Wünsche, Anna (2018) Tidally induced internal waves and mixing on the Angolan shelf. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 51 pp.

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Abstract

The variability of the tropical Angolan system is largely determined by dynamics generated in the equatorial Atlantic. Poleward propagating upwelling and downwelling coastally trapped (Kelvin) waves (CTW) modulate the seasonal cycle in the biological productive ecosystem. Shipboard observations and short term mooring observations recorded during upwelling and downwelling CTW periods are evaluated. Aim of this study is to identify internal waves as main source of energy for mixing on the Angolan shelf during upwelling CTW periods. Therefore, I investigate temporal variability of temperature, salinity and of the flow as well as dissipation rates of turbulent kinetic energy (TKE) and stratification between 12.0◦ E and 13.5◦ E at 11◦ S below the mixed layer. Furthermore, I use spectral analysis to assess temporal variability of internal waves at the continental slope. Analyses of dissipation rates of TKE from microstructure measurements reveal particular importance of the continental slope to mixing. At the continetal slope, dissipation rates of TKE reach magnitudes of up to O(10−5)m2/s−3 being of 1000 times higher compared to dissipation rates of TKE further offshore. Furthermore, enhanced turbulent dissipation coincides with reduced buoyancy frequency at the slope, which in turn facilitates mixing. Moreover, mixing is more pronounced during the upwelling period in July 2013 than during October 2015. Spectral analysis of time series of the vertical velocity component reveals internal waves of frequencies of ∼2 cph observed in both upand downwelling CTW periods. However, internal waves during upwelling phase in June 2018 are more energetic than during downwelling CTW phase in October 2015. Finally, enhanced mixing on the Angolan shelf can be associated with energetic internal waves. Energy provided by internal waves yield turbulent dissipation and mixing. Contrary to expectations, observations in October 2016 reveal high surface salinity and enhanced mixing during a downwelling CTW phase. In consideration of the results presented in this study and additional data regarding surface fluxes, I find that either a shift in the CTW periods, surface salinity increase due to freshwater fluxes or both may have modulated observations in October 2016.

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