A process study of interactions between a warm eddy and the Kuroshio Current in Luzon Strait: The fate of eddies

Journal of Marine Systems 194 (2019) 66–80

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A process study of interactions between a warm eddy and the Kuroshio Current in Luzon Strait: The fate of eddies Shengmu Yanga,b, Jiuxing Xinga, Jinyu Shengc, Shengli Chena, Daoyi Chena,b,

T

⁎

a

Shenzhen Key Laboratory for Coastal Ocean Dynamic and Environment, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China School of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China c Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada b

ARTICLE INFO

ABSTRACT

Keywords: Mesoscale eddies Eddy-current interaction The Kuroshio Current Luzon Strait Numerical model Process study

Satellite observations reveal many mesoscale eddies in the West Pacific Ocean (WPO) that propagate westwards and eventually interact with the Kuroshio Current. Examination of global ocean and sea ice reanalysis data in years 2008–2015 suggests trajectories of these mesoscale eddies over the Kuroshio zone can be categorized into three different patterns: ~63% of mesoscale eddies dissipating during the eddy-current interaction, ~33% moving to the north along the Kuroshio and only ~4% passing through the Kuroshio and Luzon Strait (LS) to enter to the South China Sea (SCS). A three-dimensional ocean circulation model based on the MIT General Circulation Model (MITgcm) is used to study the evolution of a westward propagating mesoscale eddy during the eddy-current interaction. Thirteen numerical experiments are conducted with the circulation model driven by currents specified at the southern and northern open boundaries to represent the influence of the Kuroshio. A mesoscale eddy is initialized to the east of the Kuroshio and the model is integrated for 70 days in each experiment. Model results suggest that the northward-flowing Kuroshio Current and the seamount topography within LS form a barrier for the westward propagating eddies to enter the South China Sea (SCS). Non-linear interactions between the Kuroshio Current, local topography and westward propagating mesoscale eddies can generate localized eddies in LS which could be shed into the SCS. Furthermore, the eddy-current interaction is found to be one of mechanisms for generating a multi-eddy structure in LS region.

1. Introduction Mesoscale eddies propagate westwards under the influence of the planetary β effect (Sutyrin et al., 2003; Chelton et al., 2011). Most of these mesoscale eddies eventually interact with the western boundary currents (or western boundaries) except for those dissipated during their propagations. Zhai et al. (2010) suggested that the western boundary acts as a “graveyard” for the westward-propagating mesoscale eddies based on results produced by a reduced-gravity model and satellite altimetry data. The eddy-current interaction has been studied for both the evolution of mesoscale eddies and meander formation processes of the Kuroshio to the east of Taiwan (Jan et al., 2017; Zhang et al., 2001), the south of Japan (Waseda et al., 2002, 2003; Miyazawa et al., 2004, 2008), and over the Kuroshio extension region (Waterman et al., 2011). The Kuroshio Current (referred to as the Kuroshio hereafter) is a western boundary current in the West Pacific Ocean (WPO), which

evolves from the North Equatorial Current in the offshore deep waters east of the Philippines (Zheng et al., 2011). The Kuroshio has the characteristics of warm temperatures, high salinity, and high current speeds. The Kuroshio flows northward along the east coast of the Luzon Island, passes by Luzon Strait (LS), the east coast of Taiwan Island, and Okinawa Trough in the East China Sea, and returns back to the WPO through Tokara Strait (Hu et al., 2008) (Fig. 1). LS is a large strait along the western boundary of the Pacific Ocean. Within LS, there are two large-size north-south ridges known as Lan-Yu Ridge to the east and Heng-Chun Ridge to the west. The Pacific Ocean is on the eastern side and the South China Sea (SCS) is on the western side of LS, making LS to be the main channel for exchanging of the mass, momentum and energy between the WPO and SCS (Hu et al., 2012; Zheng et al., 2011). Several studies were conducted in the past to investigate physical dynamics associated with the Kuroshio intrusion in LS, such as the characteristics of the Kuroshio's penetration in LS (Yuan et al., 2006; Liang et al., 2008; Hwang et al., 2007; Nan et al., 2015), and eddy

⁎ Corresponding author at: Shenzhen Key Laboratory for Coastal Ocean Dynamic and Environment, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China. E-mail address: [email protected] (D. Chen).

https://doi.org/10.1016/j.jmarsys.2019.02.009 Received 2 August 2018; Received in revised form 23 February 2019; Accepted 26 February 2019 Available online 01 March 2019 0924-7963/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Major topographic features of Luzon Strait (LS) and its adjacent northwest Pacific Ocean, with the main path of the Kuroshio (black thick dashed line).

shedding from the Kuroshio bend (Jia and Liu, 2004; Jia et al., 2005; Nan et al., 2011; Li et al., 1998). A loop current is formed when the northeast monsoon deflects the Kuroshio through LS. Based on wind observations and satellite-derived sea-surface temperature (SST) images in LS, Farris and Wimbush (1996) suggested that a loop current development is largely determined by the strength of wind stress. In addition to the wind forcing, perturbation of westward propagating eddies is also one of the important factors for the Kuroshio path variability (Zheng et al., 2017). In the LS region, the Kuroshio is more prone to deformation under disturbances because of the highly-varying local topography in this region. The current wisdom is that the path and pattern of the Kuroshio in LS are determined by perturbations coming from the Northern Pacific Ocean. During its interaction with eddies, the Kuroshio behaves like an unsteady flow with its stream path frequently modified, in a way of cutting off, meandering and branching (Zheng et al., 2017). Direct velocity observations of the Kuroshio at its entrance to LS demonstrated that the Kuroshio is modulated strongly by impinging of westward propagating eddies (Lien et al., 2014; Tsai et al., 2015). Therefore, the eddy-current interaction plays a significant role in the modification of the local circulation system in LS. In addition, the strength of the Kuroshio is a dominant factor for the eddy evolution in the region (Sheu et al., 2010). Previous studies indicated that the eddy movement depends strongly on the relative strength of the initial eddy to the current (Vandermeirsch et al., 2003a). Significant efforts have been made in tracking propagation paths and determining statistics of mesoscale eddies in LS and its adjacent northwest Pacific Ocean from satellite observations or/and numerical results. Sheu et al. (2010) suggested that some mesoscale eddies can pass through the Kuroshio and enter the SCS though LS based on numerical results produced by a three-dimensional (3D) ocean circulation model. Zheng et al. (2011) found that an anticyclonic mesoscale eddy passed through LS in June and July 2004. It is still a controversial issue, however, whether a mesoscale eddy can move freely through LS and enter the SCS from the northwestern Pacific. Furthermore, there is a knowledge gap about the material exchange processes in addition to the

energy transfer during an eddy passing through LS (Guo et al., 2007). Many of previous studies focused on the variation of the Kuroshio, and only a few studies investigated westward propagating eddies in the eddy-current interaction in LS. In this study, global ocean and sea ice reanalysis data from 2008 to 2015 are first analyzed to investigate the patterns of mesoscale eddies in the Kuroshio region. A 3D ocean circulation model with an idealize setup is then used to investigate influences of the strength of Kuroshio and the local topography on eddycurrent interactions in the LS region in this study. This paper is organized as follows. Section 2 describes the methods used in the study including the global ocean and sea ice reanalysis data and a 3D ocean circulation model. The model results are presented in Section 3. Discussion on the main mechanisms of the eddy-current interaction is given in Section 4. Summary is given in Section 5. 2. Method 2.1. Reanalysis data of ECCO2 The high-resolution global-ocean and sea-ice reanalysis data known as the Estimating the Circulation and Climate of the Ocean Phase II (ECCO2) in years 2008–2015 are used to determine the distribution of mesoscale eddies in LS. The ECCO2 data syntheses were produced by using the least squares fit of a global full-depth-ocean and sea-ice configuration of the MITgcm to the available satellite and in-situ data (Menemenlis et al., 2008). The ECCO2 data syntheses have been used for many studies in the past such as quantifying the role of the oceans in the global carbon cycle, understanding the recent evolution of the polar oceans, monitoring time-evolving term balances within and between different components of the Earth system, and for many other scientific applications (Menemenlis et al., 2008). The ECCO2 sea level data set (ECCO2-SSED, http://apdrc.soest. hawaii.edu/index.php) has a horizontal resolution of 1/4° and temporal resolution of one day. This data set is suitable for identifying and tracking mesoscale eddies with radii about 50–200 km over the study 67

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region. Based on the methodology suggested by Zheng et al. (2011), a mesoscale eddy is identified over an area with the closed contour line in the sea surface height anomaly (SSHA) field. Two conditions must be satisfied for identification a mesoscale eddy: (i) the SSHA of the eddy center should have a closed contour line with a value > 7.5 cm, and (ii) the eddy radius should be > 50 km. By defining the eddy center to be the position with the maximum amplitude of the SSHA within the area occupied by the eddy, the eddy trajectory is determined by tracking the movement of the eddy center in the continuous time (Doglioli et al., 2007; Chaigneau et al., 2008).

The model horizontal resolution is set to 5 km. In the vertical direction, there are 28 z-levels with the model resolution of 50 m in the upper 1000 m and coarser resolution in the lower 1000 m. The model domain has four open boundaries. The Orlanski radiation condition (Orlanski, 1976) is applied to the model variables at the model open boundaries. In the model, the lateral (horizontal) eddy viscosity coefficient is set to a value of 100 m2/s, and the vertical eddy viscosity coefficient is set to 5.0 × 10−4 m2/s. For the temperature equation, the vertical eddy diffusivity coefficient is set to 10−4 m2/s and horizontal eddy diffusivity coefficient is set to zero. The Coriolis parameter fo = 5.0 × 10−5 s−1 and the planetary parameter β = 2.0 × 10−11 m−1 s−1. The main reason to use a β plane in this study is that the β effect is the main force for the propagation of a mesoscale eddy (Yang et al., 2017). To simplify the process study in this paper, the Kuroshio in the model is initialized by a steady and vertically uniform flow specified along the central part of the southern open boundary over the eastern side of Luzon Island, with a width of 100 km. A warm mesoscale eddy is initialized with an axisymmetric Gaussian-type profile over an area to the east of the Kuroshio (see more details in Section 2.3). The previous study made by Jan et al. (2017) suggested that the horizontal gradient of stratification in the Kuroshio zone is much smaller than the counterpart in the eddy zone. For the model initial condition, the initial temperature in the far-field of the warm eddy is set to be horizontally uniform. The temperature decreases linearly with the depth in the upper 1000 m and is uniform of 4 °C below 1000 m. The sea surface height is set to zero in the initial condition and is quickly adjusted dynamically after the model starts. There is no surface heat flux and wind stress applied in the model. Due to small changes in vertical salinity of the Kuroshio (< 0.5 psu) (Zhang et al., 2016), salinity is set to be a constant of 35 (psu) for simplicity in the model. The schematic of the model setup and initial temperature distributions in the vertical are shown in Fig. 3. Thirteen numerical experiments are conducted to investigate the interaction between a mesoscale eddy and the Kuroshio (Table 1). These thirteen experiments can be divided into seven different groups in terms of influences of the Kuroshio and local topography on the eddy-current interactions. The first and third groups of experiments are used to simulate the “background” fields without any mesoscale eddy. The second group of experiments is used to simulate the circulation and eddy-current interaction under no seamount conditions. The fourth group of experiments is used to simulate the circulation and eddycurrent interaction with two idealized seamounts in LS. The fifth and sixth groups of experiments are to study the influence of the position and the size of the mesoscale eddy on the eddy-current interaction with

2.2. The circulation model and setup The hydrostatic version of the MITgcm (Adcroft et al., 2011) is used, based on the argument that the non-hydrostatic dynamics play only a minor role in the mesoscale dynamic processes to be considered in this study. The model equations in the MITgcm are expressed in z-coordinates and discretized using a staggered Arakawa C grid. Finite volume techniques are employed for the treatment of irregular geometries. The MITgcm has been used in simulating a wide range of physical oceanographic phenomena, from the ocean deep convection on the spatial scales of meters to basin-scale circulation patterns in the global ocean (Adcroft et al., 2011). The model domain covers a rectangular region of 1000 km by 900 km with two types of idealized topographic features used in this study. In the first type, the model topography has a flat ocean bottom of 2000 m with two rectangular islands to represent approximately Luzon Island to the south and Taiwan Island to the north, respectively (Fig. 2). The horizontal dimensions are 100 km × 300 km and 100 km × 200 km respectively for the former and latter idealized islands. The region between two idealized islands presents approximately Luzon Strait. The meridional width of Luzon Strait is set to 400 km in the model topography. As mentioned in the introduction, the local topography in LS has two parallel north-south seamounts known as Lan-Yu and Heng-Chun Ridges (Fig. 1). Lu and Liu (2013) found that the local bottom topography in LS exerts a control of gap-leaping behaviour of the Kuroshio, which efficiently blocks the westward propagating Rossby waves and eddy energy to get into the SCS. The seamounts adjacent to Taiwan and Luzon islands have a relatively higher elevation with the mean depth of 1000 m. In the second type of idealized topography used in this study, two idealized cylindrical seamounts with an elliptic section are added into the LS region to represent approximately the large-scale features of the Lan-Yu and Heng-Chun Ridges respectively (Fig. 2). The water depth on the top of two idealized seamounts is set to 1000 m.

Fig. 2. Schematic of model topography with flat ocean bottom and two idealized Islands for Luzon Island to the south and Taiwan Island to the north for the first type of model topography. The region between the two islands is Luzon Strait. Two idealized seamounts are added to the LS region in the second type of model tomography. The water depth is 1000 m on top of two seamounts and 2000 m elsewhere.

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Fig. 3. (a) Schematic of the model setup of the northward boundary flow, with the red arrow denoting the movement tendency of a warm mesoscale eddy. (b) The initial temperature and current conditions of the warm mesoscale eddy, with the black dashed lines and the blue solid lines denoting respectively the temperature distribution and east-west current speed contours. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

two seamounts in LS. The last group (experiment M) is the model run with a cold mesoscale eddy initialized to the east of the Kuroshio. Model results in experiment M are presented in the Appendix.

velocity is ~0.9 m/s, and the maximum surface elevation is ~0.5 m after quickly dynamic adjustment. Both open boundary values for temperature and velocity are determined using Orlanski radiation open boundary conditions. The model is integrated for 80 days from the initial state and model results are used to examine the physical processes associated with the eddy-current interactions in LS and adjacent waters.

2.3. The warm eddy structure The idealized warm mesoscale eddy is initialized in the related numerical experiments in terms of an axisymmetric Gaussian-type profile for the initial model temperature. This Gaussian-type profile is determined from various temperature observations including the moored observations (Zhang et al., 2013), Argo float data and the merged data products of satellite altimeters (Chen et al., 2010). The following equation is used to fit to the available temperature observations over southwest of Taiwan (Zhang et al., 2013):

T(x, y, z) = Tb (z) + az e

x 2 + y2

3. Results 3.1. Patterns of trajectories We first examine trajectories of warm and cold meso-scale eddies identified from the ECCO2-SSED for the 8-year period 2008–2015 (Fig. 4). During this period, total 225 eddies (with 108 warm and 117 cold eddies) were identified in the LS region (16°-28°N, 116°-128°E). Among the 108 warm eddies identified, 82 of them propagated into the Kuroshio zone and can be divided into three groups: dissipating in the interaction with the Kuroshio (~62.2%), moving northwards along the Kuroshio (~34.2%), and passing through the Kuroshio to enter the SCS (~3.6%). For the 117 cold eddies, 79 of them propagated into the Kuroshio zone and also can be divided into three groups: dissipating in the interaction with the Kuroshio (~63.3%), moving northwards along the Kuroshio (~32.9%), and passing through the Kuroshio to enter to the SCS (~3.8%). This indicates that the trajectories of all mesoscale eddies during this 8-year period in the Kuroshio zone can be categorized as three movement patterns: dissipating in the Kuroshio zone (~63% on average), moving northward along the Kuroshio Current (~33%), and crossing the Kuroshio to enter to the SCS (~4%).

(1)

2L2

where Tb(z) is the background temperature, az is a function parameter which decreases linearly with depth (−z) and L is a constant with a value of 1.5 × 104 m, (x, y, z) are position coordinates with respect to the center of the eddy, and the z-axis is positive upward. The initial velocity field within the warm mesoscale eddy is calculated using the thermal wind equation with the zero velocity set at the flat ocean bottom. The density distribution is calculated from an equation of state (Jackett and Mcdougall, 1995). Fig. 3b shows the temperature and azimuthal velocity distributions along the cross-section through the eddy center. The initial warm mesoscale eddy has 150 km in diameter occupied in the top 1000 m. The maximum surface Table 1 The list of numerical experiments. Group 1 2 3 4 5 6 Cold eddy

Case A B C D E F G H I J K L M

Topography

Eddy position

Without seamounts Without seamounts Without seamounts Without seamounts With seamounts With seamounts With seamounts With seamounts With seamounts With seamounts With seamounts With seamounts Without seamounts

No eddy No eddy East of the central LS East of the central LS No eddy No eddy East of the central LS East of the central LS Southeast of the central LS Southeast of the central LS East of the central LS East of the central LS East of the central LS

69

Eddy diameter

Kuroshio speed

– – 120 km 120 km – – 120 km 120 km 120 km 120 km 200 km 200 km 120 km

Uc = 0.25 m/s Uc = 0.50 m/s Uc = 0.25 m/s Uc = 0.50 m/s Uc = 0.25 m/s Uc = 0.50 m/s Uc = 0.25 m/s Uc = 0.50 m/s Uc = 0.25 m/s Uc = 0.50 m/s Uc = 0.25 m/s Uc = 0.50 m/s Uc = 0.25 m/s

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Fig. 4. Trajectories of warm (red) and cold (blue) eddies identified from the ECCO2-SSED data for the period 2008–2015 with open circles denoting the initial position of eddies in three cases of (a) eddies dissipating in the Kuroshio zone (~63%), (b) eddies moving northward along the Kuroshio Current (~33%), and (c) eddies crossing the Kuroshio to enter to the SCS (~4%). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Although only a small percentage (3.6–3.8%) of eddies passing through the Kuroshio, its influence on the SCS cannot be ignored.

Luzon and Taiwan Islands and an early development of a loop current in LS. At this time, there are two small-size cyclonic eddies with one off the northern coast of Luzon Island and the other off the southeastern coast of Taiwan Island. There are also two smaller-size anticyclonic eddies both over the eastern side of the Kuroshio at this time. In experiment A, the cyclonic eddy off the northern coast of Luzon Island becomes stronger and larger but still attaches to the loop current by day 30 (Fig. 5b). This cyclonic eddy pinches off from the loop current around day 40. By comparison, in experiment B, the eddy off the northern coast of Luzon Island already pinched off from the loop current in LS to enter the SCS by day 30 (Fig. 5d). As the cyclonic eddy shedding off over the western side of LS, the loop current shifts northwards and a new mesoscale cyclonic eddy is generated over the eastern side of Taiwan. Instead of generation and shedding of these local eddies into the SCS, this paper focuses mainly on the fate of a westward propagating mesoscale eddy in the eddy-current interaction. Hence, the model results in experiment A and B shown in Fig. 5 are considered to be the “background” fields, which will be extracted from model results in group 2 of numerical experiments C and D to examine the eddy shedding through LS induced by the interaction between the Kuroshio and the warm eddy.

3.2. Groups 1 and 2: model results without seamounts Previous studies suggested that the Kuroshio has significant seasonal variability, which is relatively stronger in the summer and weak in the winter (Liang et al., 2008). In this study two different Kuroshio speeds (Uc = 0.25 m/s and 0.50 m/s) are used to represent the weak (in the winter) and strong (in the summer) Kuroshio cases respectively. As a strong narrow boundary flow passes over a wide strait such as LS, the nonlinear interaction between the boundary flow (i.e., the Kuroshio) and local topography (such as piece-wise straight coastlines and sharp corners of two idealized islands shown in Fig. 2) leads to developments of meanders and mesoscale eddies in the velocity and hydrography fields. Before the study of the eddy-current interaction, we first discuss the model results without any mesoscale eddy specified initially in the model domain with the flat ocean bottom in experiments A and B (Fig. 5). The model results at day 10 in both experiments A and B (Fig. 5a and c) feature intense northward currents along the eastern coast of

Fig. 5. Surface currents (vectors) and sea levels (color shading) from experiment A (upper rows) and B (lower rows), at model day 10 (left columns) and 30 (right columns) after the start of the model integration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 6. (a–c) Surface currents (vectors) and sea levels (color shading) from experiment C and (d-e) differences in surface currents (vectors) and vorticity (color shading) between experiments C and A, at model day 10 (left columns), 30 (middle columns) and 50 (right columns) after the start of the model integration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

track the trajectory and the evolution of the warm mesoscale eddy from the current and sea level fields, due to the complicated circulation features. In this study, therefore, the warm eddy movement and structure are tracked from the simulated temperature field. Fig. 7 presents the simulated subsurface temperatures at 100 m in experiments C at six different times after the start of the model integration. In the first 30 days, the warm mesoscale eddy, which is represented by the large subsurface temperature anomaly in Fig. 7, moves southwestwards into the Kuroshio zone. Although the size of the warm mesoscale eddy decreases gradually with time for the first 50 days, the horizontal structure of the warm eddy represented by the shape of the subsurface temperature anomaly remains the nearly circular shape. The subsurface temperature distributions of the warm mesoscale eddy differ significantly at days 50 and 70 (Fig. 7e and f). At day 50 (Fig. 7e), the warm eddy interacts with the newly-formed anticyclonic eddy in the current loop and deforms under the horizontal velocity shear effect of the Kuroshio. At about day 70, the horizontal shape of the warm mesoscale eddy transforms into a long strip of filaments from the circular shape (Fig. 7f), indicating that the warm mesoscale eddy gradually dissipates and merges with the loop anticyclone of the Kuroshio over southern LS. The model results in the strong Kuroshio case in experiment D are shown in Fig. 8. At day 30 (Fig. 8b), the newly-formed cyclonic eddy off the northern coast of Luzon Island pinches off from the loop current in LS to enter the SCS. The growth and migration of this cyclonic eddy restrict the westward expansion of the loop current into the SCS. The newly-formed cyclonic eddy off the eastern side of Taiwan Island also strengthens gradually due to the form drag of topography. The sea surface current differences between experiments D and B demonstrate that anticyclonic eddies are formed in the Kuroshio zone and off the northern coast of Luzon Island. By day 30 (Fig. 8d), the anticyclonic eddy off the northern coast of Luzon Island intensifies and pinches off from the loop current to enter the SCS. There are several small-scale eddies generated along the western side of the loop current. The model results in experiment D indicate that when the Kuroshio is strong, the eddy-current interaction prompts the generation of new eddies and current meanders in LS.

We now examine the model results in the weak Kuroshio case in the model domain without seamounts (experiment C). At the beginning of the model integration, the center of the warm mesoscale eddy with a diameter of 120 km is located to the east of the central LS at about 100 km east of the Kuroshio. This warm eddy moves southwestwards under the combination of the planetary β effect and nonlinear hydrodynamic effect. At day 10, the warm eddy moves into the Kuroshio zone and interacts with the Kuroshio. The Kuroshio starts to bend to the west and intrude into LS (Fig. 6a). By day 30, a loop current has been formed in LS due to the push of the southwestward propagating warm mesoscale eddy (Fig. 6b). Because of the strong horizontal velocity shear existing in the area between the northward Kuroshio and the eastern coast of Luzon Island, a cyclonic eddy is generated locally off the northern coast of Luzon Island. This cyclonic eddy is strengthened to push the southern part of the loop current within LS to further north (Fig. 6c). The anticyclonic eddies over the eastern side of the Kuroshio move initially northwards and then northwestwards into a meander (Fig. 6b). As the loop current becomes stronger and evolves to a big loop eddy, the warm mesoscale eddy dissipates due to its interaction with the loop current (Fig. 6c). Fig. 6d–f presents differences in sea surface currents between experiments C and A at days 10, 30 and 50 respectively. These sea surface current differences are used to represent the influence of the warm mesoscale eddy on the Kuroshio during the eddy-current interaction. At day 10, the warm mesoscale eddy maintains roughly its initial structure and has small effects on the Kuroshio (Fig. 6d). At day 30, two new anticyclonic eddies are formed locally around the northeast corner of Luzon Island (Fig. 6e). As the eddy-current interaction intensifies, at day 50, a branch of the Kuroshio enters the SCS through southern LS, and a new eddy pair (one is cyclone and the other is anticyclone) is generated locally to the southwest of Taiwan Island (Fig. 6f). The model results demonstrate that the westward propagating warm mesoscale eddy in the weak Kuroshio case with the flat bottom can enter LS and modify the loop current within the Strait. The simulated sea surface currents and sea levels have significant temporal and spatial variability in the LS region and adjacent waters during the eddy-current interaction period. It is not easy, however, to 71

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Fig. 7. Surface currents (vectors) and subsurface water temperatures (color shading) at 100 m depth from experiment C at model day (a) 10, (b) 20, (c) 30, (d) 40, (e) 50, and (f) 70 respectively. The white dotted line in each panel represents the trajectory of the centers of the warm eddy, with the initial position of the eddy denoted by the red star. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The subsurface (100 m) temperatures shown in Fig. 9 demonstrate that the trajectories of the warm eddy and evolution of its structure in the strong Kuroshio case (experiment D) differ significantly from those in the weak Kuroshio case, with a small portion of warm waters in the warm mesoscale eddy is separated and transported northwards by anticyclonic eddy in Kuroshio from day 20 to 25 in experiment D. The remainder of warm waters in the warm mesoscale eddy continues to move southward and evolves to an elliptic shape. This demonstrates that the structure of the warm mesoscale eddy is dynamically unstable and the eddy dissipates gradually with time in the interaction with the strong Kuroshio. A comparison of model results in experiments C and D indicates that the interaction of the warm mesoscale eddy with the Kuroshio differs

for different speeds of the Kuroshio in the flat ocean bottom case (Figs. 6 and 8). If the Kuroshio is relatively weak, a loop current is formed over the LS region. This loop current leads to the generation of a new anticyclonic eddy over the area to the west of northern LS during the eddy-current interaction. This newly-formed anticyclonic eddy eventually pinches off from the loop current to propagate freely over the northern SCS. Due to the horizontal velocity shear effect of the loop anticyclonic eddy, the warm mesoscale eddy in the weak Kuroshio case is merged into the Kuroshio loop as a long strip of filaments over the southern LS region. If the Kuroshio is relatively strong, the Kuroshio makes some a small loop over the northern part of LS, and the mesoscale warm eddy does not enter LS. Instead, the warm mesoscale eddy in the strong Kuroshio case propagates southwards to the eastern side of Fig. 8. (a–b) Surface currents (vectors) and sea levels (color shading) from experiment D and (c–d) differences in surface currents (vectors) and vorticity (color shading) between experiments D and B, at model day 10 (left columns), and 30 (right columns) after the start of the model integration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 9. Surface currents (vectors) and subsurface water temperatures (color shading) at 100 m depth from experiment D at model day (a) 10, (b) 20, (c) 25, and (d) 35. The white dotted line in each panel represents the trajectory of the centers of the warm eddy, with the initial position of the eddy denoted by the red star. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the Kuroshio and dissipates gradually during the eddy-current interaction.

of Luzon Island, and the other off the southeastern coast of Taiwan Island) and two new anticyclonic eddies (over the eastern side of the Kuroshio). These four new eddies are relatively stronger in the strong Kuroshio case (experiment F) than in the weak Kuroshio case (experiment E) at the same times. Similarly, the model results in experiments E and F are considered as background fields and are extracted from model results in other numerical experiments to examine the influence of a warm mesoscale eddy on the Kuroshio. Fig. 11a–c presents the simulated surface currents and sea levels at days 10, 30 and 50 respectively in the weak Kuroshio case in experiment G. The main path of the Kuroshio shifts to the western part of LS due to the influence of the warm mesoscale eddy at the early stage of the model integration in this experiment (Fig. 11b). Different from the weak Kuroshio case without seamounts in experiment C, the new cyclonic eddy generated locally off the northern coast of Luzon Island migrates northward within LS and constrains the full development of

3.3. Groups 3 and 4: presence of seamounts In this section, model results in groups 3 and 4 of numerical experiments (Table 1) are analyzed to examine the interactions of the warm mesoscale eddy, Kuroshio and local bottom topography in LS. The model results in the seamount cases without any eddies are discussed first. Fig. 10 presents simulated sea levels and surface currents at days 10 and 30 for two different speeds of the Kuroshio in the model domain with two idealized seamounts in LS but without any warm mesoscale eddy (experiments E and F). Similar to model results in the flat ocean bottom case in experiments A and B, the model results of experiments E and F also feature two new cyclonic eddies (one off the northern coast

Fig. 10. Surface currents (vectors) and sea levels (color shading) from experiment E (upper rows) and F (lower rows), at model day 10 (left columns) and 30 (right columns) after the start of model integration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 11. (a–c) Surface currents (vectors) and sea levels (color shading) from experiment G and (d–f) differences in surface currents (vectors) and vorticity (color shading) between experiments G and E, at model day 10 (left columns), 30 (middle columns) and 50 (right columns) after the start of model integration. The white ellipses represent positions of two seamounts. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the loop current in the Strait. By day 50 (Fig. 11c), the newly-formed cyclonic eddy pinches off from the Kuroshio and propagates into the SCS to the west of Taiwan Island. With the shedding of this cyclonic eddy on the western side of LS, the main axis of the Kuroshio changes to a straight line from the meander in LS and an anticyclonic eddy is generated locally to the east of Taiwan Island. Because of the advection of the Kuroshio, the newly-formed anticyclonic eddy on the eastern side of Taiwan Island moves northwards along the coast (Fig. 11c). Fig. 11d–f shows differences in sea surface currents between experiments G and E at three different times. This figure demonstrates clearly the development and propagations of new eddies off the northern coast of Luzon Island and eastern coast of Taiwan Island, eddy

shedding in the SCS to the southwest of Taiwan Island and multi-eddy structures in LS due to the eddy-current interaction. Fig. 12 presents the subsurface (100 m) temperature evolution in experiment G. Because of no loop anticyclone formation in LS, the horizontal structure of the warm mesoscale eddy deforms to an elliptic shape due to the blocking and shearing of the Kuroshio at the early stage of the eddy-current interaction (Fig. 12b). Part of waters in the warm mesoscale eddy is shed into the Kuroshio as filaments around day 40 (Fig. 12c). The remaining major part of this warm eddy is restored to the circular shape and moves southwards against the Kuroshio along the eastern edge of the Kuroshio. For the strong Kuroshio case in experiment H, similarly, a new Fig. 12. Surface currents (vectors) and subsurface water temperatures (color shading) at 100 m depth from experiment G at model day (a) 20, (b) 30, (c) 40, and (d) 50. The white dotted line in each panel represents the trajectory of the centers of the warm eddy, with the initial position of the eddy denoted by the red star. The white ellipses represent positions of two seamounts. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 13. (a–c) Surface currents (vectors) and sea levels (color shading) from experiment H and (d–f) differences in surface currents (vectors) and vorticity (color shading) between experiments H and F, at model day 20 (left columns), 30 (middle columns) and 40 (right columns) after the start of model integration. The white ellipses represent positions of two seamounts. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

cyclonic eddy is generated off the northern coast of Luzon Island and then moves northwards along the Kuroshio to the northern part of LS. At the early stage (Fig. 13a) of the eddy-current interaction, two new anticyclonic eddies are generated over the Kuroshio zone by the advection of currents. By day 40 (Fig. 13c), these two newly-formed anticyclonic eddies both move northwards along the Kuroshio. Fig. 13d–f presents differences in surface currents between experiments H and F at days 20, 30, and 40, respectively. By day 10, a new eddy

pair has been formed over the southeastern region of LS during the eddycurrent interaction (Fig. 13d). As the interaction continues, multiple smallscale eddies are generated in southern LS by day 30 (Fig. 13e). By day 40, the multi-eddy structures move northwards (Fig. 13f). Combined with the topographic effect and non-linear eddy-current interaction, the small-scale eddies are intensified and move around Taiwan Island. Fig. 14 presents the subsurface temperatures and eddy trajectories in experiment H. The trajectory of the warm mesoscale eddy in the

Fig. 14. Surface currents (vectors) and subsurface water temperatures (color shading) at 100 m depth from experiment H at model day (a) 10, (b) 20, (c) 25, (d) 30, (e) 35, and (f) 40. The white dotted line in each panel represents the trajectory of the centers of the warm eddy, with the initial position of the eddy denoted by the red star. The white ellipses represent positions of two seamounts. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 75

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strong Kuroshio case is complicated and differs significantly from the trajectory of the eddy moving southward in experiment G. At the early stage of the interaction between the warm mesoscale eddy and strong Kuroshio in experiment H, the warm eddy moves southwards against the Kuroshio along the eastern edge of the Kuroshio. After entering the Kuroshio zone, the warm eddy turns to north because of the Kuroshio advection. When the warm eddy interacts with anticyclonic eddies generated earlier in the Kuroshio zone, the warm mesoscale eddy is stretched and deformed by the intense shear effect at days 10–25. Fig. 14d demonstrates that the main part of the warm eddy recovers to the circular shape at eastern side of LS by day 30. As a result, most of the waters in the warm mesoscale eddy moves northward along with the Kuroshio while only a small part of warm waters shed off as filaments during the interaction. In addition, due to the impact of seamounts in LS, the strong Kuroshio mainly passes LS by gap-leaping (Yuan et al., 2006; Sheremet and Kuehl, 2007). No new cyclonic eddies are generated off the eastern coast of Taiwan Island in this case. The model results presented in this section demonstrate that the bottom topography with idealized seamounts in LS has a great influence on both the Kuroshio path and eddy-current interaction. Due to the blocking effect of topography in LS, the full development of the loop current in the LS region is constrained. When the Kuroshio is relatively weak, the warm mesoscale eddy keeps moving southwards against the Kuroshio. When the Kuroshio is relatively strong, the warm mesoscale eddy moves northwards along the Kuroshio. For the westward propagating mesoscale eddy, the existence of seamounts in LS reduces the possibility of mesoscale eddies passing through the Kuroshio and LS into the SCS.

L), no loop current is formed during the eddy-current interaction. As the newly-formed cyclonic eddy off the northern coast of Luzon Island is shed into the SCS, a new anticyclonic eddy is formed off the eastern coast of Taiwan Island. At the same time, the Kuroshio intrudes into the SCS as a tributary. The model results of the large-size warm eddy case (group 6) discussed above demonstrate that the eddy size (or intensity) affects the processes of the eddy-current interaction. When the horizontal scale (or intensity) of the warm mesoscale eddy is large, the eddy-current interaction prompts the intrusion of the Kuroshio as a tributary instead of the loop current at LS. Additional numerical experiments using a large model domain (1200 km × 900 km) were conducted to examine the model sensitivity to the influence of the model open boundary locations on the eddycurrent interactions presented in this paper. The model results in this large model domain (not shown) are the essentially same as those presented above. 4. Discussion The above analyses of model results in different experiments demonstrate that propagations and dissipations of a mesoscale warm eddy are affected significantly by the interactions between the Kuroshio and the local topography. The generation and dynamic behaviours of new eddies in the eddy-current interaction are examined next by vorticity diagnosis. The vorticity budget equation without mixing/dissipation is given by (Stevens, 1979)

3.4. Groups 5 and 6: sensitivity of eddy positions and sizes

t

In this section, we examine the model sensitivity of simulated eddycurrent interactions to the initial positions and sizes of the warm mesoscale eddy. In group 5 of numerical experiments I and J, a mesoscale warm eddy with the diameter of 120 km is initialized at (X = 620 km, Y = 480 km) to the southeast of the central LS. At the early stage of the integration in experiments I and J, the simulated surface circulation (not shown) also features two new cyclonic eddies and two new anticyclonic eddies. When the Kuroshio is weak, the model results in experiment I (not shown) are very similar to the case with the warm mesoscale eddy initialized to the east of the central LS at (X = 620 km, Y = 680 km) (experiment G). Due to the blocking and shearing effect of the Kuroshio, the warm mesoscale eddy is deformed and dissipated gradually during the eddy-current interaction. For the strong Kuroshio in experiment J, as the newly-formed anticyclonic eddy in LS moves northwards and interacts with Taiwan Island, a small new cyclonic eddy is formed off the western coast of Taiwan Island. The model results in group 5 are highly similar to the case with the warm mesoscale eddy initialized to the east of the central LS (group 4), which indicates that the position of the warm mesoscale eddy does not have the essential influence on the eddy shedding through LS induced by the interaction between the Kuroshio and a warm mesoscale eddy. In group 6 of experiments K and L, a large-size warm eddy with a diameter of 200 km is initialized at (X = 620 km, Y = 680 km) to the east of the central LS. For the weak Kuroshio (experiment K), the model results in this case (not shown) also feature two new cyclonic eddies in addition to the warm mesoscale eddy. One newly-formed cyclonic eddy is located off the northern coast of Luzon Island which eventually pinches off to enter the SCS, and the other off the eastern coast of Taiwan Island. Under the influence of the large-size warm eddy, smallsize new cyclonic eddies are formed locally off the northern coast of Luzon Island, and over the south side of the warm eddy. As the newlyformed cyclonic eddy in LS migrates to the north, a pair of new eddies is generated locally and a flow intrudes into the SCS as a tributary of Kuroshio at south of Taiwan. When the Kuroshio is strong (experiment

=

u

x

+v

y

w u y z

+

w

z

v

f y

u v + x x

(f + )

w v x z

(2)

where ξ is the relative vorticity and given

(

v x

u y

), u, v and w are

eastward, northward and vertical velocities respectively, and f is the planetary vorticity (or Coriolis parameter). Based on the scale analysis, the vorticity equation can be simplified by

t

=

u

x

+v

y

(f + )

u v + x x

(3)

which means that the temporal variation of the relative vorticity is mainly controlled by advection and vorticity stretching. Fig. 15 shows distributions of the advection and vorticity stretching terms for the strong Kuroshio case in the model domain without seamounts in LS at day 10. Both the advection term and vorticity stretching term have similar distributions for areas of two newly-formed cyclonic eddies off the eastern coast of Taiwan Island and the northern coast of Luzon Island. The negative values occur over the southern side of cyclonic eddies and the positive values over the northern side. The vorticity variation over time indicate that these two newly-formed cyclonic eddies migrate northwards. By comparison, the influence of the vorticity stretching term is a slightly stronger on the cyclonic eddy than the advection term. Over the area of the mesoscale warm eddy to the east of central LS, the values of advection term are positive over the northern side and negative over the southern side of the mesoscale eddy. As a result, the anticyclonic warm eddy moves southwards under the effect of advection. The values of vorticity stretching in the anticyclonic warm eddy are mostly positive, which means the intensity of the mesoscale eddy is weakened. Since the values of the advection term are much larger than vorticity stretching term over the area occupied by the mesoscale anticyclonic warm eddy, the movement of this anticyclonic warm eddy in the eddy-current interaction is mainly controlled by advection effect. Vandermeirsch et al. (2003b) studied the interaction between an 76

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Fig. 15. Distributions (×10−10s−2) of the advection term and vorticity stretching term for the strong Kuroshio case in the model domain without seamounts in LS (experiment D) at model day 10.

eddy and a zonal free jet with the constant potential vorticity by using potential vorticity (PV) analysis. Because of the conservation of potential vorticity (PV), dynamics of the Kuroshio and mesoscale eddies can be described in term of the potential vorticity anomaly (PVA) for a stratified fluid (Vandermeirsch et al., 2003a):

PVA = (f + )

f0

z

0

z

northern coast of Luzon Island derives from the water on the left side of the Kuroshio which both have positive PAV values. 5. Summary The eddy-current interaction in Luzon Strait (LS) and adjacent waters was investigated in this study based on the reanalysis data and numerical model results. The analysis of the global ocean and sea ice reanalysis data from 2008 to 2015 suggested that trajectories of 225 mesoscale eddies over the Kuroshio zone can be categorized as three different patterns: ~63% of mesoscale eddies dissipating during the eddy-current interaction, ~33% moving to the north along the Kuroshio, and only ~4% passing through the Kuroshio to enter the South China Sea (SCS).This indicates that only a small number of mesoscale eddies to the east of the Kuroshio zone was able to cross the Kuroshio and pass through LS. A three-dimensional ocean circulation model based on the MITgcm with an idealized model setup was used to simulate the dynamic processes of the eddy-current interaction. Thirteen numerical experiments were conducted with the model driven by boundary currents specified at the model southern and northern open boundaries. A mesoscale (warm or cold) eddy was initialized to the east of the Kuroshio and the model was integrated for 70 days in each experiment. The model results indicate that the western boundary flow and the seamount topography in LS form a barrier for the westward propagating mesoscale eddies to enter the SCS, which explains why only a very small number of mesoscale eddies were observed to enter the SCS from the northwest Pacific Ocean based on the ECCO2 sea level data. Our model results also demonstrated that non-linear interactions between the Kuroshio Current, local topography and westward propagating mesoscale eddies can generate localized eddies in LS which could be shed into the SCS. Therefore, eddy-current interactions are very important for generating a multi-eddy structure in LS. These new eddies induced by the eddycurrent interaction have a great effect on the evolution of a westward propagating eddy. Based on the model results and related analyses, two possible ways can be identified for the movement of a mesoscale (warm or cold) eddy into the SCS. One is that the eddy is strong enough to maintain its basic structure and the trend of movement in the interaction with the Kuroshio. The other way is that the changes of the velocity field during the eddy-current interaction promote the westward propagation of eddies in LS. In this study, numerical experiments were conducted using a simple setup for the Kuroshio Current, the mesoscale eddy and vertical stratification in the model domain with piece-wise straight coastlines and idealized seamounts in LS. The effect of wind stress, tides and other external forcing on the eddy-current interaction was not included.

(4)

where f = f0 + βy, θ is the active layer potential temperature and θ0 is the background potential temperature field. To gain better understanding of the mechanism for the eddy-current interaction, the PVA field is also examined for the strong Kuroshio case without seamounts in LS (experiment D). Furthermore, the Okubo–Weiss method (O-W parameter) (Okubo, 1970; Weiss, 1991) is used to identify the eddy. The Okubo-Weiss parameter W is given by

W = sn 2 + ss 2 sn =

u x

2

v v u ; ss = + y x y

(5) (6)

where sn and ss represent the strain and shear deformation; and u and v are eastward and northward velocities respectively. In this study, we use W < − 0.2σw to define the core region of the eddy, where σw is the standard deviation of W in the study region. Initially, the warm mesoscale eddy contains two parts: the inner part with the negative PVA and the outer annulus with the positive PVA. For the Kuroshio, facing downstream direction of this current, the negative PVA values occur over the right side and the positive PVA values over the left side of the Kuroshio. Fig. 16 presents distributions of the PVA calculated from model currents in experiment D. At day 10 (Fig. 16a), the warm mesoscale eddy moves to the southwest to interact with the Kuroshio gradually. The warm eddy deforms into a D shape and starts to migrate to the south against the Kuroshio. As the warm eddy continues to push slowly against the Kuroshio, by day 20 (Fig. 16b), the inner anticyclonic eddy (AE) with the negative PVA has evolved into an elliptical shape while the outer annulus with the positive PVA has stretched away of the warm eddy. The water on the right side of the Kuroshio with the negative PAV values evolves into an anticyclonic eddy over eastern LS. The newly formed anticyclonic eddy has the similar size to the warm mesoscale eddy. The eddy-current interaction then evolves into the eddy-eddy interaction. Based on the momentum conservation, when two newly-formed anticyclonic eddies with similar strengths interact with each other, the two eddies move clockwise. As a result, the warm eddy moves southwards while the anticyclonic eddy (AE) moves northwards as shown in Fig. 16c. Due to the advection of the Kuroshio, the AE moves continually northwards, and the mesoscale warm eddy moves southwards (at day 30, Fig. 16d). The PVA field shows that part of waters in the cyclonic eddy off the 77

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Fig. 16. PVA distributions (color image) at 100 m depth calculated from model results in experiment D at model day (a) 10, (b) 20, (c) 25 and (d) 30 after the start of the model integration. The black solid lines represent the O-W parameter with a value of −0.1σw, where σw is the standard deviation of the parameter in the study region. Abbreviations are used for the anticyclonic eddy (AE), cyclonic eddy (CE) and mesoscale warm eddy (WE) respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Realistic topography, stratification, external forcing should be considered in the future studies of the eddy-current interactions in the study region.

Government [201411201645511650; KQJSCX20170720174016789]. JS is supported by the NSERC and OFI. We also sincerely thank the editor and two reviewers for their insightful and constructive comments and suggestions.

Acknowledgements This work was supported by the National Basic Research Program of China (973 Program) [grant 2014CB745001], the Environmental Protection Special Funds for Public Welfare [201309006], and Shenzhen

Declaration of interest None.

Appendix A. A case for a cold eddy Jan et al. (2017) studied the interaction between the Kuroshio and a cyclone and an anticyclone east of Taiwan respectively, and they found that a cyclonic eddy and an anticyclonic eddy have the opposite effect on changes of sea level height, isopycnal depth, and the Kuroshio through-flow transport. Here we examine the interaction process between a cold mesoscale eddy and the Kuroshio under the same model conditions with the warm mesoscale eddy. Model results show that when the cold mesoscale eddy interacts with the weak Kuroshio in Luzon Strait, the dynamic process differs from the warm eddy case. Within the first 30 days, the model results feature two anticyclonic eddies in the Kuroshio zone (one in the current loop and the other in upstream) and two cyclonic eddies with one off the northern coast of Luzon Island and the other off the eastern coast of Taiwan Island (Fig. A1b). These two newly-formed anticyclonic eddies propagate northwards and merge into a meander. The newly-formed cyclonic eddy off the Luzon Island gradually pinches off to enter the SCS while the newly-formed cyclonic eddy off the eastern coast of Taiwan Island still attaches to the coastline. Fig. A1d–f present differences in surface currents between the present experiment M with a cold eddy initialized, and experiment A without the cold eddy, at days 10, 30, and 50 respectively. The surface current differences show that the cold cyclonic eddy prompts the generation and shedding of the cyclonic eddy off the northern coast of Luzon Island, the formation of loop current in LS, and growth of the cyclonic eddy off the eastern coast of Taiwan Island. By comparison between experiments C and M, the model results show that the effect of cold mesoscale eddy on the Kuroshio is the opposite of the effect of the warm mesoscale eddy in Luzon Strait. The cold eddy, which represented by the shape of the subsurface temperature anomaly in Fig. A1g–i, is always moving on the east side of the Kuroshio while its strength attenuates continuously during the eddy-current interaction.

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Fig. A1. (a–c) Surface currents (vectors) and sea levels (color shading) from experiment M, (d–f) differences in surface currents (vectors) and vorticity (color shading) between experiments M and A, and (g–i) subsurface water temperature (color shading) at 100 m depth at model day 10 (left columns), 30 (middle columns) and 50 (right columns) after the start of model integration. The white dotted line represents the trajectory of the centers of the clod eddy, and the red star denotes initial position of the eddy.

60, 1063–1069. Jia, Y., Liu, Q., Liu, W., 2005. Primary study of the mechanism of eddy shedding from the Kuroshio bend in Luzon Strait. J. Oceanogr. 61, 1017–1027. Li, L., Nowlin, W.D., Jilan, S., 1998. Anticyclonic rings from the Kuroshio in the South China Sea. Deep-Sea Res. I Oceanogr. Res. Pap. 45, 1469–1482. Liang, W. D., Yang, Y. J., Tang, T. Y., and Chuang, W. S.: Kuroshio in Luzon Strait, J. Geophys. Res.: Oceans, 113, 2008. Lien, R.C., Ma, B., Cheng, Y.H., Ho, C.R., Qiu, B., Lee, C.M., Chang, M.H., 2014. Modulation of Kuroshio transport by mesoscale eddies at Luzon Strait entrance. Journal of Geophysical Research: Oceans 119, 2129–2142. Lu, J., Liu, Q., 2013. Gap-leaping Kuroshio and blocking westward-propagating Rossby wave and eddy in Luzon Strait. Journal of Geophysical Research: Oceans 118, 1170–1181. Menemenlis, D., Campin, J., Heimbach, P., Hill, C., Lee, T., Nguyen, A., Schodlok, M., Zhang, H., 2008. ECCO2: high resolution global ocean and sea ice data synthesis. AGU Fall Meeting. . Miyazawa, Y., Guo, X., Yamagata, T., 2004. Roles of mesoscale eddies in the Kuroshio paths. J. Phys. Oceanogr. 34, 2203–2222. Miyazawa, Y., Kagimoto, T., Guo, X., and Sakuma, H.: The Kuroshio large meander formation in 2004 analyzed by an eddy-resolving ocean forecast system, J. Geophys. Res.: Oceans, 113, 2008. Nan, F., Xue, H., Xiu, P., Chai, F., Shi, M., and Guo, P.: Oceanic eddy formation and propagation southwest of Taiwan, J. Geophys. Res.: Oceans, 116, 2011. Nan, F., Xue, H., Yu, F., 2015. Kuroshio intrusion into the South China Sea: a review. Prog. Oceanogr. 137, 314–333. Okubo, A., 1970. Horizontal dispersion of floatable particles in the vicinity of velocity singularities such as convergences. Deep Sea Research & Oceanographic Abstracts 17, 445–454. Orlanski, I., 1976. A simple boundary condition for unbounded hyperbolic flows. J. Comput. Phys. 21, 251–269. Sheremet, V.A., Kuehl, J., 2007. Gap leaping western boundary current in a circular tank model. J. Phys. Oceanogr. 37, 1488. Sheu, W.-J., Wu, C.-R., Oey, L.-Y., 2010. Blocking and westward passage of eddies in Luzon Strait. Deep-Sea Res. II Top. Stud. Oceanogr. 57, 1783–1791.

References Adcroft, A., Dutkiewicz, S., Ferreira, D., Heimbach, P., Jahn, O., Maze, G., 2011. MITgcm User Manual, 1–451. Chaigneau, A., Gizolme, A., Grados, C., 2008. Mesoscale eddies off Peru in altimeter records: identification algorithms and eddy spatio-temporal patterns. Prog. Oceanogr. 79, 106–119. Chelton, D.B., Schlax, M.G., Samelson, R.M., 2011. Global observations of nonlinear mesoscale eddies. Prog. Oceanogr. 91, 167–216. Chen, G., Hou, Y., Chu, X., Qi, P., 2010. Vertical structure and evolution of the Luzon Warm Eddy. Chin. J. Oceanol. Limnol. 28, 955–961. https://doi.org/10.1007/ s00343-010-9040-3. Doglioli, A., Blanke, B., Speich, S., and Lapeyre, G.: Tracking coherent structures in a regional ocean model with wavelet analysis: application to Cape Basin eddies, J. Geophys. Res.: Oceans, 112, 2007. Farris, A., Wimbush, M., 1996. Wind-induced kuroshio intrusion into the South China Sea. J. Oceanogr. 52, 771–784. Guo, J.S., Yuan, Y.L., Xiong, X.J., Guo, B.H., 2007. Statistics of the mesoscale eddies on both sides of Luzon Strait. Adv. Mar. Sci. 25, 139–148. Hu, X., Xiong, X., Qiao, F., Guo, B., Lin, X., 2008. Surface current field and seasonal variability in the Kuroshio and adjacent regions derived from satellite-tracked drifter data. Acta Oceanol. Sin. 27, 11–29. Hu, J., Zheng, Q., Sun, Z., Tai, C.K., 2012. Penetration of nonlinear Rossby eddies into South China Sea evidenced by cruise data. Journal of Geophysical Research: Oceans 117. Hwang, J.-S., Dahms, H.-U., Tseng, L.-C., Chen, Q.-C., 2007. Intrusions of the Kuroshio Current in the northern South China Sea affect copepod assemblages of Luzon Strait. J. Exp. Mar. Biol. Ecol. 352 (2), 12–27. Jackett, D.R., Mcdougall, T.J., 1995. Minimal adjustment of hydrographic profiles to achieve static stability. J. Atmos. Ocean. Technol. 12, 381–389. Jan, S., Mensah, V., Andres, M., Chang, M.H., Yang, Y.J., 2017. Eddy-Kuroshio interactions: local and remote effects. J. Geophys. Res. 122. Jia, Y., Liu, Q., 2004. Eddy shedding from the Kuroshio bend at Luzon Strait. J. Oceanogr.

79

Journal of Marine Systems 194 (2019) 66–80

S. Yang, et al. Stevens, Duane E., 1979. Vorticity, momentum and divergence budgets of synoptic-scale wave disturbances in the tropical eastern Atlantic. Mon. Weather Rev. 107 (5), 535–550. Sutyrin, G., Rowe, G., Rothstein, L., Ginis, I., 2003. Baroclinic eddy interactions with continental slopes and shelves. J. Phys. Oceanogr. 33, 283–291. Tsai, C.J., Andres, M., Jan, S., Mensah, V., Sanford, T.B., Lien, R.C., Lee, C.M., 2015. Eddy-Kuroshio interaction processes revealed by mooring observations off Taiwan and Luzon. Geophys. Res. Lett. 42, 8098–8105. Vandermeirsch, F.O., Carton, X.J., Morel, Y.G., 2003a. Interaction between an eddy and a zonal jet: part I. One-and-a-half-layer model, Dynamics of Atmospheres. Oceans 36, 247–270. Vandermeirsch, F.O., Carton, X.J., Morel, Y.G., 2003b. Interaction between an eddy and a zonal jet: part II. Two-and-a-half-layer model. Dynamics of Atmospheres and Oceans 36, 271–296. Waseda, T., Mitsudera, H., Taguchi, B., Yoshikawa, Y., 2002. On the eddy-Kuroshio interaction: evolution of the mesoscale eddy. Journal of Geophysical Research: Oceans 107. Waseda, T., Mitsudera, H., Taguchi, B., Yoshikawa, Y., 2003. On the eddy-Kuroshio interaction: meander formation process. Journal of Geophysical Research: Oceans 108. Waterman, S., Hogg, N.G., Jayne, S.R., 2011. Eddy–mean flow interaction in the Kuroshio Extension region. J. Phys. Oceanogr. 41, 1182–1208. Weiss, J., 1991. The dynamics of enstrophy transfer in two-dimensional hydrodynamics.

Physica D Nonlinear Phenomena 48, 273–294. Yang, S., Xing, J., Chen, D., Chen, S., 2017. A modelling study of eddy-splitting by an island/seamount. Ocean Sci. 13, 837. Yuan, D., Han, W., and Hu, D.: Surface Kuroshio path in Luzon Strait area derived from satellite remote sensing data, J. Geophys. Res.: Oceans, 111, 2006. Zhai, X., Johnson, H.L., Marshall, D.P., 2010. Significant sink of ocean-eddy energy near western boundaries. Nat. Geosci. 3, 608–612. Zhang, D., Lee, T.N., Johns, W.E., Liu, C.-T., Zantopp, R., 2001. The Kuroshio east of Taiwan: modes of variability and relationship to interior ocean mesoscale eddies. J. Phys. Oceanogr. 31, 1054–1074. Zhang, Z., Zhao, W., Tian, J., Liang, X., 2013. A mesoscale eddy pair southwest of Taiwan and its influence on deep circulation. Journal of Geophysical Research: Oceans 118, 6479–6494. https://doi.org/10.1002/2013jc008994. Zhang, Z., Tian, J., Qiu, B., Zhao, W., Chang, P., Wu, D., Wan, X., 2016. Observed 3D structure, generation, and dissipation of oceanic mesoscale eddies in the South China Sea. Sci. Rep. 6. Zheng, Q., Tai, C.-K., Hu, J., Lin, H., Zhang, R.-H., Su, F.-C., Yang, X., 2011. Satellite altimeter observations of nonlinear Rossby eddy–Kuroshio interaction at Luzon Strait. J. Oceanogr. 67, 365–376. https://doi.org/10.1007/s10872-011-0035-2. Zheng, Q., Xie, L., Zheng, Z., Hu, J., 2017. Advances in studying mesoscale eddy in South China Sea, Advances in Marine. Science 35, 131–158.

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