Project Summary and Objectives

Summary

The main goal of the proposal is to provide a deeper understanding of physical mechanisms that control the hydrodynamic regimes associated with submarine canyons and to evaluate their impact on mass transfer between the continental shelf and open ocean. This generic goal comprises three main objectives:

• To develop a set of mathematical and laboratory models simulating the main driving mechanisms and quantifying the characteristics of mass transport through submarine canyons. To estimate the influence of these canyons on: (i) the mesoscale structure of barotropic and stratified currents, including gravity currents and (ii) the transport over the canyons. To assess the models ability to reproduce the evolution, main features and final stages of water flows in vicinity of canyons.
• To perform detailed theoretical and experimental analysis of density flows on a sloping bottom, in an ambient non-rotating fluid. To explain the mechanism of gravity current splitting due to interaction with the pycnocline and to find an effective penetration depth of density and sediment flow in stratified fluid for different canyon geometry.
• To inter-compare the theoretical and laboratory approaches and to check the results against observational data sets by matching mathematical studies and laboratory experiments with similar basic oceanographic parameters of some specific areas. In particular, to apply the results for a better understanding the observations related with the hydrographic structure and sediment transport in two submarine canyons in the Balearic Sea (North Western Mediterranean): the Foix and Palamos canyons, where data from long-term experimental studies of currents and sediment transport are currently available.

Background

The investigation of shelf/deep sea exchanges is one of the most actual and challenging problems in modern oceanography. The complex morphology of shelf/slope regions, the responses to forcing, both local and remote, and the internal dynamical instabilities generate a wide range of processes over a broad range of scales [Brink and Robinson, 1998]. The hydrodynamic regime over shelf/slope areas has a large impact on the evolution and structure of the coastal marine ecosystems and plays an important role in the globalbio-geochemical cycling. Usually the coastal waters are trapped within the shelf zone by strong currents located above the continental slope and following the contours of constant depth. Such currents considerably reduce the shelf/deep seas exchanges [Fedorov, 1986]. Thus, a key element of coastal shelf/slope processes is the influence of the bottom topography. Topography affects the ocean circulation in many ways. The topography guides and blocks the water circulation, supports and traps some modes and it exerts a stress on mean flows causing mixing. In particular, submarine canyons commonly present in many coastal areas intersect the paths of shelf currents providing a way for cross shelf/slope motions. Experimental studies have evidenced how submarine canyons are the preferential pathways for water and particulate matter exchanges between the coastal and deep sea areas [Durrieu de Madron, 1994; Puig and Palanques, 1998]. The off/onshore sea flows associated with canyons enhance horizontal, vertical water displacements and mixing with significant influence on the shelf/deep sea ecosystem interactions [Hickey, 1995, Gili et al. 1999, Della Tomasa et al., 2000]. On the other hand, the detachment of nepheloid layers and the downward transport of organic matter from the shelf toward deeper areas through the canyons are plausible contributions to the deep sea production [Puig et al., 1998].

The influence of submarine canyons was the subject of some international research programs during the 70's and 80's. Most of the interest was focused on studies of the influence of tides, internal waves and other high frequency motions on sediment distribution and re-suspension in canyons [Drake et al., 1978; Shepard et al., 1979; Walsh et al. 1988]. However, due to the inherent difficulties to obtain observational data in submarine canyons (small dimensions, steep bottom topographies, highly non-linear and variable flow patterns, etc.) detailed descriptions of the spatial structure and evolution of physical, geochemical and biological properties are relatively scarce [Durrieu de Madron, 1994; Hickey, 1997]. Recent numerical and laboratory models have been very helpful for better describing the different processes associated with topographic effects [Boyer and Davies, 2000] and circulation within submarine canyons [Klinck, 1996, Chapman and Gawarkiewicz, 1995, Ardhuin et al, 1999]. However, a comparative analysis of seemingly identical experiments with many different numerical codes for the circulation associated to canyons, have shown quantitative and qualitative differences further increased when stratification is considered [Haidvogel and Beckmann, 1998]. Recent laboratory experiment [Didkovskii et al., 2000] has shown that rotation, stratification, bottom friction and canyon geometry are the key elements to control the flow in submarine canyons. In addition, small scale processes related to the generation of deep nepheloid layers together with the dynamics of bottom gravity currents, which are one of the widespread forms of flow in the submarine canyons responsible of much of sediment transport, are still not well known. Much of the theoretical work on gravity currents not related directly to the submarine canyons has been based on the stream tube approach [Smith, 1975], which does not account for the internal dynamics of the dense flow. An attempt to overcome these limitations has been made in recent years by use of primitive equation models (e.g. Jungclaus and Backhaus, 1994) although these models do not resolve adequately processes in the bottom boundary layer. Some of these problems were solved in a theoretical approach by Shapiro and Hill, (1997) and Hill et al., (1998). Bottom gravity currents upon the horizontal and inclined bottom with and without sediments have been studied in laboratory conditions by many authors (e.g. Simpson, 1982; Garcia, 1993; Sheremet et al. 1998). However, the interaction of the bottom gravity current with a pycnocline, intersecting the bottom, a phenomenon which is widespread in the ocean, is still poorly studied [Gritsenko, Yurova, 1999]. A proper understanding of the dynamic of such flows is crucial for a realistic modelling of mass transfer through submarine canyons.

Significant improvements in understanding the physics shall be made before the model results can match the complex dynamics present around and within real submarine canyons. Thus the challenge of the proposed research is to go deeper in the physics and to enhance predictive capabilities of numerical models of such dynamical processes. The major innovation lies in the joint and complementary approach that consists of interrelated laboratory, theoretical and field data analysis. This approach will provide a more complete view of flow configuration around and within submarine canyons. Due to the local nature of the processes, they can not be extrapolated to all worldwide canyons and therefore, they should be applied to specific canyon morphology in order to compare theoretical results with field data analysis. Emphasis will be put in two canyons in the north-east Spanish shelf (Northwestern Mediterranean basin): the Foix and the Palamos canyons. These are very well documented canyons with abundant data on physical and bio-geochemical measurements. Their different locations, morphology and hydrodynamic conditions will be analysed during the proposed research and studied on the basis of mathematical and laboratory modelling.

References

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