Three Dimensional Numerical Simulations of Internal Tides in the Angolan
Upwelling Region
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 upward 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 continental 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
coast and gradually decreases offshore thereby contributing to the
establishment of cross-shore temperature gradients. The available
seasonal coverage of hydrographic data is used to design simulations to
investigate the influence of seasonally varying stratification
characterized by low stratification in austral winter and high
stratification in austral summer. The results show that IT
characteristics, such as their wavelengths, sea surface convergence
patterns and baroclinic structure, have substantial seasonal variations
and additionally strong spatial inhomogeneities. However, seasonal
variations in the spatially-averaged generation, onshore flux and
dissipation of IT energy are weak. By evaluating the change of potential
energy, it is shown, nevertheless, that mixing due to ITs is more
effective during austral winter. We argue this is because the weaker
background stratification in austral winter than in austral summer acts
as a preconditioning for IT mixing.