Three-dimensional circulation in northern South China Sea during early summer of 2015
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Abstract: Using the hydrographic data obtained during two nearly simultaneous surveys in June 2015, we carried out semi-diagnostic calculations with the help of a finite element model and a modified inverse method, to study the circulation in the northern South China Sea (NSCS) during the early summer of 2015. A number of new circulation features were found. (1) In most of the observation region, a large, basin-scale anticyclonic gyre appeared south of the 50-m isobath, which contained anticyclonic eddies. One anticyclonic eddy existed from the sea surface to 50-m depth, whose center showed no tilt, while the center of another eddy tilted eastward from the sea surface to 500-m depth. In the eastern part of the observation region, which is west of the Dongsha Islands, there was a sub-basin-scale cyclonic gyre containing a cyclonic eddy whose center tilted southward from the sea surface to 200-m depth. (2) There was a cross-continental slope current (CCSC) in the area southwest of the Dongsha Islands. Its volume transport was about 2.0×106 m3/s. (3) From the estimated order of magnitude of the stream function equation, the joint effect term of the baroclinity and relief (JEBAR) and β-effect term are two important dynamic mechanisms affecting the variation of the circulation in the NSCS. (4) The JEBAR, as a transport-generating term, resulted in the dynamic mechanism determining the pattern of the depth-averaged flow across the contours of potential vorticity fH–1. Furthermore, we show that the negative values of the JEBAR were the most dominant dynamic mechanism, causing the CCSC southwest of the Dongsha Islands to deflect from the isobaths and veer toward the deep water. The CCSC around the Dongsha Islands was located further southwest during the early summer of 2015 than during the fall of 2005 (revealed by a published study), which suggests that the location of the CCSC around the Dongsha Islands may vary with season.
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Figure 1. Bottom topography (m) of the South China Sea and observation stations of the two surveys in June 2015. The dots denote the open cruise from 21 to 30 June 2015, and the triangles denote the slope cruise from 13 to 25 June 2015. The polygon connecting the exterior markers indicates the observational domain.
Figure 2. Temperature-salinity (T-S) diagram from all hydrographic stations of the two surveys. Black and red are used to distinguish the two surveys, while the monthly climatological profiles of observed temperature and salinity in June for the SCS (15°N, 115°E) and Pacific (15°N, 125°E) are in blue and purple, respectively.
$ {\sigma }_{t} $ (kg/m3) is the density anomaly of a water sample when the total pressure on it has been reduced to atmospheric pressure.
Figure 3. Salinity distributions at surface (a), 50 m (b), 200 m (c), and 500 m (d) measured in June 2015. “HS” indicates higher-salinity water, “LS” indicates lower-salinity water, “D” indicates the Dongsha Islands, and purple contours indicate isobaths.
Figure 4. Temperature distributions at surface (a), 50 m (b), 200 m (c), and 500 m (d) measured in June 2015. “HT”indicates higher- temperature water, “LT” indicates lower- temperature water, “D” indicates the Dongsha Islands, and purple contours indicate isobaths.
Figure 5. Mean wind field derived from the ERA-Interim Reanalysis for the survey period. The polygon indicates the computational domain, and “D” indicates the Dongsha Islands.
Figure 6. Averaged weekly composite sea-level anomaly (m) from 9 to 29 June 2015. The polygon indicates the computational domain, and “D” indicates the Dongsha Islands.
Figure 7. Boxes of the modified inverse method and horizontal grid of the numerical model. The transports (106 m3/s) listed at the boundaries of the computational region were calculated using the modified inverse method.
Figure 8. Volume-averaged kinetic energy (a) and phase speed of density (b) as a function of time.
Figure 9. Sea-surface elevations (m) from the semi-diagnostic calculation. “W1” and “W2” indicate two anticyclonic eddies, and “C1” indicates the cold eddy.
Figure 10. Distributions of horizontal velocity from the semi-diagnostic calculation at surface (a), 25 m (b), 50 m (c), 100 m (d), 200 m (e), and 400 m (f). Red contours indicate isobaths, “W1” and “W2” indicate two anticyclonic eddies, and “C1” indicates the cold eddy.
Figure 11. Distribution of stream function and volume transport (106 m3/s) from the semi-diagnostic calculation. Red contours indicate isobaths, and “D” indicates the Dongsha Islands.
Figure 12. Transport per unit width (m2/s) for the semi-diagnostic calculation. Red contours indicate isobaths, and “D” indicates the Dongsha Islands.
Figure 13. Distributions of vorticity terms (s−2) near the Dongsha Islands during the early summer of 2015 from the semi-diagnostic calculation. Term I (JEBAR) (a), Term II (b), and Term III (c) on the right-hand side of Eq. (11). Red contours indicate isobaths, and “D” indicates the Dongsha Islands.
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