Separation of an Upwelling Current Bounding the Juan de Fuca Eddy

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.

Automated summary

Observations on the southern Vancouver Island shelf reveal a significant exchange of water between the shelf and offshore areas, particularly near a semi-permanent recirculation feature known as the Juan de Fuca Eddy. This eddy occurs where the shelf widens abruptly due to a bank, and consists of a mixture of offshore and coastal water. The water in this eddy is well-mixed but maintains some stratification, either due to rapid mixing or a long residence time. Despite its uniformity, there is a narrow temperature-salinity front offshore that shows no sign of instability. Cross-front transport and hydraulic jumps are observed during a survey over a bank. Upstream of the eddy, there is an equatorward along-shelf current that separates from the shelf and is replaced by offshore water in the lee of the bank, as evidenced by satellite images of cool coastal water being ejected offshore.