Separation of an Upwelling Current Bounding the Juan de Fuca Eddy

Abstract

AbstractObservations 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.