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. 1‐D 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 (∼32.5°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 subinertial 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 anticyclonic 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.

Mixing and Phytoplankton Growth in an Upwelling System

“…Substantial variability in the ratio of nitrate supply to primary production has been described (Messié et al, 2009), indicating that regulators other than nitrate supply must be at play. In agreement with previous studies (Hardman-Mountford et al, 2003;Rossi et al, 2009;Fearon et al, 2020), our results indicate that turbulent mixing may control phytoplankton growth at the different time-scales involved in the physical-biological coupling in these systems.…”

“…By using Finite Size Lyapunov Exponents and satellite data, Rossi et al (2009) described a global negative correlation between surface horizontal mixing and chlorophyll. Fearon et al (2020) used a 1D modeling approach to predict vertical mixing from wind speed in St Helena Bay (Bengala upwelling system), and emphasized the role of land-sea breeze in the development of phytoplankton blooms. As far as we know direct observations of microstructure turbulence have been never used to investigate the role of mixing in phytoplankton bloom formation in EBUS.…”

Local winds and encroaching currents drive summertime subsurface blooms over a narrow shelf

“…When terrestrial sources can be ignored, the input of “new” nutrients into the continental shelf euphotic zone can be controlled by advection, for example in wind‐forced upwelling regions (Lucas et al 2014), or by mixing and diffusion, for example at tidal fronts (Moore 2003) or areas with energetic internal waves (Sharples et al 2001; Lucas et al 2011b). At certain locations and times, advective and diffusive fluxes can be tightly coupled (Beaird et al 2020), and combine to result in high levels of primary productivity (Omand et al 2012; Lucas et al 2014; Pitcher et al 2014; Fearon et al 2020).…”

The influence of the land–sea breeze on coastal upwelling systems: locally forced vs internal wave vertical mixing and implications for thermal fronts

“…Simpson et al, 2002). In the case of land-sea breeze forcing near the critical latitude, the surface near-inertial currents can be largely attributed to the diurnal anticyclonic rotary component of the winds (τ ac ), as this is the component of the forcing which rotates at the same frequency and in the same direction as the forced surface inertial oscillations (Fearon et al, 2020). Simple linearly damped slab models have been widely used to model the surface mixed layer response to wind forcing, in some cases showing reasonable agreement with observations of near-inertial rotary currents (e.g.…”

“…The influence of the land-sea breeze on these systems is therefore typically assumed to be of low importance. High amplitude diurnal-inertial currents are however known to enhance shear-driven vertical mixing in coastal upwelling systems near the critical latitude, as revealed from observational evidence (Aguiar-González et al, 2011;Lucas et al, 2014) and numerical experiments (Fearon et al, 2020). Here, we build on the 1D-vertical model experiments of Fearon et al (2020) by introducing a 2D-vertical model which includes the cross-shore dimension, allowing us to more fully explore the role of land-sea breeze forcing in the context of coastal upwelling systems near the critical latitude.…”