Three Dimensional Numerical Simulations of Internal Tides at the
Continental Slope and Shelf off Angola
Abstract
In austral winter, biological productivity at the Angolan shelf reaches
its maximum. The alongshore winds, however, reach their seasonal minimum
suggesting that processes other than local wind-driven upwelling
contribute to near-coastal cooling and nutrient supply, one possibility
being mixing induced by internal tides (ITs). Here, we apply a
three-dimensional ocean model to simulate the generation, propagation
and dissipation of ITs at the Angolan slope and shelf. Model results are
validated against moored acoustic Doppler current profiler and other
observations. Simulated ITs are mainly generated in regions with a
critical/supercritical slope typically between the 200- and 500-m
isobaths. Mixing induced by ITs is found to be strongest close to the
shore and gradually decreases offshore thereby contributing to the
establishment of a cross-shore sea surface temperature gradient. The
available seasonal coverage of hydrographic data is used to design
sensitivity simulations. Seasonal variations in stratification results
in substantial temporal differences in IT characteristics, such as their
wavelengths, sea surface convergence patterns and baroclinic structure,
additionally showing strong spatial variations. However, seasonal
variations in the domain-integrated generation, onshore flux and
dissipation of IT energy are weak. By defining a parameter - the
relative change of the vertical density gradient - to evaluate the
relative mixing strength, it is shown, nevertheless, that mixing due to
ITs is more effective at weakening the stratification during austral
winter. We argue this is because less energy is required to mix the
water column in austral winter than in austral summer.