Modelling of Meddy-topography interactions

Description of the research project

Field observations have confirmed the existence of Meddies, not only near the southern Iberian coast but also in the Canaries basin. A Meddy (=Mediterranean eddy) is an isolated drifting eddy of horizontal scale 50 - 100 km and a vertical thickness of 800 m, containing water mass properties typical of Mediterranean outflow water. These subsurface vortices have an anticyclonic rotation (=opposite to the rotation of the earth) and they drift at typically 1 - 2 km per day. As they do so, the vortices encounter rather complex bottom topography, which affects their dynamics. In some cases (for example, along the western coast of Portugal) the topographic control is very evident; the trajectories of acoustically-tracked floats trapped within the Meddies reveal clearly the steering effect of submarine canyons, the shelf edge itself and significant ridges such as the Gorringe Bank. Particularly dramatic topographic controls are exerted by Cape St. Vincent, where some Meddies are deflected to drift northwards along the Portuguese coast and others being directed south-west into the Canaries basin. Within the Canaries basin the disturbance to the Meddy motion is provided by isolated and multiple seamounts, as well as island groups of the Canaries archipelago. In all cases, the details of the interactions of the Meddy structures with the topographic features are not well-understood, though the consequences of the interaction are very important for the transport of salt from the Mediterranean to the Atlantic.

The objective of the proposed project is to consider rather idealised bottom topography cases (shelf edge, bank, isolated seamounts) and to apply an existing numerical model developed by the applicant to study the topographic influence on two-dimensional monopolar vortices typical of Meddy structures. An integral part of the study is the laboratory simulation of the eddy-topography interactions and the comparisons between the predictions of the model and the laboratory data. In addition, since the work is related to previous and MAST (=Marine Science and Technology) III-planned field investigations of Meddy trajectories in the Canaries-Azores basin, comparisons are possible between model results with oceanic data. Since Meddies are meso-scale features, processes affecting their dynamics are often sub-grid scale for large ocean models of the region; the present study will therefore complement any modelling studies planned within the Canaries-Azores targeted project. The possibility to directly combine both numerical and laboratory modelling aspects of the problem is an innovative feature of the study, as well as the opportunity to extend a so-far numerical modelling background to include practical aspects of laboratory modelling.

The research work will consist of two main components, namely (i) the application of an existing numerical code (developed by the applicant during his post-doc. research in Eindhoven) to cases in which smooth bottom topography is present, and (ii) the completion of a series of laboratory simulation experiments of eddy-topography interactions using the rotating stratified tow-tank facility in Professor Davies' laboratory in Dundee. The latter is a tank filled with a stratified ('layered') fluid which can rotate and which is fitted with a belt arrangement that permits topographic features to be translated uniformly and horizontally at prescribed velocities along the base of the tank. (...)

The project is most clearly relevant to the Canaries-Azores Regional Seas targeted project of MAST III, though the existence of vortices resembling Meddies has also been claimed for other regional seas such as the Baltic and the Red Sea and in both of these regions, topographical effects are thought to be significant. The results of the proposed study will therefore also have more general applicability to other oceanic areas.

The participation of the applicant in the project will expose him to the activities of a very active group in laboratory modelling of oceanic flows, in another part of Europe. In this way the participation will bring benefits to both parties by the introduction and sharing of new ideas. For this particular project, there will be a very valuable technology transfer in both directions; the Dundee group is particularly strong in laboratory fluid dynamics and the applicant has complementary skills in numerical modelling. The joint involvement will aid harmonisation of scientific research in an important area of ocean modelling and enables a problem of direct interest to a targeted MAST project to be explored. The postdoctoral training afforded by the stay in the Dundee laboratory will enhance significantly the scientific credentials of the applicant and will equip him with additional skills for his future career development.

This is a slightly adapted version of the text of the proposal.
Here are some details on the numerical modelling and laboratory experiments.

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