Turbulence Observations in the Northern Bight of Monterey Bay from a Small AUV

In this manuscript we show that in shallow water an autonomous underwater vehicle (AUV) can be used to quantify the horizontal and vertical structure of turbulence. We present data obtained during the Layered Organization in the Coastal Ocean (LOCO) experiment, which took place in a very shallow region of Monterey Bay at nighttime in the summer of 2006. The AUV employed was the T-REMUS vehicle, which contains a variety of the microscale and finescale sensors as well as supporting “hotel” sensors which monitor its position and performance. The vehicle was run in a 5-degree yoyo mode and, using the Rockland Microstructure Measurement System (RMMS), was able to obtain direct estimates of the turbulent dissipation rate on a vertical scale of 0.5 m. The yoyo sampling scheme allowed the finestructure and microstructure data to be obtained with an average horizontal sampling distance of 150 m. We examine the spatial structure of turbulence over a horizontal range of 2.5 km and throughout most of the water column, from 1 m from the surface to 4 m above the bottom. Eight hours of such data were collected.The experiment took place during a time period of very light wind forcing, little air sea exchange, and weak tidal flow. Nonetheless, strong turbulence, characterized by a turbulent kinetic energy dissipation rate of ε > 10− 7W/kg and buoyancy Reynolds number of Reb > 103, was observed throughout the experimental region. The turbulence, for the most part, was patchy and typically confined to the surface and bottom boundary layer regions. However, towards the end of the experiment a spatially continuous region of turbulence of at least 2.5 km extent was observed in the upper part of the thermocline. This occurred during the time period when an internal wave train appeared in the experimental area. Evidence is presented which suggests that the internal wave induced vertical strain gradient was responsible for producing this turbulent field.