An AUV survey in the littoral zone: Small-scale subsurface variability accompanying synoptic observations of surface currents

A survey of small-scale subsurface variability within the synoptic observational field of an ocean surface current radar (OSCR) using an autonomous underwater vehicle (AUV) is described. The survey involved observation of a developing upper mixed layer in a littoral zone off southeast Florida, on the edge of a strong Florida current during the summer of 1999. Complimentary in situ observations from a bottom-mounted acoustic Doppler current profiler (ADCP), conductivity-temperature (CT) chain arrays, atmospheric measurements from a surface buoy, and CTD and ADCP observations from a surface ship provided the background to the survey. The AUV, the Ocean Explorer, equipped with a CTD, downward and upward looking ADCPs, and a small-scale turbulence package, was used to conduct a continuous 12-h survey of small-to-fine-scale variability within a few grid cells of the surface current radar field. The vehicle repeatedly sampled the same grid in a set pattern at a fixed mid-water depth. Maps of developing spatial distribution of current, salinity, temperature, and rate of dissipation have been developed using the AUV-based observations. The observed features in the current field compare well with the OSCR and the bottom-mounted ADCP measurements. The observations revealed a strong presence of a near-tidal period (10-h) oscillation in the current field accompanying a northward propagating shear wave along the edge of the Florida current and other features, including the presence of patches with relatively high dissipation rates during the survey. The data set acquired provides simultaneous maps of subsurface structure, including rate of dissipation of kinetic energy and synoptic field observations of high-resolution surface current. The application of the AUV in this oceanographic survey represents a step in the direction of developing a multivehicle ocean sampling system.