Kuroshio East Of Taiwan Research Articles

Eddy Deformation and Kinetic Energy Conversion During the Eddy‐Kuroshio Interaction East of Taiwan

AbstractEddy‐mean flow interactions have always been of interest to researchers because they are among the causes of energy cascades. Different from most previous studies, this research concentrates on the Kuroshio Current's effect on an eddy during multiple independent elliptical eddy‐Kuroshio interactions. Eddy deformation during the eddy‐Kuroshio interaction is applied as the statistical criterion to select the typical eddies that interact with the Kuroshio east of Taiwan. There are 9 anticyclonic eddies (AEs) and 15 cyclonic eddies (CEs) that satisfy the criteria from the 1993 to 2018 satellite altimeter data. The blocking effect caused by the anti‐phase velocity at the contact interface of the CE and Kuroshio results in the eddy kinetic energy (EKE) at the CE edge being obviously inhomogeneous, while the AE does not result in the same effect. Moreover, the temporal variation in EKE during the eddy‐Kuroshio interaction precedes the AE/CE deformation by approximately 0.55/0.27 interaction cycles. The Kuroshio has different modulation mechanisms for different eddy polarities. Barotropic and baroclinic instabilities contribute to the generation of EKE, with barotropic instabilities providing more than 10 times as much as baroclinic instabilities for the AE and 4 times as much for the CE. The advection effect of the AE on the Kuroshio is in anti‐phase with the effect of the Kuroshio on the AE, which eventually makes the AE shrink. They are in phase for the CE and lead the CE to eventually become meandering.

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%.

Plankton Community Respiration and Particulate Organic Carbon in the Kuroshio East of Taiwan.

Biological organic carbon production and consumption play a fundamental role in the understanding of organic carbon cycling in oceans. However, studies on them in the Kuroshio, the western boundary current in the North Pacific Ocean, are scarce. To better understand the variations of plankton community respiration (CR) and particulate organic carbon (POC), eight cruises. which covered four seasons over a 2-year period, were surveyed across the Kuroshio at the KTV1 transect east of Taiwan. Spatially, a coastal uplift of isotherms (i.e., onshore lifting and offshore deepening) was observed along the KTV1 transect. During the uplift, the cold and nutrient-rich deep waters shoal to shallow water and enhance phytoplankton growth, resulting in higher values of phytoplankton, POC, and plankton CR on the onshore side. In this study, phytoplankton was dominated by picophytoplankton including Prochlorococcus, Synechococcus, and picoeukaryotes. Plankton CR was low, and its mean depth-normalized integrated rate (the upper 100 m water depth) ranged from 7.07 to 22.27 mg C m-3 d-1, to which the picophytoplankton and heterotrophic bacteria contributed the most. The mean depth-normalized integrated value of POC ranged from 12.7 to 21.6 μg C L-1. POC is mainly associated with phytoplankton biomass with a mean carbon ratio of chlorophyll a/POC ≈ 1.03. All results suggest that plankton CR and POC variations may be associated with picoplankton dynamics in the Kuroshio.

Radiocesium in the Taiwan Strait and the Kuroshio east of Taiwan from 2018 to 2019

The release of anthropogenic radiocesium to the North Pacific Ocean (NPO) has occurred in the past 60 years. Factors controlling 137Cs (half-life, 30.2 year) and 134Cs (half-life, 2.06 year) activity concentrations in the Kuroshio east of Taiwan and the Taiwan Strait (latitude 20° N–27° N, longitude 116° E–123° E) remain unclear. This study collected seawater samples throughout this region and analyzed 134Cs and 137Cs activity concentrations between 2018 and 2019. A principal component analysis (PCA) was performed to analyze the controlling factors of radiocesium. Results of all 134Cs activity concentrations were below the detection limit (0.5 Bq m−3). Analyses of water column 137Cs profiles revealed a primary concentration peak (2.1–2.2 Bq m−3) at a depth range of 200–400 m (potential density σθ: 25.3 to 26.1 kg m−3). The PCA result suggests that this primary peak was related to density layers in the water column. A secondary 137Cs peak (1.90 Bq m−3) was observed in the near-surface waters (σθ = 18.8 to 21.4 kg m−3) and was possibly related to upwelling and river-to-sea mixing on the shelf. In the Taiwan Strait, 137Cs activity concentrations in the near-surface waters were higher in the summer than in the winter. We suggest that upwelling facilitates the vertical transport of 137Cs at the shelf break of the western NPO.

Stronger Intraseasonal Variability Observed Below the Seasonal Thermocline in the Kuroshio East of Taiwan During 2014 and 2015

AbstractIntraseasonal variability in the Kuroshio east of Taiwan was studied using moored observations at 23°N, 122°E from November 17, 2014 to September 5, 2015, high‐resolution ocean model data, and altimetry data. It is found for the first time that the observed meridional velocity has a peak signal at 50–60 days with a significant maximum variation at 200–600 m depth. Model outputs show that the 50–60‐day variability at the mooring station is related to meandering of the Kuroshio at 23°N. Both surface eddies, which can extend downward to ∼1,000 m depth, and coastal cold water upwelling associated with cyclonic eddies from deep layers can greatly change the isopycnal slope below ∼200 m depth at 23°N, 122°E, and their joint effect results in the maximum variation at 200–600 m depth. Ocean eddies southeast of Taiwan propagate northward along the current axis of the Kuroshio and have a large impact on intraseasonal variability. Case analysis indicates that the formation of ocean eddies or upwelling near the coast may be related to the Lüdao and Lanyu islands southeast of Taiwan or Kuroshio transport along the southern tip of Taiwan. In addition, the evolution of subsurface meridional velocity from mooring observations is inconsistent with that of the surface but is closely related to the evolution of local temperature below the seasonal thermocline. The maximum intraseasonal variability below the seasonal thermocline highlights the complex dynamics in the Kuroshio east of Taiwan.

Resource availability affects temporal variation of phytoplankton size structure in the Kuroshio east of Taiwan

AbstractThe on‐going discussion concerning how environmental factors determine phytoplankton size structure has centered around two hypotheses: (H1) The resource‐size relationship predicts that normalized biovolume size spectrum (NBSS) slopes for phytoplankton are progressively shallower with increasing resource availability and (H2) The temperature‐size relationship predicts that phytoplankton NBSS slopes steepen with increasing water temperature. To test these hypotheses, we examined 72 phytoplankton assemblage collections in the Kuroshio east of Taiwan. Total phytoplankton biomass was used as a proxy for resource availability instead of nutrients because nutrients are depleted and do not represent resource availability for oligotrophic seas. We found no significant relationship between NBSS slopes with temperature, providing little support for the temperature‐size rule. In contrast, a positive relationship between NBSS slopes and total biomass for most of the year lends general support to the resource‐size relationship, except during the winter and early spring. To explain this exception, we hypothesize that resource pulses occurring during the cold seasons are used more efficiently by small cells and promote faster growth of small relative to large phytoplankton because these pulses take place after a long period of resource depletion in oligotrophic seas; thus, the NBSS slopes become much steeper than would be expected from a positive resource‐size relationship. This deviation can be considered as nonsteady state in terms of phytoplankton size structure relative to resources. Nevertheless, we cannot rule out the possibility that grazing effects also play an important role in controlling phytoplankton size structure.

Nutrient flux and transport by the Kuroshio east of Taiwan

The Kuroshio is the western boundary of the North Pacific Ocean. To estimate the nutrient flux and transport in the upper 250m, measurements were conducted along a transect across the Kuroshio at 23.75° N, east of Taiwan. The results showed that the Kuroshio flowed northward at a maximum velocity varying between 0.7 and 1.4ms−1 at a depth of 100m. The flow volume transport in the upper 250m ranged from 13.43 to 17.51Sv, accounting for 51–67% of the transport above 1000m. The shoreward sloping isopycnals were revealed to be favorable for carrying nutrients onto the East China Sea shelf. The “nutrient stream” was located primarily at depths from 300 to 600m, and the maximum flux ranged from 4.2 to 7.9mmolm−2s−1 for nitrates and from 0.4 to 0.7mmolm−2s−1 for phosphates. Nutrient transport in the upper 250m varied within the range of 16.8–31.1kmols−1 for nitrates and 0.8–3.9kmols−1 for phosphates. This study suggests that 1) nitrate transport in the upper 250m increases and 2) phosphate transport above 1000m decreases during downstream transport, implying that the Kuroshio interacts with nearby ecosystems during its downstream journey.

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Fluctuations of the thermal fronts off northeastern Taiwan

[1] A high-resolution sea surface temperature (SST) data derived from several satellites is used to investigate the variability of the thermal front off northeastern Taiwan. Hidden by a dominant annual cycle, the SST data cannot reveal the thermal front fluctuation in the form of Hovmoller diagram. An innovative methodology has been applied to the SST satellite imagery to derive the SST Standardized Index (SSTSI), capable of revealing the frontal variability with multiple time scales. Principal component analysis shows that the SSTSI variation consists mainly of two modes. Mode 1 represents a strong annual cycle related to the seasonal reversal of the monsoonal winds. The temperature gradient is enhanced in winter and a cold dome is observed off northern Taiwan in summer. Mode 2 is highly correlated with the upstream Kuroshio variability. The shoreward (seaward) migration of the thermal front takes place when the Kuroshio transport weakens (strengthens). The results are consistent with transports estimated by tidal gauge measurements, satellite altimeter-based sea level anomaly, and surface flow patterns derived from high-frequency radars. Mode 2 is coherent with the Kuroshio transport through the East Taiwan Channel at periods of 120 and 45 d with a time lag of 40 and 11 d, respectively. This 120 d fluctuation is due to the interaction between westward-propagating eddies and the Kuroshio east of Taiwan, while the 45 d signal arises from the Kuroshio's self-instability. The interannual variations of the SST pattern in winter and summer are also discussed.

Generation of a trigger meander for the 2004 Kuroshio large meander

A generation process of a small meander southeast of Kyushu, which triggered the subsequent Kuroshio large meander in 2004, has been investigated using an ocean data assimilation and prediction system. The assimilation result reveals that the small meander is the most developed one since 1993. A cyclonic eddy propagating westward from the Kuroshio recirculation (KR) and a disturbance from the East China Sea (ESC) that accompanies a high potential vorticity (high‐PV) anomaly play important roles for the generation of the small meander. The cyclonic eddy triggers an initial cyclonic anomaly southeast of Kyushu. When the high‐PV anomaly passes through the Tokara Strait, a large positive vorticity is generated because the stretching of the water column is enhanced. The positive vorticity is brought in the cyclonic anomaly and accumulates there. As a result, the small meander is generated. From two sensitivity experiments an occurrence condition is determined by the cyclonic eddy from the KR by triggering the initial small‐scale meander, and the disturbance from the ECS contributes to the horizontal scale of the small meander. Tracing back the disturbance further to the upstream region of the Kuroshio, it is revealed that the disturbance is a frontal wave with a large horizontal scale generated by collision between a strong anticyclonic eddy and the Kuroshio east of Taiwan. The frontal wave propagates along the Kuroshio accompanying high‐PV generated around the continental shelf edge by viscosity. Thus the large frontal wave could contribute to the generation of the developed trigger meander by transporting a larger amount of high‐PV anomaly to the Tokara Strait.

Uncertainty reduction of estimated geostrophic volume transports with altimeter observations east of Taiwan

The common geostrophic estimation of ocean current velocity uses only water temperature and conductivity profiles. The geostrophic volume transport of a western boundary current, like the Taiwan Current (Kuroshio east of Taiwan), between the coast and its eastern boundary can be easily estimated based on hydrographic survey data. But the eastern boundary of the Taiwan Current is very uncertain due to extremely variable hydrographic conditions. This uncertainty is strongly correlated with the propagating mesoscale eddies originating from the interior of the western North Pacific Ocean. The uncertainty of estimated transport can be greatly reduced if eddy distribution is considered when determining the integration boundaries with the assistance of satellite altimeter measurements. Eight hydrographic surveys east of Taiwan between November 1992 and June 1996 are demonstrated in this study. The average geostrophic transport of the Taiwan Current with a reference set to 1000 dbar at 22°N between the east coast of Taiwan and 124°E is 22.9 ±14.2 Sv and changes to 22.1 ± 8.3 Sv, the uncertainty of which is nearly halved after taking account of the eddy distribution. The estimation uncertainty is insensitive to vertical displacements of the reference level within the depth range between 800 and 2000 dbar.

The Kuroshio East of Taiwan: Modes of Variability and Relationship to Interior Ocean Mesoscale Eddies

Abstract Observations from the World Ocean Circulation Experiment PCM-1 moored current meter array in the East Taiwan Channel are analyzed and combined with TOPEX/Poseidon altimetry data and the Parallel Ocean Climate Model simulation to study Kuroshio variability and relationships to westward propagating sea surface height anomalies in the Philippine Sea. Approximately 60% of the total subinertial velocity and temperature variance in the Kuroshio east of Taiwan is associated with so-called “transport” and “meandering” modes revealed from empirical orthogonal function analysis. The transport mode is dominated by a 100-day peak, while the most coherent energetic meandering signals are found in three limited frequency bands centered near periods of 100 days, 40 days, and 18 days. The detailed structure of the meanders is studied by frequency domain EOF analysis, which also reveals a higher frequency meander centered near 10 days confined to the western side of the channel. On the 100-day timescale, the Kuro...

The Kuroshio East of Taiwan: Moored Transport Observations from the WOCE PCM-1 Array

Observations from the WOCE PCM-1 moored current meter array east of Taiwan for the period September 1994 to May 1996 are used to derive estimates of the Kuroshio transport at the entrance to the East China Sea. Three different methods of calculating the Kuroshio transport are employed and compared. These methods include 1) a “direct” method that uses conventional interpolation of the measured currents and extrapolation to the surface and bottom to estimate the current structure, 2) a “dynamic height” method in which moored temperature measurements from moorings on opposite sides of the channel are used to estimate dynamic height differences across the current and spatially averaged baroclinic transport profiles, and 3) an “adjusted geostrophic” method in which all moored temperature measurements within the array are used to estimate a relative geostrophic velocity field that is referenced and adjusted by the available direct current measurements. The first two methods are largely independent and are shown to produce very similar transport results. The latter two methods are particularly useful in situations where direct current measurements may have marginal resolution for accurate transport estimates. These methods should be generally applicable in other settings and illustrate the benefits of including a dynamic height measuring capability as a backup for conventional direct transport calculations. The mean transport of the Kuroshio over the 20-month duration of the experiment ranges from 20.7 to 22.1 Sv (1 Sv ≡ 106 m3 s−1) for the three methods, or within 1.3 Sv of each other. The overall mean transport for the Kuroshio is estimated to be 21.5 Sv with an uncertainty of 2.5 Sv. All methods show a similar range of variability of ±10 Sv with dominant timescales of several months. Fluctuations in the transport are shown to have a robust vertical structure, with over 90% of the transport variance explained by a single vertical mode. The moored transports are used to determine the relationship between Kuroshio transport and sea-level difference between Taiwan and the southern Ryukyu Islands, allowing for long-term monitoring of the Kuroshio inflow to the East China Sea.