Main Stream Of Kuroshio Research Articles

Kuroshio path variability inferred from satellite-derived sea surface topography in the northwestern Pacific

The Kuroshio has a fundamental impact on the regional oceanography of the northwestern Pacific. But identification of the Kuroshio path (KP), an abstraction of the course along which the Kuroshio mainstream moves, has not yet been established in a systematic manner. We optimally track the KP and study its variability in the northwestern Pacific south of ~31°N, where eddy activity is rich. An automatic contour method based on maximum surface geostrophic velocity along a given satellite-derived dynamic topographic isoline is applied and its performance is evaluated. Our results are robust and can be further used to derive kinematical, statistical, and spectral properties of the flow field of the Kuroshio upstream. We improve the identification method by tracing two separate KPs in different subdomains. The existence of an alignment or mismatch of these two retrieved KPs hints at the arrival of an approaching eddy. The highly variable and distorted KP east of Luzon Strait and Taiwan is due to eddy impingement. Most of the variability along the KP stems from energy with time scales of ~30–200 days and 1 year. A more consistent KP is seen north of ~26°N, with increasing surface currents of up to ~1 m s−1 before entering through the Tokara Strait. Such regional differences result from the various impacts of impinging mesoscale eddies on the Kuroshio, mainly due to blockage by the Ryukyu Islands. Our optimally determined KP is in line with the historical shipborne subsurface velocity measurements revealing the Kuroshio velocity core and observations of strong surface currents of > ~0.5 m s−1 by shore-based high-frequency radar (HFR) from locations along the east coast of Taiwan. Supportive evidence of concurrent KP distortion shows that HFR-derived vortex-like flow patterns are related to mesoscale eddies impinging from regions east of the radar footprint. Our work has value as a supplement to the data from radar operational routines, and will help interpret and diagnose these complicated HFR observations east of Taiwan.

Oceanic Eddy Detection and Analysis from Satellite-Derived SSH and SST Fields in the Kuroshio Extension

Vigorous mesoscale eddies are broadly distributed in the Kuroshio Extension and can generally be identified from sea surface height (SSH) and sea surface temperature (SST) fields. Nevertheless, the changes in SSH and SST caused by mesoscale eddies and their seasonal correlation in the Kuroshio Extension are not clear, as well as the difference between identified eddy results from the two data. Combining in situ Argo float profiles data, the correlation between SSH anomaly (SSHA) and SST anomaly (SSTA) signals in mesoscale eddies are analyzed. The result shows that SSTA–SSHA signals inside eddies are generally more correlated in winter than in summer. Argo subsurface temperature anomalies θ′ and SSHA signals inside eddies show a high correlation, with a regression coefficient θ′/SSHA of about 7 °C·m−1, while correlations of Argo θ′–SSTA inside eddies are low. Generally, the lifetime and propagation distance of SSTA-based eddies are shorter and smaller than those of SSHA-based eddies, which may be related to the rapid changes in SSTA field and the interference of small-scale oceanic signal in the SST field. Comparing with SSHA-based eddies, which exist primarily around the region of the Kuroshio mainstream (33°–36°N), SSTA-based eddies are concentrated in the Oyashio Extension (39°–42°N), where SST gradient is large, and changes in SST fields caused by mesoscale eddies are more obvious and more likely to be captured by satellites there. In addition, the geographical distributions of SSHA- and SSTA-based eddy amplitudes are consistent with the absolute dynamic topography and SST gradient.

Impingement of Subsurface Anticyclonic Eddies on the Kuroshio Mainstream East of Taiwan

AbstractThis study shows that North Pacific subtropical mode water (STMW) characterized by low potential vorticity can approach and impinge on the Kuroshio mainstream east of Taiwan. The results of the analyses of observations and output from eddy‐resolving models show that the STMW appears with the frequency of 32.1% and 19.9% in the Kuroshio east of Taiwan. A case study with the output of the ocean general circulation model for the earth simulator during October 2015–March 2016 indicates that mode water moves southwestward toward the Kuroshio in accordance with a vertically lens‐shaped subsurface anticyclonic eddy (SAE). After the shoreward edge of the SAE impinges on the offshore edge of the Kuroshio, the northward velocity, especially subsurface velocity, of the Kuroshio increases significantly in the subsurface layer and the Kuroshio central position (KCP) extends eastward. While the SAE moves and dissipates, the offshore edge of the Kuroshio moves inshore, and then transport decreases. Both the northward velocity and the KCP changes result in the Kuroshio transport change during the impingement of SAE in the case. Statistic analysis further demonstrate that eddies those can trap moving STMW are dominated by SAEs. When SAEs encounter the Kuroshio, their subsurface northward velocity increases by 67.8% and the KCP moves eastward, and thus the northward transport of the Kuroshio increases by 43.4%.

Vertical Structure and Seasonal Variability of Shear on the Southwestern Continental Slope of the East China Sea

The vertical structure and seasonal variability of shear were examined using nearly three years of mooring ADCP (acoustic Doppler current profiler) data on the southwestern continental slope of the East China Sea (ECS). Shear spectra suggest that the sub-inertial currents (SICs); near-inertial waves (NIWs); and diurnal (D1), semidiurnal (D2), and tridiurnal (D3) internal tides (ITs) dominate the local shear field. The shear exhibits a remarkable surface-intensified pattern with high values occurring mostly in the upper 200 m. Significant seasonal variations can be found in the shear, but with differences between the upper (50–200 m averaged) and lower layers (210–570 m averaged). Satellite altimeter data indicate that the meander of the Kuroshio mainstream and the Kuroshio intrusion affect the seasonal variation of total shear by mainly influencing the shear caused by SICs. In addition, the shear efficiency (SE) of D2 ITs is obviously less than that of NIWs and that of D1 and D3 ITs via analyzing the kinetic energy (KE) densities and shear caused by these motions, since the predominant mode of the former is the first baroclinic mode, while the latter is dominated by higher baroclinic modes with large vertical wavenumbers. Moreover, the SE of incoherent ITs is relatively stronger than that of coherent ITs as a result of a larger proportion of high baroclinic modes in the incoherent component compared to the coherent component, based on modal decomposition.

Variation of Internal Tides on the Continental Slope of the Southeastern East China Sea

The semidiurnal internal tides (ITs) on the continental slope of the southeastern East China Sea (ECS) exhibited abrupt enhancement in November of 2017. This enhancement resulted from the intensification of the coherent semidiurnal ITs. Coherent and incoherent semidiurnal ITs had a comparative energy contribution in October; however, coherent semidiurnal ITs dominated with a variance contribution of 90% in November. The variance contribution of vertical modes of the semidiurnal ITs varied between October and November, and the mode with most variance contribution changed from the second mode to the first mode. Altimeter data and the observed background currents indicated that the Kuroshio mainstream meandered and abruptly intruded into the ECS in November. The upper layer background currents were significantly related to the kinetic energy of the semidiurnal ITs, and the correlation coefficient between them reached 0.81. The frequent occurrences of the Kuroshio intrusion have suggested that the ITs in the ECS are susceptible to the modulation of the Kuroshio current. Numerical modeling and predication of ITs should consider the meander of the Kuroshio mainstream.

Paleoproductivity and climate evolution in the Kuroshio mainstream area over the past millennium

A variety of biomarkers of sediment core KET-1 were studied to reconstruct both paleoproductivity and phytoplankton community structure variations and to reflect the local climate change of Kuroshio mainstream of east Taiwan over the past millennium. The results showed that sterols in sediments derived from the mixed terrestrial and marine inputs, with the riverine input from eastern Taiwan as the main terrestrial sterol source. The upward increase of terrestrial organic matter over the past millennium suggested the increasing riverine input of eastern Taiwan. The variation of the marine primary productivity was mainly influenced by the nutrient supplements from the riverine input. The changes of brassicasterol to dinosterol ratio indicated a phytoplankton community shift from dinoflagellate-dominated to diatom-dominated since 1500 CE. The increasing terrestrial inputs and the change of phytoplankton community change over time were related to “dry-humid” climate change.

Transportation of Diazotroph Community From the Upstream to Downstream of the Kuroshio

AbstractThe Kuroshio is a strong western boundary current in the North Pacific, in which N2 fixation is an important source of new nitrogen. In order to understand the dynamics of diazotrophs in the Kuroshio, we used 454‐pyrosequencing of nifH gene amplicons to unravel and compare the diazotroph communities along the main path of the Kuroshio (23.44° N, 121.99° E–34.3° N, 140.1° E) with its adjacent waters. The diazotroph communities in the geographically distanced stations along the Kuroshio mainstream were highly similar, while those in the Kuroshio adjacent waters were highly varied, indicated that diazotroph communities were actively transported in the Kuroshio. Along the Kuroshio, the UCYN‐A1, UCYN‐C, and Gamma‐A were rather evenly distributed along the Kuroshio, whereas the UCYN‐A2 and Trichodesmium were mainly distributed in the Kuroshio upstream. The diazotroph communities shifted from cyanobacteria dominated in the Kuroshio upstream to proteobacteria (mainly Bradyrhizobium and sulfate‐reducing bacteria) dominated in the downstream. Among all cyanobacterial diazotrophs detected, UCYN‐A1 was the most sustained group in the Kuroshio with >103 gene copies per L and that may explain its extended distribution to higher latitude regions as compared with other cyanobacterial diazotrophs. Considering that the Kuroshio is a hot spot of unicellular cyanobacterial diazotrophs, we have adopted DNA‐directed RNA polymerase subunit beta (rpoC1) gene as a biomarker to further explore their phylogeny. Comparing the data set of nifH and rpoC1 gene fragments, we provided new insights into the phylogeny of the recently defined UCYN‐B2 and poorly understood UCYN‐C groups.

Plutonium in the western North Pacific: Transport along the Kuroshio and implication for the impact of Fukushima Daiichi Nuclear Power Plant accident

This study examined Pu source terms in the western North Pacific (WNP) based on data collected in 2014 and 2015. The basin wide 240Pu/239Pu atom ratios ranged from 0.227 to 0.263 with an average value of 0.244 ± 0.011, consistently higher than that of global fallout (~0.180). The spatial distribution of 240Pu/239Pu atom ratios showed higher values within the Kuroshio region, the main western ocean boundary current, as compared to the zone off of the Kuroshio. There was also an overall decreasing trend of 240Pu/239Pu along the Kuroshio path to its extensions. 239+240Pu activities in surface seawater exhibited a wide range from 1.15 to 4.30 mBq m−3 and their spatial distribution showed an increasing trend with latitude. Unlike the 240Pu/239Pu atom ratios, which had heavier isotopic compositions in the Kuroshio mainstream compared to the zone off of the Kuroshio, the 239+240Pu activities were higher outside the Kuroshio than within the Kuroshio. These patterns in both Pu isotopic ratios and activities point towards a unique close-in fallout source, which levels down in its source term and has a high degree of scavenging during its transport along the Kuroshio, and can be traced back to a precursor, the North Equatorial Current, which originates near the Pacific Proving Grounds (PPG) with characteristically higher 240Pu/239Pu atom ratios. High 240Pu/239Pu atom ratios found in the zone outside the Kuroshio were transported from the Kuroshio via the formation and circulation of North Pacific Intermediate Water. We further revealed, using a simple two end-member mixing model, that the PPG source contributed 60 ± 13% of the Pu in the Kuroshio zone and 45 ± 10% in the zone off of the Kuroshio. Both the comparison of Pu isotopic composition in the WNP within a radius of 500 km or 1000 km off the Fukushima Daiichi Nuclear Power Plant (FDNPP) between prior to and post the accident and a simple first order mass balance calculation in terms of atmospheric deposition and release suggest that the Pu originating from the FDNPP accident, if any, was either negligible, or the input flux of 239+240Pu was too small to significantly alter the Pu isotopic composition in the ambient seawater.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Sources and burial of particulate organic matter in the Kuroshio mainstream and its response to climate change over the past millennium

Sedimentation rate and biogenic elements of a sediment core KET-1 recovered from Kuroshio Mainstream, east of Taiwan, were determined to investigate variation of organic carbon burial and its response to climate fluctuations. The results showed that the organic carbon burial flux was mainly depended on the sedimentation rate and the input of river particles had a dilution effect on total organic carbon (TOC) content of the sediment. Based on the organic carbon burial flux, the entire depositional process could be divided into three stages, which exactly corresponded to three specific climate periods over the past millennium. During the Medieval Warm Period (MWP, ad 1150–1250), the strong East Asian summer monsoon (EASM) caused more river input to the sea, resulting in high sedimentation rate and high organic carbon burial flux, meanwhile the dilution effect of river particles as well as low marine primary productivity resulted in relatively low proportion of marine organic matter (MOM) in the sediment, which was reflected by low TOC content. During the Little Ice Age (LIA, ad 1400–1835), lower organic burial flux and higher marine primary productivity as well as higher TOC content in the sediment were accompanied by weakened EASM. In the Modern Period (MP, ad 1835–present), however, the correlation between organic carbon burial and climate fluctuations that was found to be significant during the MWP and LIA had been greatly changed, and anthropogenic forcing was thought to be the primary driving factor. Overall, the climate variability and anthropogenic activities over the past millennium dominated the organic carbon burial in the sediment along the Kuroshio mainstream.

Carbon Chemistry in the Mainstream of Kuroshio Current in Eastern Taiwan and Its Transport of Carbon into the East China Sea Shelf

Comprehensive carbon chemistry data were measured from the mainstream of Kuroshio, off eastern Taiwan, in May 2014. Results indicated that variations of pH@25 °C, POC, ΩCa, DIC, pCO2 and RF were closely related to the characteristics of various water types. Phytoplankton photosynthesis played important roles in DIC variation in Kuroshio Surface Water (KSW), whereas the DIC variation in Kuroshio Subsurface Water (KSSW) was probably influenced by the external transport of DIC-enriched water from the South China Sea. Vertical profiles of hydrological parameters and carbonate species indicated that the Kuroshio Current off eastern Taiwan could intrude into the ECS shelf as far as 27.9° E, 125.5° N in spring. What is more, the KSW, KSSW and Kuroshio Intermediate Water (KIW) could convey DIC into the East China Sea (ECS) with flux of 285, 305 and 112 Tg C/half year (1 Tg = 1012 g), respectively. The relevant flux of POC was 0.16, 2.93 and 0.04 Tg C/half year, respectively. Consequently, the intrusion of Kuroshio could probably exert a counteracting influence on the potential of CO2 uptake in the ECS, which needs further study.

Intrusion of the Kuroshio into the South and East China Seas

The northward-flowing Kuroshio often intrudes westward and modulates the water masses of the South and East China Seas. These intrusions transcend multiple scales in time and space, which we demonstrate here using various independent data sets. There are two hot spots of intrusion, one in the Luzon Strait and the other off northeast Taiwan, which occur synchronously when the upstream Kuroshio weakens during winter. Beyond seasonal time scales, the two intrusions were not synchronous during 1993–2013. While intrusions into the South China Sea echoed the Pacific Decadal Oscillation, the intrusion northeast of Taiwan decreased markedly before 2002 but regularly reached the shelf thereafter. This change was due to the influence of westward impingements of cyclonic eddies from the open ocean on the Kuroshio main stream in place of anticyclonic eddies. During 1993–2001, decreasing cyclonic eddy impingements moved the Kuroshio away from northeast Taiwan, weakening the Kuroshio intrusion onto the East China Sea shelf. Thereafter, enhanced cyclonic eddy impingement during 2002–2013 weakened the Kuroshio transport, moving it closer to the shelf and enhancing its intrusion into the East China Sea.

Dissolved barium as a tracer of Kuroshio incursion in the Kuroshio region east of Taiwan Island and the adjacent East China Sea

From May to June 2014, the geochemical characteristics of dissolved barium (Ba) in sea water and its influx from the Kuroshio into the East China Sea (ECS) were studied by investigation of the Kuroshio mainstream east of Taiwan Island and the adjacent ECS. This allowed for the scope and extent of the Kuroshio incursion to be quantitatively described for the first time by using Ba as a tracer. The concentration of Ba in the Kuroshio mainstream increased gradually downward from the surface in the range 4.91–19.2 μg L−1. In the surface layer of the ECS, the Ba concentration was highest in coastal water and gradually decreased seaward, while it was higher in coastal and offshore water but lowest in middle shelf for bottom layer. The influx of Ba from Kuroshio into the ECS during May to October was calculated to be 2.19×108 kg by a water exchange model, in which the subsurface layer had the largest portion. The distribution of Ba indicated that Kuroshio upwelled in the sea area northeast of Taiwan Island. The north-flowing water in the Taiwan Strait restrained the incursion of Kuroshio surface water onto the ECS shelf, while Kuroshio subsurface water gradually affected the bottom of the ECS from outside. The results of end member calculation, using Ba as a parameter, showed that the Kuroshio surface water had little impact on the ECS, while the Kuroshio subsurface water formed an intrusion current by climbing northwest along the bottom of the middle shelf from the sea area northeast of Taiwan Island into the Qiantang Estuary, of which the volume of Kuroshio water was nearly 65%. Kuroshio water was the predominant part of the water on the outer shelf bottom and its proportion in areas deeper than the 100 m isobath could reach more than 95%. In the DH9 section (north of Taiwan Island), Kuroshio subsurface water intruded westward along the bottom from the shelf edge and then rose upward (in lower proportion). Kuroshio water accounted for 95% of the ocean volume could reach as far as 122°E. Ba was able to provide detailed tracing of the Kuroshio incursion into the ECS owing to its geochemical characteristics and became an effective tracer for revealing quantitative interactions between the Kuroshio and the ECS.

Eddy formation and surface flow field in the Luzon Strait area during the summer of 2009

The formation of mesoscale eddies and the structure of the surface flow field in the Luzon Strait area were examined using in-situ CTD data, Argo float data, and multi-satellite remote sensing data collected from May to August 2009. The results show that vigorous water exchange between Kuroshio water and South China Sea (SCS) water began to emerge over the 200 m water column throughout the strait. Based on an objective definition of surface currents, float A69 tracked an anti-cyclonic eddy southwest of Taiwan Island under a Lagrangian current measurement. The salinity inside the anti-cyclonic eddy was higher than in typical SCS water but lower than in Kuroshio mainstream water, indicating that this eddy was induced by Kuroshio frontal intrusion through the Luzon Strait and into the SCS. From hydrographic data, we propose that continuous horizontal diffusion with high-salinity characteristics in the subsurface layer could extend to 119A degrees E or even further west. The high-temperature filament, large positive sea level anomaly and clockwise geostrophic current all confirmed the existence of this warm eddy in May and June. A strongly negative wind stress curl maintained the eddy until it died. The surface flow field during July and August was rather complicated. Float A83 described an east-west orientated shuttle run in the 20A degrees N section that was not reported by previous studies. At the same time, float A80 indicated a Kuroshio bend into the north-central region of Luzon Strait but it did not cross 120.5A degrees E. The water mass rejoining the Kuroshio mainstream from the southern tip of Taiwan Island was less saline, indicating an entrainment of water from SCS by the Kuroshio bend.

Transient modulation of Kuroshio upper layer flow by directly impinging typhoon Morakot in east of Taiwan in 2009

AbstractThis study deals with the modulation of the Kuroshio upper layer flow (KULF) in response to the passage of Typhoon Morakot in 2009, using Regional Oceanic Modeling System (ROMS) and in situ measurements from Argos drifters and Argo floats. The analysis of the simulated current fields near the typhoon track revealed an intermittency phenomenon of the KULF, which was almost shut down for at least 6 hours. The process begun 2 days prior to the approach of typhoon center due to blockage of the KULF by the steadily northerly winds, and lasted for more than 2 days, simultaneously shifting the Kuroshio main stream (KMS) path. When the Morakot gradually moved closely to the Kuroshio, the KMS shifted vertically from the surface layer to deeper layer of 50 – 100 m depth, and the maximum current speed in the KMS decreased from more than 1.3 m s−1to less than 1.1 m s−1. When the Morakot center approached about 100 km to the original position of the Kuroshio, the KULF spread eastward for 1.5 degrees at 24°N. When the Morakot center moved to the original position of the KMS, the Kuroshio abruptly rushed with a maximum speed near 1.4 m s−1. Meanwhile, an offshore cool jet originating from southeastern tip of Taiwan was generated and extended northward along the Kuroshio. In the cool jet, the lowest temperature reached about 5°C lower than the ambient waters. Modeled current variations and the cool jet during the Morakot passage were validated by in situ measurements.

Satellite views of the episodic terrestrial material transport to the southern Okinawa Trough driven by typhoon

AbstractUsing satellite‐derived water transparency (alias Secchi depth) images, we found clear signals of terrestrial material transport to the southern Okinawa Trough triggered by the Typhoon Morakot in August 2009. Three sources were identified: one is from the eastern coast of Taiwan, another is from the western coast of Taiwan, and the other is from the coast of mainland China. Carried by northward flows, typhoon‐triggered terrestrial materials from both sides of Taiwan's coasts were transported to the region northeast of Taiwan. Moreover, the terrestrial material from the coast of mainland China could cross the Taiwan Strait and be further transported to the region northeast of Taiwan. These typhoon‐induced terrestrial materials off northeastern Taiwan could then be transported to the southern Okinawa Trough along the western edge of the Kuroshio. In addition to the particulate terrestrial material transported, nutrients might also be transported to the Kuroshio main stream. A significant phytoplankton bloom was observed along the Kuroshio path for about 300 km off northeast of Taiwan. Our results indicate that episodic cyclone‐driven terrestrial material transport could be another source of mud in the southern Okinawa Trough.

Variability of island-induced ocean vortex trains, in the Kuroshio region southeast of Taiwan Island

This study examines the horizontal scale and spatial characteristics of island-induced ocean vortex trains (IOVTs) in the Kuroshio region southeast of Taiwan Island using European remote sensing satellite ERS-1 SAR imagery. US Aqua satellite Moderate Resolution Imaging Spectroradiometer (MODIS) data are used to analyze the sea surface temperature (SST) features of the study area. Seasonal composites of SST images show that the IOVTs are current-induced vortexes rather than wind-induced ones. Furthermore, using the HYbrid Coordinate Ocean Model/Navy Coupled Ocean Data Assimilation (HYCOM/NCODA) system that generated current and sea surface height anomaly data, the temporal and spatial variability of the Green Island IOVTs is analyzed. The variability of IOVTs within this region shows a distinct seasonality. This seasonal variability of IOVTs is closely associated with the shoreward shift of Kuroshio mainstream driven by the winter northeasterly monsoon. This scenario is verified by vector empirical orthogonal function analysis focused on the weak IOVT period in 2012. In addition to meandering of the Kuroshio, westward-propagating mesoscale eddies and the arrival of typhoons play an important role in modifying the variability of IOVTs at intraseasonal timescale.

Optimal precursor of the transition from Kuroshio large meander to straight path

We used the conditional nonlinear optimal perturbation (CNOP) method to explore the optimal precursor of the transition from Kuroshio large meander (LM) to straight path within a barotropic inflowout-flow model, and found that large amplitudes of the optimal precursor are mainly located in the east of Kyushu, which implies that perturbations in the region are important for the transition from LM to straight path. Furthermore, we investigated the transition processes caused by the optimal precursor, and found that these processes could be divided into three stages. In the first stage, a cyclonic eddy is advected to the formation region of the Kuroshio large meander, which enhances the LM path and causes a cyclonic eddy to shed from the Kuroshio mainstream. This process causes the LM path to change into a small meander path. Subsequently, the small meander is maintained for a period because the vorticity advection is balanced by the beta effect in the second stage. In the third stage, the small meander weakens and the straight path ultimately forms. The positive vorticity advecting downstream is responsible for this process. The exploration of the optimal precursor will conduce to improve the prediction of the transition processes from LM path to straight path, and its spatial structure can be used to guide Kuroshio targeted observation studies.

Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front

Direct observations of microstructure near the Kuroshio Front were conducted in August 2008 and October 2009. These show negative potential vorticity (PV) in the mixed layer south of the front, where directly measured turbulent kinetic energy dissipation rates are an order magnitude larger than predicted by wind‐scaling. These elevated dissipation rates scale better with an empirical scaling, which considers local wind and Ekman buoyancy flux driven by downfront wind. Near‐zero PV in the thermocline under the Kuroshio mainstream is observed at 200–300 m depth, with dissipation exceeding open ocean thermocline values by factors of 10–100. Overall, the large turbulent dissipation rates measured in the Kuroshio can be categorized into two groups, one characterized by low Richardson number along the Kuroshio Front thermocline, and the other characterized by high stratification away from the Kuroshio mainstream. The former is attributed to mixing by unbalanced frontal ageostrophic flows, and the latter is attributed to internal wave breaking. On average, both groups appear in regions of large horizontal density gradients. Observed thermohaline structure shows low salinity tongues from the surface to over 300 m depth and deep cold tongues, extending upward from 500 to 100 m depth in a narrow (20 km) zone, suggesting down and upwelling driven by geostrophic straining, which is confirmed by Quasigeostrophic‐Omega equation solutions. This implies that adiabatic along isopycnal subduction and diabatic diapycnal turbulent mixing acting in tandem at the Kuroshio Front likely contribute to NPIW formation.

A three-dimensional mixed finite-difference Galerkin function model for the oceanic circulation in the Yellow Sea and the East China Sea in the presence of M 2 tide

This paper, as a sequel to Lee et al. (Continental Shelf Research 20 (2000) 863) describes the simulation of the oceanic current in the Yellow Sea (YS) and the East China Sea (ECS) with forcings of M 2 tide as well as oceanic flows prescribed at the open boundary. The model is three dimensional and barotropic, and uses a finite-difference approximation in the horizontal plane and function expansions in the vertical direction. The bottom stress is represented by the conventional quadratic friction law and the vertical eddy viscosity takes a flow-related form. A radiation condition is employed along the open boundaries to handle the M 2 tide and oceanic flows simultaneously. From a series of numerical calculations with M 2 tide forcing only, the bottom friction coefficient, 0.0035, has been found as an optimum value with which RMS errors (amplitude, phase lag) are calculated as 16.4 cm, 19.5°. Calculations have also been carried out to investigate the effects of using an empirical function expansion for the current profiles below the main stream of Kuroshio. Despite the bias of the tidal propagation and the associated flux, the tidal chart has been calculated with tolerable accuracy. The model calculation confirms the results of Exp. 4 of Lee et al. (Continental Shelf Research 20 (2000) 863), in that the tide-enhanced bottom friction effectively blocks the penetration of northwestward flow into the YS known as the Yellow Sea Warm Current (YSWC). The presence of small gyres, however, complicates the circulation near the southern YS and west of Cheju Island and tidal residual currents omnipresent at the shallow sea region off the Chinese coast between 32°N and 34.5°N also contribute to the suppression of the formation of the YSWC. The distribution of the sea surface elevation averaged over the M 2 tidal period is qualitatively in good agreement with that of Yanagi et al. (Continental Shelf Research 17 (1997) 655), calculated from the TOPEX altimetric data. Calculation with an inflow of 25.1 Sv through open boundaries on both sides of Taiwan shows that 1.15 Sv goes through Korea/Tsushima Strait, 0.36 Sv through the north of Tokara Strait (between Kyushu and Yakushima Island), 20.95 Sv through Tokara Strait and 2.77 Sv through open boundaries along the Ryukyu Islands. Calculations with fortnightly variation in M 2 open boundary forcings show that the oceanic circulation as well as tidal residuals vary significantly in the southern Yellow Sea; penetration of oceanic flows into the Yellow Sea occurs deeper at the neap tide than at the spring tide. The seasonal variation in the volume transport of the oceanic inflows/outflows without baroclinic effects has little effect on the exchange between the YS and the ECS.

Continental Slope Flow Northeast of Taiwan

Hydrographic observations and current measurements with a Shipboard Acoustic Doppler Current Profiler over the continental shelf–slope junction northeast of Taiwan during 10–17 August 1994 allow the construction of the mesoscale flow pattern generated by the collision of the Kuroshio and a stretch of the continental shelf that has turned to run nearly east–west. The pattern is made up of a deflected Kuroshio mainstream to the east, an intrusion of Kuroshio water onto the continental shelf region, a counterclockwise circulation over Mien-Hwa Canyon (MHC) immediately northeast of Taiwan, a deep southwestward countercurrent along the northern wall of MHC, and a seaward outflow of continental shelf water around the northern coast of Taiwan. The hydrography features a cold dome over the west side of MHC that consisted of subsurface Kuroshio water. A temperature–salinity plot of all the station data shows the incorporation in the neighborhood of Taiwan of continental shelf water into the Kuroshio.