Instability of the Mediterranean Water undercurrents southwest of Portugal: effects of baroclinicity and of topography

Abstract

Southwest of Portugal, in situ data show that part of the mesoscale variability of the Mediterranean Water undercurrents is triggered by topographic effects (near capes and submarine canyons), and is driven by a dominantly baroclinic instability. The vertical alignment of these undercurrents observed at sites of eddy formation is indeed favorable to baroclinic instability. This instability is materialized by the formation of filaments and of small eddies in the Portimão canyon and of meddies near Cape Saint Vincent and near the Estramadura Promontory. Hydrological data and float trajectories reveal that these eddies have a baroclinic structure. Near Portimão canyon, the variation of the potential vorticity of the undercurrents again proves their sensitivity to baroclinic and also to barotropic instabilities, enhanced by the canyon. Finally, a model of stationary coastal current explains the variations of its horizontal structure over a canyon. © 2000 Ifremer/ CNRS/IRD/É ditions scientifiques et médicales Elsevier SAS Résumé-Instabilité des sous-courants d'eau méditerranéenne au sud-ouest du Portugal : effets conjoints de la baroclinicité et de la topographie. Au sud-ouest du Portugal, des données in situ montrent qu'une partie de la variabilité méso-échelle des sous-courants d'eau méditerranéenne est déclenchée par des effets topographiques au voisinage des caps et des canyons sous-marins et est générée par une instabilité essentiellement barocline. L'alignement vertical de ces sous-courants, observé aux sites de formation de tourbillons, est en effet favorable à l'instabilité barocline. Cette instabilité est matérialisée par la formation de filaments et de petits tourbillons lenticulaires dans le canyon de Portimão, et de ''meddies'' au cap Saint Vincent et près du promontoire d'Estrémadure. Les données hydrologiques et les trajectoires de flotteurs lagrangiens révèlent que ces tourbillons ont une structure barocline. Près du canyon de Portimão, la variation de la vorticité potentielle des souscourants montre leur sensibilité à l'instabilité barocline, et également barotrope, amplifiées par le canyon. Enfin, un modèle de courant cô tier stationnaire permet d'expliquer ses variations de structure horizontale sur un canyon. © 2000 Ifremer/CNRS/IRD/É ditions scientifiques et médicales Elsevier SAS Mediterranean Water / undercurrents / baroclinic instability / meddies / NE Atlantic Eau méditerranéenne / sous-courants / instabilité barocline / ''meddies'' / Atlantique nord-est

Hydrology and circulation in the Strait of Hormuz and the Gulf of Oman—Results from the GOGP99 Experiment: 2. Gulf of Oman

“…If baroclinic instability is responsible for their growth, it could be triggered by a topographic spur. And indeed, the 200-m isobath is indented offshore near 24°25 0 N, 57°10 0 E (a similar effect was observed on the Mediterranean outflow southwest of Portugal; see the Surdrift buoy trajectories of Chérubin et al [2000]). If time variation of the PGW outflow is responsible for the formation of meanders, their local amplification could be favored by the presence of a cyclonic motion upstream.…”

Hydrological Structure, Circulation and Water Mass Transport in the Gulf of Cadiz

“…Potential vorticity gradients have reversals in sign (not shown), which allow barotropic or baroclinic instability of the undercurrents. The Portimão canyon is a well-known site of meddy and MW cyclone generation ( [13][14][15]). These eddies often recirculate cyclonically in the Gulf, due to cyclone-meddy pairing, and thus contribute both to the regional-scale circulation in this layer, and to the presence of the MW tongue, via diffusion.…”

Mesoscale eddies in the Gulf of Aden and their impact on the spreading of Red Sea Outflow Water

“…There are at least three possible reasons: lack of a strong undercurrent, direction of monopole propagation on a β-plane and the presence of strong mesoscale eddies in the GOA. Several mechanisms have been proposed for the formation of meddies: separation of the Mediterranean Undercurrent from topography at a sharp corner (Bower et al, 1997), mixed baroclinic/barotropic instability of the undercurrent itself (Cherubin et al, 2000) and dipole formation as the undercurrent interacts with a deep canyon (Serra et al, 2005). With no undercurrent in the GOA, there seems to be no energy source to generate reddies.…”