-derived wind stress curl. Our results indicated a positive correlation between the integrated phytoplankton chlorophyll a and vertical nutrient fluxes in the offshore region of the nSCS during the study period. We generally found an increasing role of turbulent diffusion but a decreasing role of curl-driven upwelling in vertical transport of nutrients from the coastal ocean zones to the offshore pelagic zones. Elevated nutrient fluxes near Dongsha Islands supported high new production leading to net growth of the phytoplankton community, whereas the low fluxes near the southwest of Taiwan had resulted in a negative net community growth leading to decline of a surface phytoplankton bloom. Overall, phytoplankton dynamics in the large part of the nSCS could be largely driven by vertical nutrient fluxes including turbulent diffusion and curl-driven upwelling during the spring inter-monsoon period.

AB112. Long-term result of ileal-ureter substitution in ureteral urothelial carcinoma patients: a single-center experience

ObjectiveIn the southwest coast of Taiwan, the high prevalence rate of upper tract urothelial carcinomas (UTUCs) has been described. Renal preserving surgeries are of great importance when ureteral urothelial carcinoma (UC) happens in solitary kidney, in bilateral upper urinary tract simultaneously, or in patients with chronic kidney disease. We present the single center experience of total ureterectomy plus ileal-ureteral substitution for the ureteral UCs in the southwest of Taiwan over the last two decades.MethodsWe retrospectively reviewed patients who had undergone total ureterectomy plus ileal-ureteral substitution for ureteral UC at our center from January 1988 to December 2013. Chart review was done to obtain the information including age, sex, underlying medical diseases, oncology status before the surgery, baseline renal function, long-term renal function, duration of hospitalization, perioperative surgical complications, long-term complications, and oncological outcomes.ResultsOverall, 8 patients were treated with ileal ureteral substitution for ureteral UC, with a mean follow-up period of 66.5 months. All the resected ureters had confirmed to have UC on the final pathology reports. A total of four major complications (grade III) and 1 minor complication (grade I-II) developed according to the Clavien-Dindo complication grading system during a 30-day post-operative period. No perioperative mortality occurred in these patients. There were 3 patients (37.5%) who experienced long-term complications, including recurrent urinary tract infection and recurrent gross hematuria. At the end of follow-up, the renal function remained stable in 4 patients (50.0%), improved in 2 patients (25.0%), and deteriorated in 2 patients (25.0%). Only one of the deteriorated patient required long-term hemodialysis (12.5%). About the oncological outcome, only 1 patient (12.5%) was found to have tumor recurrence over the pelvic-ileal anastomotic site.ConclusionsIleal-ureteral substitution has been described as a reserved procedure for an extensively diseased ureter that other reconstructive procedures are not feasible. It provides good long-term outcomes for preservation of renal function, and also has acceptable safety concerns. In carefully selected patients, the procedure can also be carried out as nephro-sparing surgery for patients who have ureteral malignancy.

Mesoscale eddies transport deep-sea sediments.

Mesoscale eddies, which contribute to long-distance water mass transport and biogeochemical budget in the upper ocean, have recently been taken into assessment of the deep-sea hydrodynamic variability. However, how such eddies influence sediment movement in the deepwater environment has not been explored. Here for the first time we observed deep-sea sediment transport processes driven by mesoscale eddies in the northern South China Sea via a full-water column mooring system located at 2100 m water depth. Two southwestward propagating, deep-reaching anticyclonic eddies passed by the study site during January to March 2012 and November 2012 to January 2013, respectively. Our multiple moored instruments recorded simultaneous or lagging enhancement of suspended sediment concentration with full-water column velocity and temperature anomalies. We interpret these suspended sediments to have been trapped and transported from the southwest of Taiwan by the mesoscale eddies. The net near-bottom southwestward sediment transport by the two events is estimated up to one million tons. Our study highlights the significance of surface-generated mesoscale eddies on the deepwater sedimentary dynamic process.

Eddy characteristics in the northern South China Sea as inferred from Lagrangian drifter data

Abstract. Cyclonic and anticyclonic eddies from large scale to submesoscale in the northern South China Sea (NSCS) have been statistically characterized based on the satellite-tracked Lagrangian drifters using our developed geometric eddy identification method. There are in total 2208 eddies identified, 70% of which are anticyclonic eddies. If the submesoscale eddies are eliminated, the other eddies in the NSCS will show a 1.2:1 ratio of the number of anticyclones (210) to the number of cyclones (171). The spatial distribution of the eddies is regional: in southwest of Taiwan, the number of anticyclones dominates the number of cyclones, and most of them are the submesoscale anticyclones with small radii; in contrast, the large and medium cyclonic eddies are a little more than the same scale anticyclonic eddies in northwest of Luzon. The temporal distribution of eddy number in the NSCS has a close relation with the Asian monsoon. The number of the large and medium eddies peaks during the winter monsoon, while the submesoscale eddies are apt to generate in the summer monsoon. The spatial and temporal patterns have a good agreement with the results of the sea surface height anomaly (SSHA). The maximum and mean tangential velocities of anticyclones (cyclones) are 40 (30) cm s−1 and 25 (15) cm s−1, respectively. The calculated normalized vorticities from drifters suggest that although the mesoscale eddies may be considered in geostrophic balance, ageostrophic dynamics and centrifugal effects may play an important role for the growth and decay of the mesoscale cores.

Extreme Rainfall Mechanisms Exhibited by Typhoon Morakot (2009)

Moderate Typhoon Morakot (2009) became the most catastrophic typhoon in Taiwan on record. The MM5 numerical experiments with and without bogus data assimilation (BDA) were used to investigate the extreme rainfall mechanisms in Taiwan associated with the westbound typhoon. The BDA, based on 4DVAR, helps MM5 to maintain a more consolidated typhoon vortex and better predict the observed track after landfall, thus producing realistic extreme rainfall (about 2400 mm) at the southern and Central Mountain Range (CMR) of Taiwan. Severe rainfall in Taiwan is dominated by the CMR that hence modulates rainfall predictability.Model analyses indicate that the synoptic environment provides low-level preconditioning with large convective available energy (CAPE) in the southwest monsoon conveying belt in conjunction with the cyclonic Morakot. When Morakot passed slowly over north Taiwan, the developing Tropical Storm Goni, originating west of Hong Kong, facilitated the moist southwesterly flow to converge with the northerly cyclonic flow of Morakot. These processes contributed to enhanced rainfall in south Taiwan. In an experiment whereby the Goni vortex was initially deactivated by BDA, the southwesterly prevailing flow, southwest of Taiwan, weakened considerably and shifted southward at a later time, resulting in one-third reduction in total accumulated rainfall in south Taiwan. Conversely, total accumulated rainfall in Taiwan is greatly reduced when the initial Morakot vortex is deactivated. Removal of Taiwan topography results in a significant reduction in total accumulated rainfall by more than 50%, due to lack of orographic lift by the CMR.

A diagnostic calculation of the circulation in the upper and middle layers of the Luzon Strait and the northern South China Sea during March 1992

Wind data from NCEP and hydrographic data obtained during 8–27 March 1992 have been used to compute circulation in the Luzon Strait and the northern South China Sea using three-dimensional diagnostic models with a modified inverse method. Numerical results are as follows: the main Kuroshio is located above 800 m levels. It has two intrusive branches of the Kuroshio in the areas above 400 m. One part intrudes anti-cyclonically northwestward, then flows through the area above 200 m southwest of Taiwan and into the Taiwan Strait. The other part intrudes westward and flows cyclonically in the areas north of the cyclonic eddies, then flows southward through the southern boundary of the region. The net westward volume transport (VT) through Section at 120°15′E between Luzon Island and Taiwan Island is about 3.0 Sv, net northward VT through northern boundaries into the Taiwan Strait is about 1.4 Sv and net southward VT through southern boundaries is about 1.6 Sv, which finally flows into the Karimata and Mindoro Straits. In the areas above 400 m east of 117°15′E, the circulation is mainly dominated by the basin-scale cyclonic gyre, which consists of two cyclonic eddies. However, in the areas below 400 m east of 119°00′E, the circulation is mainly dominated by basin-scale anti-cyclonic gyre. The joint effect of baroclinity and relief and interaction between wind stress and relief are important in different area respectively for the pattern of the depth-averaged flow across contours of fH −1.

The Kuroshio near the Luzon Strait and circulation in the northern South China Sea during August and September 1994

Wind data from NCEP and hydrographic data obtained from August 28 to September 10, 1994 have been used to compute circulation in the northern South China Sea and near Luzon Strait using three-dimensional diagnostic models with a modified inverse method. The numerical results are as follows: the main Kuroshio is located above 400 m levels near Taiwan’s eastern coast and above 800 m levels away from it. Near Luzon Strait above 400 m levels a branch of the Kuroshio joins with a part of the northward current, which comes from an area west of Luzon’s western coast and intrudes northwestward, then it branchs into western and eastern parts near 20°30′ N. The eastern part flows northward into an area east of Taiwan, while its western part continues to intrude northwestward, flowing through an area southwest of Taiwan. Net westward intruded volume transport through longitude Section AB at 121°00′ E from 19°00′ N to 21° 43′ N is about 3.5 × 106 m3s−1 in a layer above 400 m levels. The anticyclonic eddies W1 and W3 exist above 700 m levels east of Dongsha Islands and below 200 m levels in the eastern part of the region, respectively. The circulation in the middle region is dominated mainly by a basin-scale cyclonic gyre, and consists of three cyclonic eddies. Strong upwelling occurs in the middle region. The joint effect of baroclinity and relief and interaction between wind stress and relief both are important for real forcing of flow across contours of fH −1 in effecting the circulation pattern.

Directional noise and correlation of signal and noise variability in the ASIAEX South China Sea Experiment

As part of the 2001 ASIAEX South China Sea experiment, known acoustic signals and noise were recorded for 16 days with co-located horizontal and vertical receiving arrays in 120-m depth water at the edge of the continental shelf. The location was south of China and southwest of Taiwan. Vertical and horizontal directionality of both signals and noise can be estimated using the two arrays. Directionality can be linked to vertical mode bandwidth. Furthermore, because of mode stripping effects, it can indicate the amount of mode coupling in the vicinity of the receiver. Previously, arrival time and total energy of the signals have been shown to respond to the strong nonlinear internal waves and internal tides of the area. Here, we investigate the response of noise energy and directionality to the waves, and compare with the response of signal energy and directionality. The results can be compared to the Shelfbreak PRIMER experiment in the Mid-Atlantic Bight, where signal and noise parameters recorded with a VLA were correlated. Correlation of signal and noise violates assumptions used in many signal processing algorithms.