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Enhanced vertical mixing in coastal upwelling systems driven by diurnal-inertial resonance: numerical experiments

Abstract : The land-sea breeze is resonant with the inertial response of the ocean at the critical latitude of 30à N/S. 1D-vertical numerical experiments were undertaken to study the key drivers of enhanced diapycnal mixing in coastal upwelling systems driven by diurnal-inertial resonance near the critical latitude. The effect of the land boundary was implicitly included in the model through the ‘Craig approximation’ for first order cross-shore surface elevation gradient response. The model indicates that for shallow water depths (<∼100 m), bottom shear stresses must be accounted for in the formulation of the ‘Craig approximation’, as they serve to enhance the cross-shore surface elevation gradient response, while reducing shear and mixing at the thermocline. The model was able to predict the observed temperature and current features during an upwelling/mixing event in 60 m water depth in St Helena Bay (∼32.5à S, southern Benguela), indicating that the locally forced response to the land-sea breeze is a key driver of diapycnal mixing over the event. Alignment of the sub-inertial Ekman transport with the surface inertial oscillation produces ‘shear spikes’ at the diurnal-inertial frequency, however their impact on mixing is secondary when compared with the diurnal-inertial resonance phenomenon. The amplitude of the diurnal anticlock- wise rotary component of the wind stress represents a good diagnostic for the prediction of diapycnal mixing due to diurnal-inertial resonance. The local enhancement of this quantity over St Helena Bay provides strong evidence for the importance of the land-sea breeze in contributing to primary production in this region through nutrient enrichment of the surface layer.
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Submitted on : Thursday, April 16, 2020 - 8:07:03 AM
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Giles Fearon, Steven Herbette, Jennifer Veitch, Gildas Cambon, Andrew Lucas, et al.. Enhanced vertical mixing in coastal upwelling systems driven by diurnal-inertial resonance: numerical experiments. Journal of Geophysical Research. Oceans, Wiley-Blackwell, 2020, 125 (9), pp.e2020JC016208. ⟨10.1029/2020JC016208⟩. ⟨hal-02544118⟩



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