Influence of a deep flow on a surface boundary current

Abstract

International audienceThe stability properties of a coastal current composed of a deep flow and of an intense counterflowing surface jet are investigated with a linear quasi-geostrophic model and with a nonlinear isopycnic shallow-water model (MICOM), both in three-layer configurations. The currents are modeled by strips of uniform potential vorticity anomaly (PVA), with opposite signs. The linear stability analysis of the current is performed with exponentially-growing modes, varying the ratio of layer thicknesses, the potential vorticity distribution (width and intensity of the PVA strips) and the perturbation wavelength. The effect of a sloping bottom is also investigated. The various nonlinear regimes are described and interpreted in terms of growth of the modal perturbations, of critical layer distributions and of interactions of PVA poles. The linear growth rates of the perturbations are essential for enough PVA of the currents to reach the critical layers before wave breaking can occur. The positions of the critical layers determine where cut-off of the PVA contours occurs. This position is shown to depend only on the lower layer thickness (the upper layer one being kept constant). The intermediate layer thickness determines only the growth rates of the perturbation. Finally, the long-time nonlinear evolutions are governed by the interaction of detached PVA poles. An oceanographic application is performed using a 4-layer configuration, representative of the Mediterranean water undercurrents flowing under the South Portugal coastal upwelling jet. The analysis of this configuration shows that even for a stable upper layer, instability-driven eddies in the lower layers can disturb the surface jet and generate large meanders and filaments, similar to the observations from south of the Iberian peninsula

On the stability of outcropping eddies in a constant‐PV ocean

“…not a motionless ocean. The addition of mean flow to the surrounding ocean is supported by results from high‐resolution ocean models (Petersen et al , ) as well as by the modifications in the instability of ocean currents that override a lower layer with mean flow (Meunier et al , ). Dewar and Killworth () showed that a Gaussian eddy in a shallow‐water model is stable when the underlying ocean co‐rotates with the eddy itself with a fairly strong tangential velocity so that the vertical shear is significantly reduced (i.e.…”

Upwelling filaments off Cap Blanc: Interaction of the NW African upwelling current and the Cape Verde frontal zone eddy field?

“…The impact of eddies on the formation and growth of filaments from coastal fronts was also observed in satellite imagery by Shapiro et al [2010] in the Black sea and by Serra et al [2010] in the southern Iberian upwelling system. Moreover, most processes recognized to trigger the filamentation of a front are associated with vortical structures, whether they originate from topographic effects [ Meunier et al , 2010; Barton et al , 2004], instability processes [ Ikeda and Emery , 1984; Marchesiello et al , 2003; Capet and Carton , 2004; Meunier et al , 2012] or external sources [ Strub et al , 1991].…”

“…Many numerical studies have focused on the dynamics of upwelling filaments, highlighting the variety of physical processes possibly responsible for their formation and evolution. The formation of steep meanders through intrinsic baroclinic instability of the upwelling front was investigated by Ikeda and Apel [1981], Ikeda et al [1989], Haidvogel et al [1991], Batteen [1997], Capet et al [2002], Marchesiello et al [2003], Capet and Carton [2004], Batteen et al [2007], and Meunier et al [2012]. They all agreed in the meander‐like nature of the filaments, but some studies differed on the role of the coast‐line geometry and the bottom topography especially in the trapping of the filaments.…”

“… Serra et al [2010] used remotely sensed data and a realistic numerical model to demonstrate that these meddies could interact with the upwelling front and generate upwelling filaments further north along the western Iberian coast. The interaction of these deep structures and the surface current were investigated in a simple model framework by Meunier et al [2012], who showed that deep eddies could interact with the surface flow even when they are separated with a mid‐depth zero‐Potential Vorticity Anomaly layer.…”

Why compensated cold‐core rings look stable

“…Stability of flows in multilayer models has been considered in a number of studies (see summary for three‐layer rectilinear QG flows in Table 1 by Olascoaga []). The growth rate of baroclinic instability was noticed to be sensitive to an intermediate layer with uniform PV between upper and deep flows [e.g., Sutyrin , ; Meunier et al ., ]. Instability of a circular QG vortex has been analyzed by Sokolovskiy [] where the problem generally depends on eight nondimensional parameters.…”