John R. Apel

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 ha...

An improved model of the ocean surface wave vector spectrum and its effects on radar backscatter

Images of the ocean surface taken with active microwave sensors often contain much information on near-surface and subsurface processes. However, their interpretations may depend on a detailed understanding of the physics of electromagnetic scatter. In scattering theory, the surface hydrodynamics enters the equations via (1) the probability distribution function for either wave heights or slopes, and (2) the two-dimensional wave height/slope autocovariance or its Fourier transform, the wave vector spectrum. This paper advances an improved model for the ocean surface wave vector spectrum based on recent work by M. A. Donelan, by M. L. Banner, and by B. Jahne and their collaborators. The model addresses the range of surface wavelengths from fully developed wind waves to the gravity-capillary region. For gravity-capillary waves, the spectral equation satisfactorily represents the observational data of Jahne et al. taken in tanks at large fetches, in the range from approximately 50 to 1500 rad/m to within the accuracy of the data. From the spectrum, the two-dimensional autocovariance of the sea surface is computed and correlation lengths and curvatures obtained. When used with a modification of Hollidays formulation of microwave radar backscatter from a Gaussian sea, it quantitatively reproduces observational cross section data taken at vertical polarization from aircraft and spacecraft over the open ocean, with differences from the field data having a mean of −0.2 dB and a standard deviation of 1.7 dB, The range of parameters for which satisfactory fits are obtained includes: wind speeds from 1.5 to 24 m/s; frequencies from approximately 5.5 to 35 GHz; and incidence angles from 0° to greater than 60°. For horizontal polarization, the scattering calculations fail rather badly for larger incidence angles, as do all theories based on the Kirchhoff approximation. Additionally, in spite of the incorporation of an anisotropic angular distribution of wave energy, the observed azimuthal variation of radar scatter is not captured, indicating that the source of that variation lies elsewhere.

A New Analytical Model for Internal Solitons in the Ocean

Abstract A new model for tidally generated internal solitons in the ocean is advanced, based on a little-known asymptotic analytical solution to the weakly nonlinear Korteweg–de Vries equation. The solution has many of the properties observed in oceanic internal solitary waves (solitons). It has been adapted to simulate the dynamics of stratified fluid flow driven by tidal forcing at continental shelves and sills, and the adaptation describes the canonical behavior of such solitary wave trains. It is shown that the solitons are positioned at the leading edge of an internal tidal bore on the shelf and that the distance between successive packets of solitons constitutes the internal tidal wavelength. A future paper will apply the model to field observations from four geographical regions.

On the use of high‐frequency acoustics for the study of internal waves and microstructure

Experimental data and theoretical calculations on the scattering of high-frequency acoustic signals from oceanic internal waves are presented. Acoustic data on internal waves are compared with simultaneous temperature (towed thermistor) data. The comparisons have shown a high degree of correspondence between the temperature and the acoustic data. Theoretical calculations for the acoustic scattering cross section σ are made by assuming that temperature fluctuations give rise to the acoustic scattering. An enhanced cross section for scattering from layered temperature fluctuations is to be expected, in agreement with the 1973 calculations of W. H. Munk and C. Garrett.

A study of oceanic internal waves using satellite imagery and ship data

Abstract Surface manifestations of oceanic internal waves have been studied in Landsat-1 and -2 data since 1972. The internal waves appear as periodic, intermittent variations in the surface optical reflectivity and are visible from spacecraft, aircraft, and surface vehicles under certain circumstances. The Landsat data suggest that the source of the waves is semidiurnal and diurnal tidal action at the edge of the continental shelf. A study of the wave characteristics yields considerable insight into the physics of their excitation, propagation, and dissipation. Packets have been observed from the Gulf of Maine to Cape Hatteras and in images taken off the U.S. and African east and west coasts, the Gulf of Mexico and the Caribbean, The Gulf of California, the Sulu Sea, and the Baltic. The internal wave groups show an orderly variation in wavelength from front to rear of the packet, due to a combination of frequency dispersion and nonlinear amplitude effects. An oceanographic cruise was carried out in synchronism with two 18-day Landsat-1 cycles, and data were taken on temperature, density variations, acoustic echoes, and surface slicks accompanying the internal waves. The data were satisfactorily correlated with spacecraft and U-2 imagery taken simultaneously.

Nonlinear Features of Internal Waves as Derived from the SEASAT Imaging Radar

The synthetic aperture radar (SAR) on Seasat has yielded well defined images of quasi-periodic internal waves in the waters on the west side of Baja California. These waves occur in groupings separated by 10–20 km, each group having two to twenty striations with wavelengths of order 400 m. At the shelf edge, they occur in water approximately 150 m deep, shoreward of several banks at the continental shelf edge whose depths are as shallow as 15 m. The wave surface signatures exhibit clear nonlinear features: higher-than-linear group velocities, and decreases in wavelength, crest length and amplitude toward the rear of the packet. Environmental data for the area have been examined and these show a well-developed mixed layer, low winds and vigorous tidal action. Thus stratification, wind speed, and bottom topography apparently combine to establish proper conditions for tidal generation of internal waves. The images have been analyzed from the standpoint of cnoidal-wave-packet theory, and it is shown that the vertical displacement of an isopycnal surface can be deduced from a combination of SAR imagery, vertical density profiles, and bottom topography. Numerical examples are given.

Seasat: A spacecraft views the marine environment with microwave sensors

Seasat-A is a new NASA satellite dedicated to oceanographic measurements of interest to a broad spectrum of the marine community. Its strong suit is an array of active and passive microwave instruments that give it the ability to view surface features on a day-night, near-all-weather basis. It will measure such features as wave heights, lengths, and directions; surface wind velocities; currents; temperatures; ice cover; and the marine geoid. Sensor capabilities and examples of their data output will be given, and the usefulness of these data for understanding the coastal marine environment will be discussed.

Acoustic intensity and phase variability caused by time‐evolving internal wave fields

Relationships between time‐evolving internal wave fields in a shallow‐water environment and both acoustic intensity and phase variability are explored through a computer simulation model developed from experimental data acquired on the continental shelf off the New Jersey coast [Apel et al., IEEE J. Ocean. Eng. 22, 465–500 (1997)]. From an analysis of SWARM (Shallow Water Acoustics in a Random Media) data, single realizations of the acoustic field are computed at an array located approximately 42 km from sources emitting at 224 and 400 Hz. Using a wide‐angle parabolic equation, the acoustic field is computed in 1‐min intervals over a 12‐h time window during which both spatially diffuse and spatially localized internal wave fields propagate through the water column. Results include time‐evolving modal power and scintillation indices, and can be interpreted in terms of mode coupling. The ability to locate a source using phase‐coherent techniques such as matched field processing will also be addressed. [Work...

Acoustic field propagation through a shallow water waveguide dominated by internal waves

The continental shelf off the New Jersey coast is a site of intense internal‐wave activity. The space–time variability of an acoustic field in this shallow‐water region was studied using oceanographic measurements along a 42‐km track from the SWARM95 [shallow‐water acoustics in a random medium] experiment. A sound–speed model based on the KdV equation was developed to include the nonlinear contribution of the internal wave field, with parameters estimated from towed yo‐yo ctd data, ship radar images of internal‐wavesurface expression and acoustic backscatter images of subsurface internal‐wave structure. The linear internal‐wave contribution was modeled by an ADCP‐derived displacement power spectrum. Bathymetry and sub‐bottom parameters were previously estimated from wave‐field inversions using chirp sonar surveys. Simulation results are presented for acoustic propagation at frequencies of 224 and 400 Hz using a wide‐angle parabolic equation method to compute realizations of the acoustic field. Results inc...

Acoustic travel time and intensity fluctuations measured in the SWARM95 experiment

During the summer of 1995, a multidisciplinary, multilaboratory experiment entitled ‘‘SWARM’’ (shallow‐water acoustics in a random medium) was conducted on the continental shelf off New Jersey. The purpose of this experiment was to examine the effects of internal waves, both linear and nonlinear, upon acoustic transmissions in the 10 to 1000‐Hz range. Towards this goal, numerous oceanographic sensors were deployed along a fixed across shelf track, together with a fixed acoustic transmission range. Shipborne acoustic sources also allowed out of plane (along shelf) studies. Results will be presented of recent data analyses showing the nature of the measured acoustic travel time and intensity fluctuations, and how they correlate to environmental forcing, particularly by internal waves and the shelfbreak front. [Work supported by ONR.]