Current observations from three moored Autonomous Temperature Line Acquisition System (ATLAS) buoys are analyzed to examine the tidal currents in the upper layer of central deep basin of South China Sea (SCS). Spectrum analysis of current data shows that diurnal and semi-diurnal tidal constituents play an important role in the total current, especially the diurnal tidal constituents. In the northern SCS deep basin, two major diurnal constituents K1and O1could contribute 33% and 21% to depth averaged current, respectively. From north to south, the magnitude of diurnal tidal constituents to the total current decrease while that of semi-diurnal tidal constituents remain similar. The magnitude, direction and contribution of most tidal currents, except the semi-diurnal tidal current in the northern part, vary significantly with water depth. This indicates that the tidal current is mainly baroclinic in upper layer of SCS deep basin.

INTRODUCTION

The South China Sea (SCS) is the largest marginal sea in the Southeast Asia, with maximum depth over 5000m in the deep basin. It has a bottom topography that makes it a unique semi-enclosed ocean basin that is temporally forced by a pronounced monsoon wind [Liu et al., 2001]. The tides have a remarkable contribution to the mixing and circulation in the SCS [Tian et al., 2003; Cai et al., 2003]. In recent years, a number of studies about SCS tidal currents had been done using coastal observations and numerical models [Mazzega and Berge, 1994; Fang et al., 1999; Yu et al., 1999; Yang and Liu, et al., 2002; Cai et al., 2003; Cai et al., 2006]. Numerical simulation from barotropic tide model shows tidal pattern is very simple and the tidal currents are very weak in the central deep basin of SCS, generally below 2 cm/s [Fang et al., 1999]. However, verifications of these numerical models are mainly based on the coastal measurements, direct observations of tidal currents in the SCS central basin had not been documented because of lack of long period direct measurements.

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