Abstract
2SUMMARY: In baroclinic fronts, jets and eddies, potential vorticity, P = (ς + f) / D, connects horizontal motions of planetary vorticity, f, and relative vorticity, ς, to vertical transport through increases and decreases of the water column height, D. In this paper we present evidence that positive changes in relative vorticity of the SW flowing Catalan Current and meandering of the coastal jet follow the 500 m isobath and are associated with topographic steering by offshore canyons. Generally, positive relative vorticity is a result of cyclonic torque that generates cyclonic flow causing the water column to upwell since both P and the volume of the flow are conserved. With coastal jets that flow over abrupt topographies, such as deep canyons, D increases and flow must take on a cyclonic circulation, favoring upwelling conditions. In the case of a westward flowing current, we would expect the flow to acquire a cyclonic vorticity upstream of the canyon but to enter at the canyon obliquely (Cushman-Roisin, 1994) causing downwelling. Upwelling in meandering coastal jets can also occur when horizontal streamlines diverge, unable to adjust to relative vorticity changes, and fluid parcels near the edge of the front separate. Using pycnocline thickness as an indicator of D, we show that deep canyons stretch the water column causing cyclonic meandering (positive relative vorticity), upstream of the canyon during the FRONTS cruises 1991 to 1993. This phenomena has been shown to cause upwelling upstream of meander troughs (Onken, 1992) which in this case would mean upstream of canyons. We hypothesize that the increased relative vorticity, associated with upwelling in the pycnocline, fuels episodic primary production across the front accounting for more than 30% of the new production in this coastal zone.
Highlights
Within the NW basin of the Mediterranean Sea, a permanent density front occurs between the BalearicIslands and the Iberian mainland (Salat and Font, 1987; La Violette et al, 1990; López García, 1991; Estrada, 1993)
The front is a prograde, meaning that it intersects the bottom shoreward of its intersection with the surface; it does not reside over any specific isobath
A Sea Bird CTD equipped with a Sea Tech fluorometer was used for the 1991 cruise and an Endeco dissolved oxygen (DO) sensor was added for the 1992 cruise
Summary
Within the NW basin of the Mediterranean Sea, a permanent density front occurs between the BalearicIslands and the Iberian mainland (Salat and Font, 1987; La Violette et al, 1990; López García, 1991; Estrada, 1993). The position of the front is seasonally variable and controlled by wind and density distributions over the shelf. The latter is dependent on river discharge, which normally peaks in the spring (Font et al, 1990, Salat et al, 2002). The Catalan current is a coastal jet within the front. Given the rough topography of this coastline and the tendency of coastal jets to follow isobaths, the mechanism of relative vorticity driven upwelling may be an important mechanism driving production in the Western Mediterranean Sea. The critical meander scale for topographic steering of the current can be estimated from the relationship
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