Evidence for enhanced primary production resulting from relative vorticity induced upwelling in the Catalan Current

Abstract

SUMMARY: 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.Key words: vorticity, meanders, Catalan Current, new production, primary production. RESUMEN: EVIDENCIA DE UN AUMENTO DE LA PRODUCCIÓN PRIMARIA DEBIDO AL AFLORAMIENTO INDUCIDO POR LA VORTICI-DAD EN LA CORRIENTE CATALANA. -En los frentes baroclínicos, los chorros y los remolinos, la vorticidad potencial, P = (ς + f) / D, conecta los movimientos horizontales de la vorticidad planetaria, f, y de la vorticidad relativa, ς, con el transporte vertical mediante aumentos y disminuciones de la altura de la columna de agua, D. En este trabajo presentamos pruebas de que los cambios positivos en la vorticidad relativa de la Corriente Catalana, que fluye hacia el SW, y la formación de meandros del chorro costero, siguen la isobata de 500 m y están asociados con la dirección topográfica definida por los cañones submarinos. Por lo general, la vorticidad relativa positiva es el resultado de un momento torsor ciclónico que genera un flujo ciclónico que provoca que la columna de agua aflore puesto que tanto P como el volumen del flujo se conservan. Con chorros costeros que fluyen sobre topografías abruptas, tales como cañones profundos, D aumenta y el flujo tiene que adoptar una circulación ciclónica, lo que favorece las condiciones de afl...

Plankton in the open Mediterranean Sea: a review

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“…1), lead to deviations of the Catalan Current, creating eddies and intrusions (filaments) of denser and saltier water onto the continental shelf (e.g. Font et al 1988, Masó et al 1990, Granata et al 2004). …”

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