Three‐Dimensional Numerical Simulations of Internal Tides in the Angolan Upwelling Region

Z. Zeng,P. Brandt,K. G. Lamb,R. J. Greatbatch,M. Claus,M. Dengler,X. Chen

doi:10.1029/2020jc016460

Abstract

AbstractIn 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 that this is because the weaker background stratification in austral winter than in austral summer acts as a preconditioning for IT mixing.

Highlights

Tropical eastern boundary upwelling systems are characterized by rich marine ecosystems (Carr & Kearns, 2003)
For location S1, the model results are generally consistent with the moored data, despite some longer-period variations superposed on the tidal currents in the observations
At station S2, there is a difference in the mean alongshore velocity of around 3.5 cm s–1 between model and observations, which likely corresponds to the presence of an alongshore current in the observations associated with the weak poleward Angola Current or intraseasonal and/ or seasonal variability (Kopte et al, 2017) that are not related to tidal dynamics

Summary

Introduction

Tropical eastern boundary upwelling systems are characterized by rich marine ecosystems (Carr & Kearns, 2003). They undergo strong intraseasonal to interannual variability dominantly associated with equatorial forcing and are often subject to intense hypoxia (e.g., Bachelery et al, 2016; Echevin et al, 2008; Mohrholz et al, 2008). The Angolan shelf hosts such a tropical eastern boundary upwelling system known for its high biological productivity and fisheries The climatological seasonal cycle of sea surface temperature (SST) in the tropical Angolan upwelling system (∼11°S) is characterized by lowest temperature during austral winter (June–September) and highest temperature during austral summer (February–April) (Figure 1(a)). The stratification in the Angolan upwelling region is strongly affected by net surface heat, freshwater fluxes, and the coastally trapped waves (CTWs) that are forced

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