A Lagrangian study of the near-surface intrusion of Pacific water into the South China Sea
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Abstract: Satellite-tracked Lagrangian drifters are used to investigate the transport pathways of near-surface water around the Luzon Strait. Particular attention is paid to the intrusion of Pacific water into the South China Sea (SCS). Results from drifter observations suggest that except for the Kuroshio water, other Pacific water that carried by zonal jets, Ekman currents or eddies, can also intrude into the SCS. Motivated by this origin problem of the intrusion water, numerous simulated trajectories are constructed by altimeter-based velocities. Quantitative estimates from simulated trajectories suggest that the contribution of other Pacific water to the total intrusion flux in the Luzon Strait is approximately 13% on average, much smaller than that of Kuroshio water. Even so, over multiple years and many individual intrusion events, the contribution from other Pacific water is quite considerable. The interannual signal in the intrusion flux of these Pacific water might be closely related to variations in a wintertime westward current and eddy activities east of the Luzon Strait. We also found that Ekman drift could significantly contribute to the intrusion of Pacific water and could affect the spreading of intrusion water in the SCS. A case study of an eddy-related intrusion is presented to show the detailed processes of the intrusion of Pacific water and the eddy-Kuroshio interaction.
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Key words:
- Kuroshio intrusion /
- Lagrangian transport /
- Luzon Strait /
- South China Sea
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Figure 1. Geographic distributions of the number of drifter observations in each 0.5°×0.5° bin between 1986 and 2014 (a), the total number of drifter observations (gray) in each year (b), the total number of drifter observations (gray) in each month (c). In a, the blue dots indicate the locations where the drifters were launched or drifted in. The black box outlines the Luzon Strait (18.6°–22.0°N, 120.4°–121.9°E). The green line approximates the mean Kuroshio core based on AVISO velocities. The black contours are 200 m and 1000 m isobaths. In b, the black bars denote the number of drogued drifters.
Figure 2. Selected drifter trajectories near the Luzon Strait. a and b. Trajectories of drifters that originated from the Pacific Ocean and reached the Luzon Strait from April–September and October–March; c. trajectories of drifters launched inside the Luzon Strait; and d. trajectories of cyclonic (blue) and anticyclonic (red) closed loops identified from drifters originating in the Pacific Ocean. Based on the different fates and the ways leaving the Luzon Strait, drifters in a–c are grouped into three types: those directly drifted into the SCS (blue), into the western Pacific after looping into the SCS (red), and into the western Pacific with no loop in the SCS (black). In b, the bold black line denotes the trajectory of the drifter with ID 7710570 and the yellow line denotes the western boundary of the strait. The bottom topography from 0 m to 1 000 m is shaded in gray with contour intervals of 200 m.
Figure 3. Seasonal cycles of quasi-Eulerian velocity fields (vectors) derived from the drifter observations. a. January–March, b. April–June, c. July–September, and d. October–December. The colors present the number of the 7-d observation windows in each 0.5°×0.5° bin. Note that the velocities were estimated only for bins with more than five 7-d observation windows.
Figure 4. The intrusion of Pacific water into the SCS. a. Intrusion probability map with 0.25°×0.25° bins. The green curve approximates the mean Kuroshio core. b. Trajectories of intrusion drifters 15 d before and after crossing the line 120.8°E (the red dash line) in the Luzon Strait. Yellow dots denote the initial positions of these trajectories. The zonal and meridional blue lines denote the KC and LSE section, respectively. c. A subset of trajectories across the KC section (18.5°N, 122.3°–124.0°E). d. A subset of trajectories across the LSE section (124.0°E, 18.5°N–23.0°N). In c and d, trajectories of drogued and undrogued drifters are marked in dark grey and grey, respectively.
Figure 5. Geographic distributions of geostrophic currents and EKE (a), Ekman currents (b), and residual velocities derived from drogued drifters (c) and undrogued drifters (d) in wintertime. The velocities were estimated only for bins with more than 5 drifter observations.
Figure 6. Schematic plot for release locations of three different groups of simulated particles (a, red dots), and a subset of 150-d trajectories of 54 particles released from the KC band and advected within the AVISO-NCEP velocity field (b). In a, the zonal and meridional blue lines denote the KC (18.5°N, 122.3°–124.0°E) and LSE (18.5°–23.0°N, 124.0°E) section, respectively. The average AVISO-NCEP velocity field is superimposed.
Figure 7. Probability maps (with 0.5°×0.5° boxes) for particles on Days 30, 60, 90, 120, and 150 (top to bottom) after they are released from the KC (left) and LSE (right) band. Simulated particles are advected within the AVISO-NCEP velocity field during 1993 to 2014.
Figure 8. Intrusion percentages as a function of time for particles after they are released from the KC (left) and LSE (right) band. Different line styles correspond to different velocity fields used to evolve simulated trajectories.
Figure 9. Probability maps (with 0.5°×0.5° boxes) for particles on Day 150 after they are released from the KC (left) and LSE (right) band. Simulated particles are advected within the AVISO-only (top), the AVISO-NCEP (middle) and the AVISO-NCEP-slip (bottom) velocity field between 1993 and 2014.
Figure 10. Time series of intrusion flux from the KC (blue line) and LSE (red line) section. Thin lines denote weekly flux and thick lines denote one-month running average flux. The yellow line marks the time of an eddy-related intrusion event.
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