Effects of coastal geometry and the formation of cyclonic/anti-cyclonic eddies on turbulent mixing in upwelling simulation

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Abstract
This paper presents a combination of theory and simulation on coastal upwelling with the aim of understanding the origin and nature of the structures found. Cyclones/anti-cyclones and thin filaments observed in satellite infrared images in upwelling regions are rather well reproduced. The instabilities are confirmed to be of mixed baroclinic–barotropic and modified Rayleigh–Taylor types. Nonlinear interactions limit the growth of the large scale structures and generate ‘fish-hook’ structures. The Rayleigh–Taylor and mixed instabilities and fish-hook structures cause sharp increases in mixing. Mixing and stirring are quantified using a mixedness parameter and energy budgets. Coastal perturbations modify the coherent structures which travel in the windward direction, changing their structure. The mechanisms of generation of these structures has been studied with simplified models but is not completely understood. We present animations derived from the simulations to investigate the process of formation of cyclonic/anti-cyclonic eddies. The simulations are based on solving the Navier–Stokes (N–S) equations in generalized curvilinear coordinates. The cape produces strong vortex stretching due to the acceleration of the flow around it. The continued vortex stretching results in vortex tearing in the cape vicinity which causes greater stirring than in the no-cape flow. These processes explain observed features of laboratory experiments and the observations on the west coast of the USA. The coastal perturbation inhibits the development of these structures and produces thin filaments extending offshore and downstream of the perturbation; these are an important feature of coastal upwelling. This article was chosen from selected Proceedings of the Second International Symposium on Turbulence and Shear Flow Phenomena (KTH-Stockholm, 27–29 June 2001) ed E Lindborg, A Johansson, J Eaton, J Humphrey, N Kasagi, M Leschziner and M Sommerfeld.