| Abstrakt: |
Observations of temperature, salinity, and oxygen on the southern Vancouver Island shelf show a large‐scale exchange of shelf water with offshore water, just offshore of a semi‐permanent recirculation, often termed the Juan de Fuca Eddy. The Eddy occupies a region where the shelf widens abruptly in the lee of a bank. The water in this Eddy is a mixture of offshore water and water from a buoyant coastal current. This water is well‐mixed along a mixing line in temperature‐salinity space, though it retains stratification, and is either rapidly mixed or has a long residence time. There is a less than 1 km wide temperature‐salinity front on the offshore side of this well‐mixed water that has no sign of instabilities. The clearest evidence of cross‐front transport is found during a tidally resolved survey over a bank. The transport is due to flows in the cross‐bank direction that also drive 50 m tall hydraulic jumps. Upstream of the Eddy, there is an along‐shelf current flowing equatorward. However, the whole current separates from the shelf before reaching the Eddy, in the lee of a bank, and is replaced by water from offshore. The separation event was also seen in sea‐surface temperatures from satellite images as a tongue of cool coastal water that is ejected offshore. Plain Language Summary: The southern Vancouver Island continental shelf is biologically productive due to high nutrient input from the Strait of Juan de Fuca and Salish Sea estuarine system and substantial cross‐shelf transport due to the complicated topography. Here, we present an intensive sampling of the Juan de Fuca Eddy region. The observations show that below the surface mixed layer, the water in the Eddy is low in oxygen, and has undergone substantial vertical and lateral mixing. In contrast to previous literature, we find that the low oxygen in the Eddy is likely because of respiration rather than being pulled from low‐oxygen water in the California Undercurrent. The observations also show a remarkable flow separation of the equatorward shelf current. The current is seen to detach and is pushed offshore. Such events are readily seen in satellite imagery, but our observations indicate that the separation extends the depth of the water column on the shelf, and that this separation may be partially driven by the local bathymetry. The separation is a very strong cross‐shelf exchange event, and transports substantial nutrient‐rich coastal water offshore to drive productivity in the deeper ocean adjacent to the continental slope. Key Points: The shelf break current along Vancouver Island separates downstream of a submarine bankOffshore water is drawn onto the shelf and forms a sharp semi‐persistent front with the Juan de Fuca EddyThe Eddy shows evidence of long residence times, and little evidence of deep‐water origin [ABSTRACT FROM AUTHOR] |
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