Internal wave measurements from a midwater float

Abstract

An instrument package has been developed that drifts along freely with the water while it repeatedly profiles ocean temperature. Profiling during drifting reduces the Doppler and fine structure effects that usually contaminate internal wave measurements. Six days of exceptionally clean internal wave records were acquired 470 km offshore of San Diego, California, in June 1973 at a nominal depth of 800 m. The resulting vertical displacement spectra decrease generally as ω−2 up to the local Brunt‐Väisälä (B‐V) frequency. Just below the B‐V frequency there is a spectral peak. Above the B‐V frequency the spectra drop sharply to low levels. Vertical coherence of internal waves over a separation of 100 m was found to be 0.8 and fairly independent of frequency up to the B‐V frequency. An internal wave model proposed by Garrett and Munk is in general agreement with the observed features. Both the measured coherence and the oscillatory nature of the spectrum near the B‐V cutoff are consistent with an energy concentration in the lower six or so modes. In addition to the internal wave records, continual changes in the thermal structure were observed by the drifting package. One repeating thermal inversion pattern occurred four times during the 6‐day thermal structure record.

Resonant triad model for studying evolution of the energy spectrum among a large number of internal waves

“…The parameter n * influences the modal band width, i.e., how quickly the energy levels of the modes decrease with mode number. A value of 3 gives results that agree with some observations [3].…”

Observations of long period waves in the tropical oceans and atmosphere

“…The observed frequency-dependence of internal wave displacement spectra at mid-latitude is also -2 (Cairns (1975)). …”

Internal wave–turbulence pressure above sloping sea bottoms

“…First, spectra from vertical current observations using ADCP above GMS show a small peak just below N and roll‐off just to about the upper limit of the above IWC range obtained from pressure observations, indicative of and coinciding with N 1 (Figure 6, red spectrum). This is because small‐scale, thin layer internal wave motions dominate vertical currents; near N‐N 1 the aspect ratio of motions is about equal to one and a small peak in w spectra is commonly found [e.g., Cairns , 1975], which has a shape similar to the stratification distribution [ van Haren and Gostiaux , 2009]. In the open ocean, the shape slopes like σ +1 from IWC toward N [ van Haren and Gostiaux , 2009], whereas in other, probably more turbulent, areas it slopes more like σ 0 [ D'Asaro and Lien , 2000a].…”