Observations of Flow and Mixing in Juan de Fuca Canyon

• Main Authors: MacCready, Parker, Alford, Matthew
• Format: Text
• Language: English
• Published: Western CEDAR 2014
• Subjects: Terrestrial and Aquatic Ecology; Pacific; Ocean acidification
• Online Access: https://cedar.wwu.edu/ssec/2014ssec/Day2/43; https://cedar.wwu.edu/cgi/viewcontent.cgi?article=1169&context=ssec

Juan de Fuca Canyon, Washington, which cuts across the continental shelf from the mouth of the Strait of Juan de Fuca to the shelf break, is a likely conduit for deep (below shelf break) Pacific water to enter the Salish Sea. This is important to the Salish Sea ecosystem because the deeper Pacific water has lower pH and dissolved oxygen. Despite its potential importance to Ocean Acidification in the Salish Sea, very few direct observations have been made in the Canyon. Here we report breaking internal lee waves, strong mixing and hydraulic control associated with wind-driven up-canyon flow near the shelf break in Juan de Fuca Canyon. Unlike the flow above the canyon rim, which shows a tidal modulation typical on continental shelves, the flow within the canyon is consistently up-canyon during our observations, with isopycnals tilted consistent with a geostrophic along-canyon momentum balance. As the flow encounters a sill near the canyon entrance at the shelf break, it accelerates significantly and undergoes elevated mixing on the upstream and downstream sides of the sill. On the downstream side, a strong lee wave response is seen, with displacements of O(100 m) and overturns tens of meters high. The resulting diffusivity is sufficient to substantially modify coastal water masses as they transit the canyon.