A case study of continental shelf waves in the northwestern South China Sea induced by winter storms in 2021
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Abstract: This study aims to investigate characteristics of continental shelf wave (CSW) on the northwestern continental shelf of the South China Sea (SCS) induced by winter storms in 2021. Mooring and cruise observations, tidal gauge data at stations Hong Kong, Zhapo and Qinglan and sea surface wind data from January 1 to February 28, 2021 are used to examine the relationship between along-shelf wind and sea level fluctuation. Two events of CSWs driven by the along-shelf sea surface wind are detected from wavelet spectra of tidal gauge data. The signals are triply peaked at periods of 56 h, 94 h and 180 h, propagating along the coast with phase speed ranging from 6.9 m/s to 18.9 m/s. The dispersion relation shows their property of the Kelvin mode of CSW. We develop a simple method to estimate amplitude of sea surface fluctuation by along-shelf wind. The results are comparable with the observation data, suggesting it is effective. The mode 2 CSWs fits very well with the mooring current velocity data. The results from rare current help to understand wave-current interaction in the northwestern SCS.
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Figure 1. Study area and locations of observation stations. Blue dots are locations of tidal gauge stations of Hong Kong (HK), Zhapo (ZP) and Qinglan (QL). Red square is the location of mooring station MO. Red dots represent cruise observation section SE. Black lines represent sea surface wind observation sections W1, W2, W3 and W4. Coordinate system x-o-y is set as x-axis perpendicular to the coastline seaward positive and y-axis parallel to the coastline leftward positive. θ (= 35°) is the y-axis direction referring to true north. Contours display water depth.
Figure 2. Time series DSLA data at tidal gauge stations: a. HK, b. ZP, c. MO and d. QL.
Figure 3. Velocity components derived from mooring observed data. a. Cross-shelf current component, b. along-shelf current component, c. barotropic current component, d. baroclinic cross-shelf current component and e. baroclinic along-shelf current component.
Figure 4. Cruise observed salinity (a), temperature (b) and squared buoyancy frequency (c) along Section SE. Red dots in a indicates stations.
Figure 5. Time series of sea surface wind component derived from CMEMS wind product. a. Along-shelf wind; b. cross-shelf wind at sections W1 (red curve), W2 (green curve), W3 (blue curve) and W4 (black curve).
Figure 6. Power spectral density (PSD) of observation data. a. PSD of DSLA at stations HK (red), ZP (green), MO (blue), and that of barotropic current of cross-shelf component (CU, magenta) and along-shelf component (AU, cyan). Green and red dashed lines in a are the 5% significance level against red noise for DSLA at HK and ZP, respectively. PSD of de-tided baroclinic current of cross-shelf (b) and along-shelf components (c). Black dashed lines represent three frequency bands 0.0177 h−1, 0.0106 h−1 and 0.0056 h−1; blue dashed line indicates the inertial frequency band (f0).
Figure 7. Wavelet transforms of DSLA at stations. a. ZP and b. MO, c. along-shelf and d. cross-shelf components of barotropic (BT) current, e. along-shelf and f. cross-shelf components of baroclinic (BC) current at the depth of 14 m. The black and blue dashed lines with arrows and squares point out events and spectral bands. The thick black lines are the 5% significance level against red noise. The power spectrum is normalized, and the unit is 1. The color code represents normalized PSD.
Figure 8. Time series of bandpass DSLA at stations HK, ZP, MO and QL with periods of 56 h (a), 94 h (b) and 180 h (c) derived from 4-order Butterworth filter. Dashed lines with shadow point out E1 and E2. Dashed black curves and arrow indicate the propagation of signals in DSLA. A 4-order Butterworth bandpass filter with frequency cutoffs (−3 dB) at 0.34 d−1 and 0.60 d−1 for 56 h period band, 0.20 d−1 and 0.30 d−1 for 94 h period band, 0.11 d−1 and 0.16 d−1 for 180 h period band.
Figure 9. XWT of DSLA for station pairs: HK–ZP (a) and ZP–QL (b). The thick black lines are the 5% significance level against red noise. The thin black lines show the cone of influence (COI). The arrows show the relative phase relationship between two time series with in-phase (anti-phase, leading and lagging) pointing right (left, down and up). Color codes are normalized power spectra. The magenta dashed lines with arrows point out signal events. The power spectrum is normalized, and the unit is 1.
Figure 10. Comparison of dispersion relation derived from this study with the Kelvin mode and the lowest mode of CSW (a); arbitrary amplitude of sea level in cross-shelf direction, the amplitude of sea level calculated from the toolbox (blue curve) and that of Kelvin mode (black curve) (b), arbitrary along-shelf velocity component in cross-shelf direction (c), and mean depth profile between ZP and MO (d). The data points in a are calculated from XWT of DSLA for station pairs. For a, theoretical dispersion relations are derived from the mean depth profile (blue curve) as shown in d. Black curve in d represents the idealized depth profile.
Figure 11. XWT of along-shelf velocity component of sea surface wind for section pairs W1–W2 (a) and W2–W3 (b). The thick line is the 5% significance level against red noise. The thin line shows the cone of influence (COI). The arrows show the relative phase relationship between two time series with in-phase (anti-phase, leading and lagging) pointing right (left, down and up). Color codes are normalized power spectra. The blue lines with arrows point out CSW events. The power spectrum is normalized, and the unit is 1.
Figure 12. XWT of along-shelf wind component at section W3 with cross-shelf (a) and along-shelf (b) velocities measured by mooring station MO. The thick line is the 5% significance level against red noise. The thin line shows the cone of influence (COI). The arrows show the relative phase relationship between two time series with in-phase (anti-phase, leading and lagging) pointing right (left, down and up). Color codes are normalized power spectra. The blue dashed lines with arrows point out CSW events. The power spectrum is normalized, and the unit is 1. The color code represents normalized PSD.
Figure 13. Sketch of the wind-driven sea surface fluctuation and Ekman transport over the continental shelf.
Figure 14. Phase lag between sea surface fluctuation and along-shelf wind from Fig. 12a (a); comparison of the DSLA at station ZP (red curve) with amplitude of sea surface fluctuation with the fixed phase of π/4 (blue curve), and amplitude of sea surface fluctuation (green curve) calculated with the phase lag in a (b).
Figure 15. Bandpass cross-shelf current (a), along-shelf current velocity (b) distribution with a central period of 90 h, and bandpass cross-shelf current (c) and along-shelf current velocity (d) distribution with a central period of 190 h.
Table 1. XWT of DSLA at different station pairs
Station Distance/km Event Period/h Time lag/h Phase speed/(m∙s-1) HK–ZP 225 E1-1 56 4.6 ± 0.5 13.6 ± 1.6 E1-2 119 9.1 ± 3.8 6.9 ± 4.9 E2-1 91 3.3 ± 0.7 18.9 ± 5.1 E2-2 181 4.3 ± 1.1 14.5 ± 5.0 ZP–QL 174 E1-1 56 5.9 ± 0.7 8.2 ± 1.1 E1-2 119 2.9 ± 1.0 16.7 ± 8.7 
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Table 2. XWT of along-shelf wind at different station pairs
Station Distance/km Event Period/h Time lag/h Phase speed/(m∙s−1) W1–W2 225 E1-1 56 –2.3 ± 0.2 –27.2 ± 2.1 E1-2 119 3.6 ± 1.2 17.4 ± 4.4 E2-1 91 –3.9 ± 0.5 –16.0 ± 1.8 E2-2 181 3.3 ± 0.1 18.9 ± 0.5 W2–W3 73 E1-1 56 –0.2 ± 0.4 –101.4 ± 67.6 E1-2 119 3.0 ± 0.4 6.8 ± 0.8 E2-1 91 0.7 ± 0.5 29.0 ± 12.1 E2-2 181 3.1 ± 0.1 6.5 ± 0.2
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