Mixing and secondary circulation in Juan de Fuca Strait




Ott, Michael William

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Estuaries, the regions where runoff of fresh water, soil, and contaminants first encounter the ocean, are also primary fishing and recreation areas. It is therefore important to understand the dynamics associated with mixing and currents within these bodies of water. Pollutants and fresh water from river runoff flow out to sea in the upper layer, while nutrient-carrying oceanic water returns beneath. While there have been many studies of the processes involved in this exchange flow, the dynamics and vertical structure of transverse flows are much less understood, despite the role these currents play in redistributing water properties and momentum throughout the estuary. One such estuary, Juan de Fuca Strait, is an ideal location in which to study estuarine exchange and the resulting cross-channel flows induced by internal friction, primarily because its length and smooth topography reduce the topograpic steering of currents. Historical current meter data from a number of deployments in Juan de Fuca Strait reveal that, while mean along-channel currents are roughly consistent with the thermal wind equation, cross-channel flows are not, particularly at mid-depths where transverse currents are largest. A momentum balance using historical sea level and current meter data suggests that the vertical eddy viscosity Aᵥ ≈ 0.02 m²s⁻¹ at interfacial depths in May. The mean circulation in Juan de Fuca Strait-depths is highly seasonal in nature, however, and larger values may be more appropriate in summer when the estuarine exchange peaks due to the freshet. Stronger friction is in turn associated with elevated mixing rates and increased transverse velocities. An Acoustic Doppler Current Profiler deployed in Juan de Fuca Strait in the summer of 1996 resolved the vertical structure of these velocities. Concurrent Current-Temperature-Depth data reveal that neither the along- nor the cross-channel currents are in geostrophic balance with the hydrographic structure, suggesting that the physical processes associatd with these currents are more localised than the five kilometre scales over which the hydrography was measured. Zooplankton within Juan de Fuca Strait comprise a significant part of the scattering cross-section upon which the ADCP depends. During their dusk migration into the euphotic zone to feed and dawn descent to escape predation, they do not act as passive backscatter targets for the Acoustic Doppler Current Profiler. Vertical migration velocities, measured from the backscatter intensity record, reached 0.03 m s⁻¹, suggesting that significant biases in the measured vertical velocity could be introduced. Little effect was actually seen in the velocity fields, however, even though the cross-sectional fraction of the zooplankton was an order of magnitude larger than the background. Mean currents in Juan du Fuca Strait reveal strong transverse flows at mid-depths, suggestive of interfacial Ekman layers. The along-channel estuarine exchange is significantly enhanced at neap tide, consistent with weaker mixing upstream. The cross-channel flows at interfacial depths are also substantially larger during neap tide, implying a fortnightly modulation of mixing rates within the strait. The ADCP was also used to measure the Reynolds stresses directly. These were found to be more than an order of magnitude larger at neap tide than during spring tide and were consistent with changes in the mean current over the spring-neap cycle. Reynolds stresses were maximal at mid-depth on the transition from ebb to flood, at which time the gradient Richardson numbers were smallest, suggesting that critical layer absorption of internal waves are important dynamically.



Oceanography, Ocean circulation, Estuaries