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Enhanced vertical mixing in coastal upwelling systems driven by diurnal-inertial resonance: numerical experiments
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  • Giles Geoffery Fearon,
  • Steven Herbette,
  • Jennifer Jackson-Veitch,
  • gildas cambon,
  • Andrew Lucas,
  • Florian Lemarie,
  • Marcello Vichi
Giles Geoffery Fearon
University of Cape Town

Corresponding Author:gfearon11@gmail.com

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Steven Herbette
Université de Bretagne Occidentale
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Jennifer Jackson-Veitch
South African Environmental Observation Network
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gildas cambon
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Andrew Lucas
Scripps Institution of Oceanography
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Florian Lemarie
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Marcello Vichi
University of Cape Town
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The land-sea breeze is resonant with the inertial response of the ocean at the critical latitude of 30°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 (<~100m), 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 60m water depth in St Helena Bay (~32.5°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 anticlockwise 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.
Sep 2020Published in Journal of Geophysical Research: Oceans volume 125 issue 9. 10.1029/2020JC016208