Seasonal and Spatial Features of Barotropic and Baroclinic Tides in the Northwestern South China Sea

Abstract

Seven seasonal moorings distributed over the shelf and slope east of Hainan Island and south of western Guangdong Province are used to investigate the seasonal characteristics of barotropic and baroclinic tides in the northwestern South China Sea (NWSCS). The barotropic K 1 current intensified during summer over the shelf sites. The diurnal baroclinic tides are much more prominent than the semidiurnal tides at each mooring site. Over the continental shelf, accompanied by large phase speeds and wavelengths, baroclinic tides are significantly enhanced in summer whereas they decrease to minimum values in winter when the water column is vertically well mixed, suggesting the key role of seasonal stratification in modulating the seasonal variation of baroclinic tide. To aid in elucidating these observations, a circulation-tide coupled model covering the whole South China Sea (SCS) is set up for baroclinic tidal energetics in the NWSCS. Comparisons with moorings, tidal gauges, altimeters, and so on show that the model reasonably predicts the barotropic and baroclinic tides and seasonal upper circulations in the SCS and NWSCS. The model results present significant summer intensification of barotropic K 1 energy in the NWSCS, which is in accordance with the observed summer-enhanced barotropic K 1 current. Strong baroclinic tides are reproduced over the summer shelf, which is related to intense stratification. The slope south of Hainan Island is a major local generation source for diurnal baroclinic tides that radiate southeastward approximately 500 km away. Both the Xisha Islands and Shenhu-Yitong shoals act as baroclinic tidal energy sink subregions. Plain Language Summary The South China Sea (SCS) is a semienclosed marginal basin wherethe diurnal barotropic tide is amplified because of the resonance, and the baroclinic tide is extremely active. Previous studies on baroclinic tides in the SCS mainly focused on the northeastern SCS and Luzon Strait. Due to limited in situ observations, the basic characteristics of baroclinic tides and their seasonality in the northwestern SCS (NWSCS) are rarely reported. Past sporadic research has demonstrated the existence of baroclinic tides in the Xisha Islands and northeastern shelf of Hainan Island by short-term observations. In the present paper, a large amount of in situ data, including seven seasonal ADCPs and CTDs, is used to identify the seasonal characteristics of barotropic and baroclinic tides over the shelf and slope east of Hainan Island and the shelf southeast of western Guangdong Province. Subsequently, a high-resolution circulation-tide coupled ocean model for SCS is used to analyze the seasonal distribution of baroclinic tide energetics in the NWSCS. Multiple sources, integrated baroclinic tidal energy fluxes and energy budgets for three important subregions, are identified by the model results, which have been validated with diverse data sets.

Tidal currents in the coastal waters east of Hainan Island in winter

“…The velocity vectors rotate clockwise at H03 and H04 and rotate anticlockwise at H05 and the MR site. The direction of the major axis of the K 1 tide is consistent with the result in Yan et al (2020), while the rotation is opposite. With regard to the M 2 constituent, the direction of the major axis changes from NW-SE off shore (at H04-05) to NE-SW onshore (at H03) in the north; it is the same direction as the K 1 tide at the MR site.…”

“…There are previous studies showing the long-range westward propagation of the internal K 1 and M 2 tide generated at the Luzon Strait based on satellite observations (Zhao, 2014) and numerical modeling (Xu et al, 2016;Yan et al, 2020), which can propagate across the deep basin and arrive at the western continental slope (though it decreases when propagating to the west and is rather small relative to that in the far east), suggesting that the energy fl ux of internal tide near the successive stations probably indicates the feature of the internal tide propagated far away from the east. Moreover, Xu et al (2016) showed remarkable barotropic to baroclinic conversion rate of K 1 tide along the northern continental slope of South China Sea, and Yan et al (2020) revealed that the slope region around 200-m depth in the southeast of Hainan Island is generation region of baroclinic M 2 tide and the barotropic to baroclinic conversion rate along the slope is signifi cant in winter, suggesting the possible eff ect from the generated internal tide along the slope on the energy fl ux and the westward propagation to the study area. However, this issue needs support of more data and is out of this paper's scope and will be investigated in the future.…”

“…Particularly, intensive observations are rare to reveal the crossshelf variation of the tidal current on the shallow shelf. Moreover, previous studies have shown that the tide type east of Hainan Island is complicated, and it is the transition region of two tide/tidal current types, irregular semidiurnal tide/tidal current and irregular diurnal tide/tidal current (Editorial Board of Marine Atlas, 2006;Yan et al, 2020). Therefore, more works are necessary to sharpen our view of the tidal current in this region.…”

“…The baroclinic K 1 tide appears to exhibit a semiannual variation with stronger signals in the summer and winter. Yan et al (2020) presented the tidal ellipses of four principal constituents from moored time series in the northeast, southeast, and south of the Hainan Island approximately along the 50-m isobath. The results showed that the major axes of all the four constituents in the northeast are nearly in the N-S direction and the maximum velocity of the semidiurnal tide is 14.4 cm/s, which is about twice that of the diurnal tide.…”

Generation of Shoreward Nonlinear Internal Waves South of the Hainan Island: Synthetic Aperture Radar Observations and Numerical Simulations

“…Li et al (2008) and Z. Xu et al (2010), Su and Pohlmann (2009), Liang et al (2019), D. Li et al (2011 and Yan et al (2020), and J. Xu, Chen, et al (2016), respectively.…”