The Impact of Submesoscale Flows on Mesoscale Agulhas Dynamics
Mesoscale eddies are central to the oceanic circulation and the global climate. Of particular importance, in this respect, are mesoscale eddies in the Agulhas region south of Africa, as they govern the inflow of warm and salty Indian Ocean waters into the Atlantic Ocean. In this dissertation, it is shown that these eddies are strengthened by submesoscale flows. This highlights the importance of including submesoscale effects for a realistic representation of the mesoscale dynamics in ocean models. First, a general circulation ocean model for the Agulhas region is improved with respect to the simulation of submesoscale dynamics by increasing the vertical and horizontal resolution and reducing the model diffusion and dissipation. Second, a model validation based on horizontal-wavenumber spectra computed from high-resolution satellite sea-surface temperature and sea-surface height measurements is performed. It demonstrates that the simulated submesoscale and mesoscale circulation in the Cape Basin are extraordinary well represented in this model. A comparison to two parallel model experiments, of which the first only resolves the largest and the second no submesoscale flows, reveals that the mesoscale spectral density of sea-surface height increases the more submesoscales are resolved. The results of an eddy detection algorithm show that this can be attributed to a strengthening of the mesoscale eddies. Third, a coarse-graining approach for the transfer of kinetic energy between spatial scales is applied to the model outputs. The results indicate that the mesoscale eddies are strengthened in spring or early summer by the absorption of submesoscale eddies resulting from baroclinic mixed-layer instability in winter. Fourth, this analysis complemented with the computation of the transfer of kinetic energy between temporal scales reveals that submesoscale eddies emerging from barotropic instabilities at the northern boundary of the Agulhas Current are important for the strength of shear-edge eddies and further of lee cyclones that propagate into the Cape Basin. The model comparison shows that the combination of both strengthening effects contributes to an increase of the surface mesoscale kinetic energy in the Cape Basin by 28 %, if submesoscale processes are resolved.
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