The surface manifestation of internal waves: Doppler sonar observations from SWAPP

Abstract

During the Surface Wave Processes Program (SWAPP, February–March 1990), a ten‐beam dual frequency Doppler sonar system was used to observe the sea surface velocity field. The sonars were mounted on the research platform FLIP and oriented at various angles in azimuth, forming a directional array. Measurements were obtained out to a range of 1100m. Here, a preliminary effort to use the array to observe that component of the sea surface velocity field associated with internal waves is presented. The internal wave signal exists beneath the far stronger signatures of the surface wave and Langmuir fields. The utility of this approach depends on the ability to isolate these competing motions from the total velocity field. The directional properties of the open ocean internal wave field remain largely unknown. The large horizontal scales of the waves, coupled with the great range of scales, render the measurement problem difficult. The desire for adequate internal wave directional measurements prompted the original development of Doppler sonar at S.I.O. in 1974. Sections of the 20‐day Patchex time series are examined, through a variety of weather and surface wave conditions. Emphasis is placed on the problem of distinguishing small‐scale waves from nonpropagating Langmuir cells. In addition, an attempt to use the full 2‐km aperture of the array to estimate the directional properties of the baroclinic tide will be discussed.