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Abstract: Using observational data from multiple satellites, we studied seasonal variations of the shape and location of the Luzon cold eddy (LCE) northwest of Luzon Island. The shape and location of the LCE have obvious seasonal variations. The LCE occurs, develops, and disappears from December to April of the next year. During this period, the shape of the LCE changed from a flat ellipse to a circular ellipse, and the change in shape can be reflected by the increase of the ellipticity of the LCE from 0.16 to 0.82. The latitude of center location of the LCE changes from 17.4°N to 19°N, and the change in latitude can reach 1.6°. Further study showed that seasonal variation of the northeast monsoon intensity leads to the change in the shape and location of the LCE. The seasonal variation of the LCE shape can significantly alter the spatial distribution of the thermal front and chlorophyll a northwest of the Luzon Island by geostrophic advection.
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Key words:
- Luzon cold eddy /
- seasonal variation /
- shape and location /
- thermal front /
- chlorophyll a concentration
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Figure 1. Topographic distribution of the South China Sea and winter climatological mean distribution of sea level anomaly (SLA) and wind vectors. The filling contour represents SLA (unit: 10−2 m). The arrows represent wind vectors (unit: m/s). The black solid line represents the −0.08 m contour of SLA. The red box encompasses 14.625°–19.875°N, 116.125°–121.125°E, which represents the region northwest of Luzon Island. HI: Hainan Island, LI: Luzon Island, TI: Taiwan Island.
Figure 2. Seasonal variations in the shape and location of the Luzon cold eddy (LCE). The filling contour represents sea level anomaly (SLA). The thick black solid line represents the edge of the LCE. The thick red solid line represents the edge of the fitting ellipse for the LCE. The black star represents the center location of the LCE. The red star, located at 21.875°N, 118.375°E southwest of Taiwan Island, represents the location along which the SLA gradient north of the South China Sea is defined. The white solid line represents the distance between the black star and red star.
Figure 3. Schematic diagram of the fitting ellipse for the Luzon cold eddy. a and b represent the minor axis and major axis of the ellipse, respectively.
$ \alpha $ represents the included angle.
Figure 4. Correspondence between ellipticity of the fitting-ellipse of the Luzon cold eddy (LCE) and intensity of wind stress curl (WSC) northwest of Luzon Island (a); correspondence between the included angle of the fitting-ellipse of the LCE and WSC northwest of Luzon Island (b).
Figure 5. Two spatial patterns from self-organizing map analysis. a. Elliptical mode of the Luzon cold eddy (LCE); b. circular mode of the LCE. White numbers in each panel denote the incidence rate of the corresponding pattern. The black thick solid line represents the edge of the LCE. The interval between isolines is
$ 5 \times {10}^{-3} $ m. SLA: sea level anomaly.
Figure 6. Distribution of incidence rate of different spatial modes of the Luzon cold eddy. a. Distribution is for elliptical mode; b. Distribution is for circular mode.
Figure 7. Correspondence between distance of the center location of the Luzon cold eddy (LCE) and sea level anomaly (SLA) gradient north of the South China Sea. The SLA gradient is obtained by dividing the difference between the SLA at the red star and the black star by the corresponding distance in Fig. 2. a. In latitude, the mean latitude of the center location of the LCE is 18°N. b. In longitude, the mean longitude of the center location is 118.6°E.
Figure 8. Seasonal variation in the wind stress curl (WSC) northwest of Luzon Island. The black solid line represents the climatological mean location of the WSC.
Figure 9. Correspondence between Luzon cold eddy (LCE) and wind stress curl (WSC) northwest of Luzon Island from winter 1993 to winter 2020. Winter is defined as the period from December to March. a. Daily variation in the WSC. The upper (lower) red dotted line represents the sum (difference) of the one-time standard deviation and the average value of the time series, and the days when the time series is bigger (smaller) than the upper (lower) red dotted are defined as positive (negative) index days. b. Composition of sea level anomaly (SLA) for positive index days, and the thick black solid line represents the −0.08 m contour of SLA. The thick black (red) solid line represents the edge of the LCE (the fitting ellipse for the LCE), whose ellipticity is 0.26 (0.69). The black star represents the center location of the LCE. The red star, located at 21.875°N, 118.375°E, southwest of Taiwan Island, represents the location along which the SLA gradient north of the South China Sea is defined. The white solid line represents the distance between the black star and red star. c. Same as b, except for negative index days, and the thick black solid line represents the −0.06 m contour of SLA. The thick black (red) solid line represents the edge of the LCE (the fitting ellipse for the LCE), whose ellipticity is 0.69.
Figure 10. Seasonal variation of the thermal front and geostrophic current anomaly (unit: m/s) northwest of Luzon Island. The black solid line in each subgraph represents the 0.1℃/(10 km) contours of the thermal front. The red solid line in g represents the 0.09℃/(10 km) contour of gradient magnitude.
Figure 11. Spatial distribution of the thermal front and geostrophic current anomaly (unit: m/s) for elliptical mode of the Luzon cold eddy (LCE) based on self-organizing map (SOM) analysis (a); circular mode of the LCE based on SOM analysis (b). The black solid line represents the 0.1℃/(10 km) contours of the thermal front.
Figure 12. Seasonal variation of chlorophyll a concentration and geostrophic current anomaly (unit: m/s) northwest of Luzon Island. The black circular or elliptic lines are used to indicate the shape and location of chlorophyll a.
Figure 13. Spatial distribution of chlorophyll a concentration and geostrophic current anomaly (unit: m/s) for elliptical mode of the Luzon cold eddy (LCE) based on self-organizing map (SOM) analysis (a); circular mode of the LCE based on SOM analysis (b). The black elliptic lines are used to indicate the shape and location of chlorophyll a.
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