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

This paper presents a combination of theory and simulation on coastal upwel ling with the aim of understanding the origin and nature of the structures found. Cyclones/anti-cyclones and thin filaments observed in satellite infr ared images in upwelling regions are rather well reproduced. The instabilit ies are confirmed to be of mixed baroclinic-barotropic and modified Rayleig h Taylor types. Nonlinear interactions limit the growth of the large scale structures and generate 'fish-hook' structures. The Rayleigh-Taylor and mix ed instabilities and fish-hook structures cause sharp increases in mixing. Mixing and stirring are quantified using a mixedness parameter and energy b udgets. Coastal perturbations modify the coherent structures which travel in the wi ndward direction, changing their structure. The mechanisms of generation of these structures has been studied with simplified models but is not comple tely understood. We present animations derived from the simulations to inve stigate the process of formation of cyclonic/anti-cyclonic eddies. The simu lations are based on solving the Navier-Stokes (N-S) equations in generaliz ed 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 stirrin g than in the no-cape flow. These processes explain observed features of la boratory experiments and the observations on the west coast of the USA. The coastal perturbation inhibits the development of these structures and prod uces thin filaments extending offshore and downstream of the perturbation; these are an important feature of coastal upwelling.