Numerical study of the circulation in a steep canyon off the Catalan coast (western Mediterranean)

Abstract

We use a limited-area fine-resolution primitive equation model to carry out a case study of the circulation in the steep and narrow Blanes Canyon, located off the Catalan coast (western Mediterranean). This area is characterized by a permanent along-slope density-driven current and strong northerly and easterly wind bursts in autumn. In a first step the slope current is allowed to adjust to the bottom topography, leading to a stationary circulation pattern which presents large vertical variations related to the canyon geometry. In the surface layer the strong stratification allows the current to flow straight over the canyon rim, ignoring bottom topography. In the intermediate layer the canyon is wide, and waters are isolated in a geostrophic anticyclonic eddy maintained by friction at the edge of the slope current. In the deep layer the canyon is narrow, and a topographic upwelling lifts bottom waters toward the canyon head. In a second step the circulation is forced by easterly (along-shore with the coast on its right) and northerly (seaward) 1-day wind bursts which induce significant vertical and cross-shore motions steered by the bottom slope. This ageostrophic circulation is substantially amplified in the canyon where the flow pattern presents large-amplitude oscillations characterized by a downwelling (upwelling) during the easterly (northerly) wind burst followed by a rebound upwelling (downwelling) as the wind relaxes. The upwelling induced by the northerly wind burst is capable of renewing waters trapped in the canyon eddy. Our results appear to be in fairly good agreement with in situ physical and biogeochemical data collected in Blanes Canyon and comparable canyons of the world ocean. The three-dimensional features obtained in this study can explain some of the observed patterns of larvae distribution and exchanges between the coastal zone and the open sea. Copyright 1999 by the American Geophysical Union.