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.
Three-dimensional structure in a tidal convergence front
Lagrangian measurements of the structure of a tidal convergence zone were made in Haro Strait, on the western boundary of the U.S. and Canada. Acoustic imaging of the ocean surface from below was used in combination with a sub-surface, acoustically tracked, neutrally buoyant float and several CTDs. Generally, regions of high acoustic back-scatter were correlated with surface convergence and downwelling, while regions of low back-scatter corresponded to divergence and upwelling. The large scale velocity gradients which appear to form this front were concentrated in a shear zone a few 10 s of m wide with vertical vorticity up to 0.15 s−1 . Associated with this is a 100 m wide region of roughly 0.1 m s−1 average downwelling and maximum downwellings of 0.2 m s−1. A rich structure of three-dimensional eddies and front-like features occurs throughout the tidal mixing region. One common feature, appearing on the surface as a circular boil, extended at least 50 m deep and consisted of a 100–150 m region of upwelling surrounded by a much narrower ring of more intense downwelling. These eddies have a turbulent Froude number near one, so their size is constrained by the stratification. As a whole, the convergence zone acted to mix the waters brought together by the converging tidal streams. Although our data are very limited, the turbulence intensities which we observed are potentially large enough to support all of the mixing required by the overall circulation in Haro Strait.