Three‐Dimensional Numerical Simulations of Internal Tides in the Angolan Upwelling Region. Issue 2 (22nd February 2021)

• Record Type: Journal Article
• Authors: Zeng, Z.; Brandt, P.; Lamb, K. G.; Greatbatch, R. J.; Dengler, M.; Claus, M.; Chen, X.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 2(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-02-22
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016460
• Languages: English
• ISSNs: 2169-9275

Abstract

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 that this is because the weaker background stratification in austral winter than in austral summer acts as a preconditioning for IT mixing. Plain Language Summary: Tropical eastern boundary upwelling regions (e.g., on the Angolan shelf and Peruvian shelf) usually have high biological productivity. Unlike further poleward, tropical eastern boundary upwelling regions are often characterized by weak winds. Maximum biological productivity on the Angolan shelf is observed in austral winter during periods of weak winds. Therefore, other factors must contribute to the seasonality in the productivity. Mixing induced by internal tides is one of these possible factors. We have designed numerical simulations to explore the generation, propagation, and dissipation of internal tides on the Angolan continental slope and shelf. It is found that the internal tides on the Angolan shelf indeed promote the appearance of cold water at the surface near the coast. Furthermore, we explore the seasonal variations of the internal tides taking into account the seasonally varying stratification at the continental slope and shelf. The results show that seasonal variations in the tidal energy available for mixing on the shelf are weak, but mixing by the internal tides is, nonetheless, more effective when the stratification is weak during austral winter. Therefore, a stronger impact of internal tide mixing on sea surface temperature and biological productivity is suggested to occur in austral winter. Key Points: Mixing induced by internal tides contributes to the establishment of a cross‐shore sea surface temperature gradient on the Angolan shelf The impact of mixing on local environment is strongest in austral winter due to the stratification being weakest at that time of year