The role of outflow geometry in the formation of the recirculating bulge region in coastal buoyant outflows
Density-driven coastal currents are a common feature in the world's coastal oceans. These currents may separate from the coastline due to variations in geometry. Past studies have shown that this separation may produce two distinctly different flow states: a continuation of the coastal current, or a recirculating gyre downshelf of, and attached to, the separation point. Laboratory experiments of coastal buoyant outflows (rotationally dominated, buoyancy driven) were undertaken to examine the role of bay geometry on the evolution of the outflow. Experiments were conducted on a rotating turntable in relatively deep water (such that the buoyant layer depth was much less than the total ocean depth). The geometry of the bay-exit was varied, both in exit angle () and in radius of curvature (rc). The width of the bay was varied such that the bay exit Kelvin number (a ratio between the width of the bay exit and the internal Rossby deformation radius) was order 1 for all experiments. A recirculating bulge (a large, anticyclonic gyre joining the coastal current to the buoyant source) was occasionally observed to form. Results are compared to the Bormans and Garrett (1989) hypothesis: this hypothesis is found to explain a portion of the results only. Geometrical arguments are presented that build upon the Bormans and Garrett hypothesis that parameterizes the magnitude of the flow separation between the buoyant fluid and the exit. A separation ratio, Γ, is defined as a ratio between the inertial turning radius of the flow and the maximum offshore distance between the separated flow and the coast. A recirculating bulge was observed to form for flows with values of Γ > 0.5. The separation ratio, Γ, is shown to be equivalent to the impact angle, Φ, of the buoyant fluid re-encountering the wall. The impact angle governs the upshelf and downshelf volume flux of the impacting fluid: recirculating bulge formation is found to occur when at least 50% of the source volume flux returns to the source region. This is equivalent to an impact angle greater than or equal to 60-degrees.
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Document Type: Research Article
Publication date: July 1, 2003
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