The Sulu Sea Internal Soliton Experiment

Abstract

We present the results of a comprehensive study of large-amplitude internal solitons generated in the Sulu Sea in the Philippines by intense tidal flow over a sharp bathymetric feature. Surface signatures of these waves have been observed in images from the DMSP, Landsat, and Nimbus-7 satellites and the space shuttle SIR-A synthetic aperture radar. A two-week experiment was conducted in the Sulu Sea in May 1980 from R.V. Oceanograher, during which a three-mooring phased array of current meters and thermistors was implanted near the wave source and at 82 km and 200 km from the source. Seventeen soliton packets were observed whose properties underwent significant change as they propagated across the sea. Vertical profiles of density, temperature, and acoustic backscatter, as well as radar and photographic surface signatures, were obtained from the ship. Data from satellites, moorings, and the ship give unprecedented information on the spatial and temporal characteristics of solitary waves, which have amplitudes as large as 90 m, wavelengths up to 16 km, periods approaching 1 hour, crest lengths in excess of 350 km, and lifetimes greater than 2.5 days. Phase speeds are near 2.5 m s?1, well above computed linear values. The solitons are accompanied by narrow stripes of breaking surface waves 1000 m wide, with heights on the order of 1 m. The waves occur in groups containing several solitons, with groups separated by 12.5- and 25-hour tidal intervals, and appear and disappear over a fortnightly cycle. The theory of solitary-wave evolution has been used to interpret the observations with the evolution equation being solved using pseudospectral technique and good agreement is obtained between theory and experiment. From an observed initial disturbance, the calculation reproduces the measurements at the distant mooring with respect to number, speed, and spacing; amplitudes and widths depend on factors not currently in the theory.