Observations of vertical turbulent nitrate flux during summer in the Great Australian Bight

Doubell, Mark; Spencer, David M.; Ruth, Paul D. van; Lemckert, Charles James; Middleton, John

doi:10.1016/j.dsr2.2018.08.007

Cited by 15 publications

(17 citation statements)

References 39 publications

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“…This implies a potential enhancement in primary productivity within an oligotrophic region. The magnitude of the nitrate flux observed in the present study is the same as that noted at the Great Australia Bight (Doubell et al, ) and in the southern side of Kuroshio front where low nitrate gradient was also observed (Kaneko et al, ). This mechanism can also be enhanced by the passage of an atmospheric system (Rumyantseva et al, ; Thomas et al, ), as well as by the net seasonal heat flux variability, not discussed in the present study.…”

Section: Discussion and Concluding Remarkssupporting

confidence: 90%

“…In theory, both shear probes should measure the same microstructures with the same magnitude, though in reality, they present differences due to the sensitivity of each sensor. The contrasts in measurements of dissipation rates of TKE from the respective shear probes can be reduced by using the mean field of TKE dissipation rate estimates (e.g., Doubell et al, ). The vertical profiles of

ε

of each shear probe show a similar vertical distribution of the turbulent motion peaks despite the difference between their magnitudes (Figures a–d).…”

Section: Turbulence Observationsmentioning

confidence: 99%

“…The magnitude of the diffusivity in the interior ocean affects a wide range of processes, from the dispersion of nonconservative properties such as nutrients to the vertical transport of salt and heat, affecting ultimately the stratification of the water column. To verify whether or not the subsurface turbulence in the BC‐IWBC domain may drive mixing processes at the interface of these two jets, the Osborn model (Osborn, ) is used to compute the vertical diffusivity (e.g., Doubell et al, ; Itoh et al, ; Kaneko et al, ):

κ_{ρ} = normalΓ \frac{ε}{N^{2}},

where

normalΓ = 0.2

is the mixing efficiency parameter (Gregg et al, ; Imberger & Ivey, ). Turbulent diffusivity was also expressed in terms of the turbulence activity parameter (

ε false/ ν N^{2}

), which exhibits a linear relationship at the stationary and intermediate turbulence regime (Shih et al, ).…”

Section: Turbulence Observationsmentioning

confidence: 99%

“…According to the turbulence activity parameter, the intermediate regime of turbulence dominates between the base of the SML and the 1,026.2 isopycnals (not shown). At those depths, vigorous patches of

κ_{ρ}

play an important role in the turbulent flux of conservative and nonconservative properties, which can be estimated from

F_{c} = - κ_{ρ} \frac{\partial C}{\partial z},

where

\partial C false/ \partial z

is the vertical gradient of a scalar property C (e.g., Doubell et al, ; Kaneko et al, ; Rumyantseva et al, ).…”

Section: Turbulence Observationsmentioning

confidence: 99%

“…To verify whether or not the subsurface turbulence in the BC-IWBC domain may drive mixing processes at the interface of these two jets, the Osborn model (Osborn, 1980) is used to compute the vertical diffusivity (e.g., Doubell et al, 2018;Itoh et al, 2010;Kaneko et al, 2013):…”

Section: Mixing and Turbulent Vertical Fluxesmentioning

confidence: 99%

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On the Role of Turbulent Mixing Produced by Vertical Shear Between the Brazil Current and the Intermediate Western Boundary Current

et al. 2020

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An intensification of the vertical shear is observed below the surface mixed layer at 21 • S due to the mutually opposing flows of the Brazil Current and the Intermediate Western Boundary Current. The propensity to develop turbulence and mixing due to vertical shear over intense stabilizing density gradients is an important characteristic of such environments. For the first time, microscale measurements were made in the Brazil Current-Intermediate Western Boundary Current system, providing direct quantitative values of the turbulent fluctuations. Peaks of relative strong dissipation rates of turbulent kinetic energy (O(10 −8 ) W/kg) were observed close to the base of the surface mixed layer. On the other hand, prominent peaks of turbulent kinetic energy dissipation rates of up to 2 orders of magnitude higher than the background were observed at deeper levels, where stratification begins to lose intensity. Analyzing such peaks, caused by intense vertical shear or weak stratification-and sometimes both-, allows a characterization of the local mixing processes and the role played by vertical exchanges of biogeochemical properties. Based on the estimated nitrate gradient and the vertical diffusivity, we show that turbulent mixing driven by vertical shear plays an important role in the supply of nitrate to the upper layer.Plain Language Summary Turbulent mixing across the density surfaces can bring nutrient-rich waters from the subsurface to the upper sunlit layer of the ocean, therefore, modulating the primary productivity in an oligotrophic ocean. Based on measurements of small-scale shear variance, we found that the interaction between the poleward-flowing Brazil Current and the Intermediate Western Boundary Current flowing underneath in the opposite direction enhances the upper-ocean mixing through shear instabilities. The destabilizing influence of the velocity shear overcomes the stabilizing effect of the stratification. The mixing on the interface between these two western boundary currents may provide an important route for local nutrient exchanges.

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Section: Discussion and Concluding Remarkssupporting

confidence: 90%

ε

of each shear probe show a similar vertical distribution of the turbulent motion peaks despite the difference between their magnitudes (Figures a–d).…”

Section: Turbulence Observationsmentioning

confidence: 99%

κ_{ρ} = normalΓ \frac{ε}{N^{2}},

where

normalΓ = 0.2

is the mixing efficiency parameter (Gregg et al, ; Imberger & Ivey, ). Turbulent diffusivity was also expressed in terms of the turbulence activity parameter (

ε false/ ν N^{2}

), which exhibits a linear relationship at the stationary and intermediate turbulence regime (Shih et al, ).…”

Section: Turbulence Observationsmentioning

confidence: 99%

κ_{ρ}

play an important role in the turbulent flux of conservative and nonconservative properties, which can be estimated from

F_{c} = - κ_{ρ} \frac{\partial C}{\partial z},

where

\partial C false/ \partial z

is the vertical gradient of a scalar property C (e.g., Doubell et al, ; Kaneko et al, ; Rumyantseva et al, ).…”

Section: Turbulence Observationsmentioning

confidence: 99%

Section: Mixing and Turbulent Vertical Fluxesmentioning

confidence: 99%

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On the Role of Turbulent Mixing Produced by Vertical Shear Between the Brazil Current and the Intermediate Western Boundary Current

et al. 2020

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

Schlosser

Lucas

Jones

et al. 2022

Limnology & Oceanography

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The ability to forecast the biological productivity of the coastal ocean relies on the quantification of the physical processes that deliver nutrients to the euphotic zone. Here we explore these pathways using observations of the coupled biological and physical variability of waters offshore of the east coast of Tasmania in the summertime. The observations include an array of moored autonomous profilers deployed over an 18-d periodproviding continuous, full-depth measurements of turbulent microstructure, temperature, velocity, and chlorophyll a (Chl a) fluorescence, complemented by shipboard nutrient measurements. Local upwelling was driven by the encroaching East Australian Current (EAC) extension onto the shelf and to a lesser extent the local winds. The interaction of the local winds and the encroaching boundary current was reflected in the shelf nutrient budget and led to a rapid increase in subsurface Chl a. Diffusive vertical fluxes had minimal impact on subsurface Chl a in the mid-shelf and outer-shelf. Upwelling-favorable winds were too weak to drive significant vertical mixing, and mixing associated with the current-driven Ekman transport was too deep compared to the euphotic zone depth. The observed subsurface Chl a did not reflect the satellite estimates of productivity. Since the EAC extension transports warm, low-nutrient surface waters from the subtropics, satellite chlorophyll measurements decreased during the same period the depth-averaged Chl a increased. This seeming paradox illustrated how long duration, full water column sampling can elucidate the coupled biological and physical processes that aid our ongoing effort to forecast the biological state of the coastal ocean.

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Mapping the long‐term influence of river discharge on coastal ocean chlorophyll‐a

Auricht

Mosley

Lewis

et al. 2022

Remote Sens Ecol Conserv

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Although the potential of river discharge to support ocean productivity and marine ecosystems is known, the specifics of this relationship are poorly understood in many regions of the world. Global estimates of river flow indicate that river discharge is decreasing due to the increasing fragmentation, extraction and regulation of rivers. This likely means that the contribution of river flow to coastal productivity and water quality is changing, potentially leading to fewer and smaller magnitude ocean fertilisation events. We developed a simple analysis method, based on Earth observation data, to investigate where coastal ocean chlorophyll-a is most strongly influenced by river discharge. The per-pixel spatiotemporal correlation technique (implemented using Python) correlates chlorophyll-a concentration (a proxy for phytoplankton biomass and indicator of primary productivity) from MODIS ocean colour data with river discharge data. The method was tested globally on 11 different rivers discharging into coastal ocean regions. Our findings suggest some of the world's largest river systems, such as the Amazon River, have zones of elevated coastal chl-a that extend hundreds to thousands of km from the river mouth. These findings suggest the influence of river discharge may have been underestimated in many coastal regions of the world. The method appears more effective for larger river systems discharging to ocean waters with less complex nutrient dynamics and weaker seasonal productivity patterns, most notably in temperate regions. Increasing our understanding of the specific areas influenced by river discharge, and the degree of influence over space and time, is an important step towards the improved river and coastal management. This method will increase the capacity of researchers to monitor how, when and where coastal waters are affected as river discharge continues to change into the future.

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Observations of vertical turbulent nitrate flux during summer in the Great Australian Bight

Cited by 15 publications

References 39 publications

On the Role of Turbulent Mixing Produced by Vertical Shear Between the Brazil Current and the Intermediate Western Boundary Current

On the Role of Turbulent Mixing Produced by Vertical Shear Between the Brazil Current and the Intermediate Western Boundary Current

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

Mapping the long‐term influence of river discharge on coastal ocean chlorophyll‐a

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