Southward Internal Tides in the Northeastern South China Sea

Abstract

The M 2 internal tides in the northeastern South China Sea are studied using satellite altimeter data from 1992-2018. By an improved mapping technique that combines plane wave analysis and two-dimensional spatial filtering, multiple internal tides are separately extracted with weak internal tides becoming detectable. The satellite results reveal for the first time a 300-km-long southward M 2 internal tidal beam in the northeastern South China Sea. The generation source is on the steep continental slope at the southern entrance to the Taiwan Strait. It ranges from 118-120 • E along 22 • N. Combining satellite-observed internal solitary waves and internal tides, it is found that the onshore radiation evolves into nonlinear solitary waves and the offshore radiation in the form of linear internal tides. Based on the 26-year-coherent satellite results, the integrated southward energy flux is 0.18 GW, about 10% of the westward energy flux from the Luzon Strait. In the northeastern South China Sea, the westward and southward internal tides form a multiwave interference field, which features significant spatial variations in the magnitude and direction of energy flux. Further analyses reveal that the steep continental slope radiates southward semidiurnal M 2 and S 2 internal tides, but not diurnal K 1 and O 1 internal tides. Plain Language Summary Internal tides are internal waves of tidal frequency occur in the interior of the stratified ocean. They mainly originate from topographic scattering of the barotropic tidal motion. Our knowledge of internal tides is mainly limited by the lack of field measurements with sufficient temporal and vertical resolution. Satellite altimetry offers a unique technique that observes internal tides from the space via their centimeter-scale sea surface fluctuations. Mapping techniques have been developed to extract internal tides from satellite altimeter data. This paper applies a newly developed mapping technique to the northeastern South China Sea and obtains an internal tide field with unprecedented details. Multidirectional internal tides from different generation sites are separately resolved. The satellite observations shed new light on barotropic-to-baroclinic conversion, internal solitary waves, multiwave interference, and internal tide energetics in the South China Sea. This paper confirms the uniqueness of satellite altimetry and the usefulness of the new mapping technique in advancing our knowledge of internal tides. This mapping technique can be applied to observe multiple internal tides in other marginal seas.

Reflection of K1 Internal Tides at the Continental Slope in the Northern South China Sea

“…Reflection of the O 1 ITs is also investigated, since their frequency is close to that of the K 1 ITs. In contrast to the K 1 ITs, no remarkable reflected waves for the O 1 ITs are detected in either altimeter or simulated results (Figure 10), agreeing with the results of Zhao (2020). As shown in Figures 10c and 10d, the eastward O 1 energy flux is almost an order of magnitude lower than that for the K 1 ITs.…”

“…A large portion of the reflected K 1 energy propagates southeastward across the deep basin, while the remainder propagates eastward toward the LS. Some features of the reflected K 1 ITs have been revealed in Zhao (2020), but results from this work provide a more comprehensive and detailed map.…”

“…In addition, local generation of ITs is estimated by calculating the barotropic tidal body force as , where Q is the tidal flux derived by the OTIS, N b is the bottom buoyancy frequency (Zhao, 2020). Large magnitude of F indicates the local generation of ITs.…”

“…In this work, to provide a clear map of the reflection of the K 1 ITs and analyze corresponding energetics, 3D numerical simulations are performed. Note that although the O 1 ITs have a frequency close to that of the K 1 ITs, no remarkable eastward waves are detected (Zhao, 2020); therefore, they are not the main focus in this work. The remainder of this paper is organized as follows.…”

“…However, owing to the long wavelength of the K 1 ITs, limited basin width and complex bottom topography in the northern SCS, precisely extracting the K 1 ITs from altimeter data is somewhat difficult. In addition, the contribution of high modes to reflected waves is absent when only mode‐1 waves are analyzed (Zhao, 2020). In other words, the reflected energy may be underestimated to some extent.…”