Mixing in Haro Strait
Abstract : The long-term goal is to better understand mixing processes in estuaries. Of particular interest are the processes at work in the Haro Strait region between Washington State and British Columbia, which are modulated by a spring/neap cycle in tidal velocities. The field phase of the Haro Strait experiment took place in the summer of 1996 and involved a collaborative effort between MIT, WHOI and IOS. Our objectives are to understand the dynamics associated with intense coastal mixing processes in which separation of tidal flows past topography leads to formation of fronts accompanied by instability and mixing. During this year, we have begun to analyze this data in two different areas. First, we wanted to better quantify the large-scale dynamics. This means estimating horizontal layer transports, rates and amounts of mixing that take place. Second, we want to use these results to understand the importance of smaller-scale mixing that occurs at convergence fronts within this region.
Real-time frontal mapping with AUVs in a coastal environment
A new oceanographic measurement concept is presented, combining a tomographic network with small autonomous underwater vehicles. Using wireless local area network technology, the acoustic tomography data are recorded and processed in real time for achieving a low-resolution estimate oceanographic parameters. The results are used to focus direct measurements by a network of small AUVs in areas of high spatial and temporal variability. By combining the high coverage but low resolution of the tomography with the high resolution capabilities of the AUVs, oceanographic phenomena with small scale dynamics controlled by large scale environmental forcing can be mapped. The feasibility of the new measurement concept has been demonstrated in connection with a June 96 experiment in Haro Strait, BC, aimed at mapping the properties coastal fronts driven by combined estuarine and tidal forcing.
Mixing Efficiencies in Turbulent Tidal Fronts: Results from Direct and Indirect Measurements of Density Flux
Abstract The authors report direct measurements of density flux at a single depth in a turbulent tidal flow, made by towing a CTD beside the vertical beam of a modified acoustic Doppler current profiler. The direct flux estimates are compared with indirect estimates of density flux based on simultaneous microscale profiler measurements of ϵ and χ, dissipation rates of turbulent kinetic energy and of temperature variance, respectively. Two mixing efficiency estimates are made using Γd from the ratios of indirect flux estimates and Γo from the ratio of direct to ϵ flux estimates. The analysis indicates that • Γd is no different from that determined in other open ocean experiments and is independent of the sign of the flux • Γ d 0) < |Γo| (flux < 0). The consequences for interpretation of ocean microstructure flux estimates are discussed.