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 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.