Cascading is a specific type of buoyancy‐driven current in which dense water formed by cooling, evaporation, or freezing in the surface layer over the continental shelf descends down the continental slope to a greater depth. We have identified, by world‐wide trawling and analysis of raw databases, several distinct mechanisms of preconditioning for cascades. We have validated, where possible, existing theories and developed simple models, which allow estimation of the parameters for preconditioning, initiating, and down‐slope fluxes as well as evolution of the temperature contrasts during cascading events. Compliance or noncompliance of observations with these simple and easy‐to‐use models has been related to the stage of cascade development and to local factors. Estimates of observed down‐slope transport rates in case studies accord with theory, which is thereby substantially validated. Typical values of cascading transport rates were in the range 0.5–1.6 m²s⁻¹. We hazard a very approximate global estimate of order 10⁵ km³yr⁻¹ or average 3 Sv based on our findings and previous studies.

Plumes of dense shelf water cascade down continental slopes in many parts of the world’s oceans and provide a mechanism for shelf–ocean exchange. In this paper a nonlinear process-orientated theory is developed and used to examine the dynamics of cascading. The theory is formulated in terms of a “1½-layer” model and incorporates bottom topography, earth rotation, internal and bottom friction, and entrainment as well as externally imposed pressure gradients. The theory occupies a niche between the stream tube class of model (which considers only bulk properties of a plume) and the full three-dimensional primitive equation approach. The model provides useful insights into the complex interplay between the controlling forces, and it allows one to recover the shape and trajectory of dense plumes as well as the three-dimensional flow field inside the bottom layer. Asymptotic limits are investigated and lead to several basic results. A typical thickness of a fully developed plume is found to be twice t... Abstract Plumes of dense shelf water cascade down continental slopes in many parts of the world’s oceans and provide a mechanism for shelf–ocean exchange. In this paper a nonlinear process-orientated theory is developed and used to examine the dynamics of cascading. The theory is formulated in terms of a “1½-layer” model and incorporates bottom topography, earth rotation, internal and bottom friction, and entrainment as well as externally imposed pressure gradients. The theory occupies a niche between the stream tube class of model (which considers only bulk properties of a plume) and the full three-dimensional primitive equation approach. The model provides useful insights into the complex interplay between the controlling forces, and it allows one to recover the shape and trajectory of dense plumes as well as the three-dimensional flow field inside the bottom layer. Asymptotic limits are investigated and lead to several basic results. A typical thickness of a fully developed plume is found to be twice t...

This paper quantifies the backward effect on the ocean currents caused by a tidal stream farm located in the open shallow sea. Recent studies in channels with 1-D models have indicated that the power potential is not given purely by the flux of kinetic energy, as has been commonly assumed. In this study, a 3-D ocean circulation model is used to estimate (i) practically extractable energy resource at different levels of rated generation capacity of the farm, (ii) changes in the strength of currents due to energy extraction, and (iii) alterations in the pattern of residual currents and the pathways of passive tracers. As well as tidal streams, the model also takes into account the wind-driven and density-driven ocean currents. Numerical modelling has been carried out for a hypothetical tidal farm located in the Celtic Sea north of Cornwall, an area known for its high level of tidal energy. Modelling results clearly indicate that the extracted power does not grow linearly with the increase in the rated capacity of the farm. For the case study covered in this paper, a 100-fold increase in the rated generation capacity of the farm results in only 7-fold increase in extracted power. In the case of a high power farm, kinetic energy of currents is altered significantly as far as 10–20 km away from the farm. At high levels of extracted energy the currents tend to avoid flowing through the farm, an effect which is not captured with 1-D models. Residual currents are altered as far as a hundred kilometres away. The magnitude of changes in the dispersion of tracers is highly sensitive to the location. Some of the passive drifters analysed in this study experience significant variations in the end-to-start distance due to energy extraction ranging from 13% to 238% while others are practically unaffected. This study shows that both energy extraction estimates and effects on region wide circulation depend on a complex combination of factors, and the specific figures given in the paper should be generally considered as first estimates.