Internal Solitary Waves in the Warm Pool of the Western Equatorial Pacific

Abstract

During the spring tides of late November 1992, early January and early February 1993, solitary internal wave packets were observed at 2°S, 156°15?E in the western equatorial Pacific. Apparently generated in the Nuguria island group (3°S, 153°E), the waves propagate northeastward at 2.4?2.8 m s?1, appearing in fixed phase with the underlying semidiurnal baroclinic tide. The initial solitary wave crests have downward displacements in excess of 60 m and peak velocities greater than 80 cm s?1. Groups of 1?3 crests are observed, with vertical structure resembling a first-mode internal wave and horizontal variability consistent with third-order comb KdV solitons. Many of the packets exhibit an overall organization reminiscent of undular bores. This borelike behavior is confined primarily to the upper 70 m, not sharing the mode-one dependency of the crests. The solitons displace the ambient equatorial currents, including the Equatorial Undercurrent, both vertically (80 m), and laterally (1?2 km), with little apparent interaction. A 161-kHz Doppler sonar mounted on the R.V. John Vickers provided ocean velocity measurements with 3-m vertical resolution and 2-min time resolution in the upper 250 m of the sea. Merged with GPS-derived ship?s navigation, the resultant depth?time records of absolute velocity enable estimation of flow streamlines, given the two-dimensional nature of the wave trains. During passage of the soliton crests, the vertical displacement of streamlines is in good agreement with the observed vertical displacement of biological scattering layers. Noticeable increases in acoustic scattering strength are associated with the passage of all soliton groups, suggesting the (turbulent) production of small-scale (0.46 cm) structure in the sound speed field. However, the shear in these mode-one solitons is small compared to ambient equatorial background shears. The minimum Richardson number intrinsic to the soliton packet is of order 15. The crests apparently trigger small-scale instabilities on the background shear. Several of the soliton packets display pronounced internal wave ?tails.? These too are apparently triggered disturbances on the preexisting flow. High-frequency shears are oriented nearly orthogonal to the low-frequency background, independent of the propagation direction of the soliton. The energy density of the larger soliton groups approaches 0.2 gigajoules per meter of crest, a value comparable to the underlying baroclinic tide, and also comparable to the energy lost from the M2 barotropic tide over a ?1500 km propagation path through the western tropical Pacific (given a mean barotropic dissipation rate of 3 ? 10?3 W m?2 for the region).