The relation of wind-driven coastal and offshore upwelling in the Benguela Upwelling System

Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface wind with different, and sometimes contrasting, impacts on coastal upwelling systems driven by alongshore wind and offshore upwelling systems driven by the local wind-stress-curl. Here, concurrently measured wind-fields, satellite-derived Chlorophyll-a concentration along with a state-of-the-art ocean model simulation spanning 2008-2018 are used to investigate the connection between coastal and offshore physical drivers of the Benguela Upwelling System (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model are fairly consistent across the entire BUS and closely followed by the Chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of sub-mesoscale activity (i.e., filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonally dependent off Lüderitz and across the southern BUS. Thus, our findings suggest this relationship is far more complex than currently thought and seems to be sensitive to climate changes with short- and far-reaching consequences for this vulnerable marine ecosystem.

On the connection between coastal Ekman upwelling and wind-stress-curl-driven upwelling off the southwest African coasts

<p>Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface atmospheric flow with different, and sometimes contrasting, impacts on coastal and open-ocean upwelling systems. Here, concurrently measured wind-fields, satellite-derived Chlorophyll-a concentration along with a state-of-the-art ocean model simulation spanning 2008-2018 are used to investigate the connection between coastal and offshore physical drivers of the Benguela Upwelling System (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model are fairly consistent across the entire BUS and closely followed by the Chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of sub-mesoscale activity (i.e. filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonal dependent off Lüderitz and across the southern BUS. Thus, our findings suggest this relationship is far more complex than currently thought and seems to be sensitive to climate changes with short- and far-reaching consequences for this vulnerable marine-ecosystem.</p>

Ekman Transport in Balanced Currents with Curvature

Ekman transport, the horizontal mass transport associated with a wind stress applied on the ocean surface, is modified by the vorticity of ocean currents, leading to what has been termed the nonlinear Ekman transport. This article extends earlier work on this topic by deriving solutions for the nonlinear Ekman transport valid in currents with curvature, such as a meandering jet or circular vortex, and for flows with the Rossby number approaching unity. Tilting of the horizontal vorticity of the Ekman flow by the balanced currents modifies the ocean response to surface forcing, such that, to leading order, winds parallel to the flow drive an Ekman transport that depends only on the shear vorticity component of the vertical relative vorticity, whereas across-flow winds drive transport dependent on the curvature vorticity. Curvature in the balanced flow field thus leads to an Ekman transport that differs from previous formulations derived under the assumption of straight flows. Notably, the theory also predicts a component of the transport aligned with the surface wind stress, contrary to classic Ekman theory. In the case of the circular vortex, the solutions given here can be used to calculate the vertical velocity to a higher order of accuracy than previous solutions, extending possible applications of the theory to strong balanced flows. The existence of oscillations, and the potential for resonance and instability, in the Ekman flow at a curved jet are also demonstrated.

Dynamics of the ITCZ Boundary Layer

This paper presents high-resolution numerical solutions of a nonlinear zonally symmetric slab model of the intertropical convergence zone (ITCZ) boundary layer. The boundary layer zonal and meridional flows are forced by a specified pressure field, which can also be interpreted as a specified geostrophically balanced zonal wind field ug(y). One narrow on-equatorial peak in boundary layer pumping is produced when the forcing is easterly geostrophic flow along the equator and two narrow peaks in boundary layer pumping are produced on opposite sides of the equator (a double ITCZ) when the forcing is westerly geostrophic flow along the equator. In the case when easterlies are surrounding a westerly wind burst, once again a double ITCZ is produced, but the ITCZs have significantly more intense boundary layer pumping than the case of only westerly geostrophic flow. A comparison of the numerical solutions to those of classical Ekman theory suggests that the meridional advection term υ(∂υ/∂y) plays a vital role in strengthening and narrowing boundary layer pumping regions while weakening and broadening boundary layer suction regions.

Wave-induced mixing and transport of buoyant particles: application to the Statfjord A oil spill

This study focuses on how wave–current and wave–turbulence interactions modify the transport of buoyant particles in the ocean. Here the particles can represent oil droplets, plastic particles, or plankton such as fish eggs and larvae. Using the General Ocean Turbulence Model (GOTM), modified to take surface wave effects into account, we investigate how the increased mixing by wave breaking and Stokes shear production, as well as the stronger veering by the Coriolis–Stokes force, affects the drift of the particles. The energy and momentum fluxes, as well as the Stokes drift, depend on the directional wave spectrum obtained from a wave model. As a first test, the depth and velocity scales from the model are compared with analytical solutions based on a constant eddy viscosity (i.e., classical Ekman theory). Secondly, the model is applied to a case in which we investigate the oil drift after an oil spill off the west coast of Norway in 2007. During this accident the average net drift of oil was observed to be both slower and more deflected away from the wind direction than predicted by oil-drift models. In this case, using wind and wave forcing from the ERA Interim archive it is shown that the wave effects are important for the resultant drift and have the potential to improve drift forecasting.

Wave induced mixing and transport of buoyant particles: application to the Statfjord A oil spill

The modelling of wave-current and wave-turbulence interactions have received much attention in recent years. In this study the focus is on how these wave effects modify the transport of particles in the ocean. Here the particles are buoyant tracers that can represent oil droplets, plastic particles or plankton, for example fish eggs and larvae. Using the General Ocean Turbulence Model (GOTM), modified to take surface wave effects into account, we investigate how the increased mixing by wave breaking and Stokes shear production as well as the stronger veering by the Coriolis–Stokes force affect the drift of the particles. The energy and momentum fluxes as well as the Stokes drift depend on the directional wave spectrum that can be obtained from a wave model or from observations. As a first test the depth and velocity scales from the model are compared with analytical solutions based on a constant eddy viscosity (e.g. classical Ekman theory). Secondly the model is applied to a case where we investigate the oil drift after an offshore oil spill outside the western coast of Norway in 2007. During this accident the average net drift of oil was observed to be both slower and more deflected away from the wind direction than predicted by empirical models. With wind and wave forcing from the ERA Interim archive, it is shown that the wave effects are important for the resultant drift in this case, and has the potential to improve drift forecasting.

Research on Emotion Body Language Model of the Humanoid Robot

In order to improve the personification of human-robot emotion interaction, this paper based on the Euclidean space emotion model and Ekman theory of emotion, deduces the general formula of emotion characteristic value, establishes a improvement model of robot which can express the robots emotion through the body language. The general formula of emotional feature value make the models suitable for the expression of various emotions, and through body language to express the emotions, improves the emotion expression model generality and adaptability.

On the Patterns of Wind-Power Input to the Ocean Circulation

Pathways of wind-power input into the ocean general circulation are analyzed using Ekman theory. Direct rates of wind work can be calculated through the wind stress acting on the surface geostrophic flow. However, because that energy is transported laterally in the Ekman layer, the injection into the geostrophic interior is actually controlled by Ekman pumping, with a pattern determined by the wind curl rather than the wind itself. Regions of power injection into the geostrophic interior are thus generally shifted poleward compared to regions of direct wind-power input, most notably in the Southern Ocean, where on average energy enters the interior 10° south of the Antarctic Circumpolar Current core. An interpretation of the wind-power input to the interior is proposed, expressed as a downward flux of pressure work. This energy flux is a measure of the work done by the Ekman pumping against the surface elevation pressure, helping to maintain the observed anomaly of sea surface height relative to the global-mean sea level.

Observations of Ekman Currents in the Southern Ocean

Largely zonal winds in the Southern Ocean drive an equatorward Ekman transport that constitutes the shallowest limb of the meridional overturning circulation of the Antarctic Circumpolar Current (ACC). Despite its importance, there have been no direct observations of the open ocean Ekman balance in the Southern Ocean until now. Using high-resolution repeat observations of upper-ocean velocity in Drake Passage, a mean Ekman spiral is resolved and Ekman transport is computed. The mean Ekman currents decay in amplitude and rotate anticyclonically with depth, penetrating to ∼100-m depth, above the base of the annual mean mixed layer at 120 m. The rotation depth scale exceeds the e-folding scale of the speed by about a factor of 3, resulting in a current spiral that is compressed relative to predictions from Ekman theory. Transport estimated from the observed currents is mostly equatorward and in good agreement with the Ekman transport computed from four different gridded wind products. The mean temperature of the Ekman layer is not distinguishable from temperature at the surface. Turbulent eddy viscosities inferred from Ekman theory and a direct estimate of the time-averaged stress were O(102–103) cm2 s−1. The latter calculation results in a profile of eddy viscosity that decreases in magnitude with depth and a time-averaged stress that is not parallel to the time-averaged vertical shear. The compression of the Ekman spiral and the nonparallel shear–stress relation are likely due to time averaging over the cycling of the stratification in response to diurnal buoyancy fluxes, although the action of surface waves and the oceanic response to high-frequency wind variability may also contribute.