Seasonal and Spatial Features of Barotropic and Baroclinic Tides in the Northwestern South China Sea. Issue 1 (22nd January 2020)

• Record Type: Journal Article
• Authors: Yan, Tong; Qi, Yiquan; Jing, Zhiyou; Cai, Shuqun
• Is Part Of: Journal of geophysical research. Volume 125:Issue 1(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-01-22
• Subjects: South China Sea -- baroclinic tide -- barotropic tide -- observation -- model -- seasonal variation; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2018JC014860
• Languages: English
• ISSNs: 2169-9275

Abstract

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 K1 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 K1 energy in the NWSCS, which is in accordance with the observed summer‐enhanced barotropic K1 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 where the 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. Key Points: Observed barotropic K1 current shows significant summer intensification in accordance with modeled intensified internal K1 energy Summer intensification and winter disappearance of internal tide over the NWSCS shelf are shown by observations and simulations The slope south of Hainan Island is a major source region for local diurnal internal tides