Variability of Internal Solitary Waves in the Northwest South China Sea

Abstract

Internal waves are waves that travel within the interior of the water column. Its existence is owned to the stratified density structure between two or continuous layers of fluids (Apel et al., 1987). Internal solitary waves (ISWs) are nonlinear internal waves, which are frequently observed all over the world oceans, where strong tides and stratification occur over varying topography features (Apel et al., 1985; Colosi et al., 2001; Osborne & Burch, 1980). They typically occur in packets at tidal intervals, suggesting that they mainly originate from the tide-topography interactions over variable topography (Gerkema et al., 1995). Depending on the different environmental conditions, there are two main mechanisms for the generation of ISWs: lee-wave mechanism (Maxworthy, 1979) and nonlinear internal tide mechanism (Lee & Beardsley, 1974). The lee-wave mechanism states that the lee-wave is formed by the ebb tide and released when the tide changes from ebb tide to flood tide, and evolves into a rank-ordered internal solitary wave (ISW) packet. By the nonlinear internal tide mechanism, internal tides spawn ISWs in three steps: initial generation of a front due to topographic blocking, nonlinear steepening of the front, and formation of a rank-order ISW packet under effects of nonlinearity and dispersion (Helfrich & Melville, 2006; Zhao & Alford, 2006). Internal solitary waves are important for many practical reasons. As they are commonly observed wherever strong tides and stratification occur next to the irregular topography, thus they are often prominent features seen in optical and radar satellite imagery of coastal waters. They can propagate over several hundred kilometers and transport both mass and momentum. They can also induce considerable velocity shears that can impose unexpectedly large stresses on offshore oil-drilling rigs and lead to turbulence and mixing. In addition, the mixing often introduces bottom nutrients into the water column, thereby fertilizing the local region and modifying the biology system therein (Jackson, 2004). ISWs in the South China Sea (SCS) have been observed at a variety of locations from Luzon Strait to the continental shelf (Cai et al., 2002; Lien et al., 2005; Liu et al., 2004; Ramp et al., 2004; Xu et al., 2010; Zhao et al., 2004). Until recently, considerable effort has been focused on the study of ISWs in the northeastern SCS (Farmer et al., 2009; Moore et al., 2007; Ramp et al., 2004). In contrast, Due to the shortage of high-quality data sets, studies of the ISWs in the northwestern SCS are quite limited and nonlinear internal waves in this region have been