Characteristics of internal waves in the South China Sea Observed by a shipboard coherent radar

In 2005 and 2007, a coherent, X-band radar was deployed in the South China Sea on two different ships. In both cases, the two parabolic antennas of the radar were fixed at grazing angles of approximately 2° looking toward the bow of the ship. The radar transmitted and received through a single antenna but alternated between the two antennas approximately every half second. One antenna was horizontally polarized and the other was vertically polarized. The data were analyzed by computing normalized radar cross sections and scatterer velocities as a function of ground range and time. Surface signatures of the internal waves were obvious in both types of image and at both polarizations as regions of enhanced cross sections or scatterer velocities. The collected imagery showed that at least two different types of internal waves exist in the South China Sea: small, nearly sinusoidal trains of waves and large soliton-like waves. These different types travel at very different speeds and interact with each other. The small nearly sinusoidal waves travelled at phase speeds near 1 m/s that increased as the small wave trains were overtaken by the faster solitons. Combined with other shipboard measurements, the radar measurements yielded the widths, maximum velocities, and strain rates of the solitons as well as the dependence of phase speed on amplitude. When the speeds of both the ship and the solitons were removed, the measurements showed that soliton full-widths at half-maximum ranged from about 0.5 to 4.5 km. These widths showed a dependence on the amplitude of the soliton. The phase speeds of the solitons also depended on their amplitude, reaching 3 m/s in deep water but only about 1.2 m/s in shallow water. CTD profiles were used to estimate an interface depth for a two-layer fluid model of the propagation of the solitons. The phase speeds predicted by this model agreed well with the observed dependence of the soliton phase speed on amplitude in both shallow and deep water.