Measurements of vertical profiles of oceanic current and Richardson number near St. Croix, U.S.V.I

Abstract

Measurements of vertical profiles of both horizontal components of ocean current were made approximately 6 kilometers from the western shore of St. Croix, U.S. Virgin Islands, with a vertical array of current meters, and using the technique of acoustically tracking slowly sinking floats. During some of the drops, the local profile of Vaisala frequency was measured by lowering a CTD (conductivity, temperature, depth) from a ship positioned directly over the descending profiler. Richardson numbers computed from these data were typicaly between 1 and 10, with values as small as 0.25 occurring occasionally. Simultaneous drops at different locations showed significant loss of similarity at separations of 2 km, and drops made at a fixed location became decorrelated in several hours. HE spectrum of internal waves in the ocean has been shown to be remarkably homogenous, as evidenced by the ability of Garrett and Munk1'2 and Desaubies3 to fit a universal spectral model to a diverse set of observations. This homogeneity of the oceanic internal wave field is suggestive of either a homogenous forcing mechanism or saturation of the wave field, or both. It has been pointed out by Wunsch4 that it is the departures from homogeneous models that should be studied in order to gain an understanding of the mechanisms by which internal waves are generated and dissipated. All of the measurements presented in this artical were made in proximity to the island of St. Croix and therefore might be expected to differ somewhat from similar observations made in the open ocean. Since it was not known how nearby topography would affect the distribution of internal wave energy with respect to frequency and vector wave number, several types of measurements were performed. To charac- terize the vertical current structure, 58 current profiler drops were made to a maximum depth of 400 m. Drops were made at a fixed location every 1 to 4 h to assess temporal variability of the profile, and simultaneous, spatially separated drops were made with two profilers to determine spatial variability. Also, an array of vector-averaging current meters was deployed to supplement the current profile measurements. The data presented here are only a fraction of those available, and were chosen to show the kind of information that was obtained and to illustrate some of the more salient features of the current structure and variability that were observed.