Symmetric Instability, Inertial Oscillations, and Turbulence at the Gulf Stream Front

Thomas, Leif N.; Taylor, John R.; D’Asaro, Eric A.; Lee, Craig M.; Klymak, Jody M.; Shcherbina, Andrey Y.

doi:10.1175/jpo-d-15-0008.1

Cited by 91 publications

(138 citation statements)

References 28 publications

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“…When Q is multiplied by the Coriolis parameter ( f = 2Ω sin

ϕ

), we theoretically expect negative values to appear in regions where symmetric instabilities could be at play, corresponding as well to areas where the gradient Richardson number ( Ri ) is <1. Ri can be defined as follows (Thomas et al, ):

R i = \frac{N^{2}}{S^{2}}

where N 2 =

\frac{\partial b}{\partial z}

and S 2 =

{\frac{\partial u}{\partial z}}^{2}

{\frac{\partial v}{\partial z}}^{2}

. N 2 is the stratification and S 2 is the squared shear.…”

Section: Methodsmentioning

confidence: 99%

Mesoscale and Submesoscale Processes in the Southeast Atlantic and Their Impact on the Regional Thermohaline Structure

et al. 2018

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The turbulent processes in the Cape Basin, the southeasternmost gate of the Atlantic Ocean, play a key role in the transport and mixing of upper to intermediate water masses entering the area from the Indian Ocean, making them especially relevant for the Indo‐Atlantic transfer of heat and salt. In this paper, two numerical simulations at different horizontal resolutions are used to study mesoscale and submesoscale dynamics, their phenomenology, their evolution, and their impact on the local water masses. Submesoscale processes seasonally affect both, the upper and intermediate layers, but there are clear dynamical differences between the two layers. Several types of instabilities underline this spatial and temporal variability. Near the surface, mixed layer instabilities occur during winter, while mesoscale‐driven instabilities, as the symmetric type, prevail in summer. The connection between these two seasonal regimes is ensured, in anticyclonic eddies and within the mixed layers, by Charney baroclinic instabilities, involved in the local formation and subduction of mode water, that we have dubbed as Agulhas Rings mode water. Intermediate depths are instead characterized by mesoscale mechanisms of density compensation and lateral stirring of the tracer variance, triggering a significant filamentogenesis whose vertical scales are comparable to those mentioned in previous studies. This leads to a particularly efficient mixing of Antarctic Intermediate Waters of Indian and Atlantic origins. Lagrangian estimates highlight the new and significant role of fine scale structures in setting the water masses properties of upper and lower thermocline waters materializing the Indo‐Atlantic exchange and therefore potentially affecting the global ocean circulation.

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“…When Q is multiplied by the Coriolis parameter ( f = 2Ω sin

ϕ

R i = \frac{N^{2}}{S^{2}}

where N 2 =

\frac{\partial b}{\partial z}

and S 2 =

{\frac{\partial u}{\partial z}}^{2}

{\frac{\partial v}{\partial z}}^{2}

. N 2 is the stratification and S 2 is the squared shear.…”

Section: Methodsmentioning

confidence: 99%

Mesoscale and Submesoscale Processes in the Southeast Atlantic and Their Impact on the Regional Thermohaline Structure

et al. 2018

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“…Periods of down‐front wind, reduce the surface buoyancy in the front, deepen the SXL, and drive entrainment (Figure ), whereas periods of up‐front wind (when the winds are opposite to the geostrophic frontal jet) increase the surface buoyancy in the front, shoal the SXL, and drive detrainment. In the real ocean, the EBF sometimes vacillates in time between positive and negative values, primarily due to variations in the wind stress direction and amplitude, but also due to variations in the horizontal buoyancy gradient in the ocean

\nabla_{h} b

[e.g., Thomas et al ., ; Thompson et al ., ]. Hence, a question arises: what is the time‐integrated effect of these vacillations in the EBF (and wind stress more generally) on SXL depths, SXL nutrient budgets, and plankton ecosystems at a front?…”

Section: Introductionmentioning

confidence: 99%

Low-frequency and high-frequency oscillatory winds synergistically enhance nutrient entrainment and phytoplankton at fronts

Whitt

Lévy

Taylor

2017

J. Geophys. Res. Oceans

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When phytoplankton growth is limited by low nutrient concentrations, full‐depth‐integrated phytoplankton biomass increases in response to intermittent mixing events that bring nutrient‐rich waters into the sunlit surface layer. Here it is shown how oscillatory winds can induce intermittent nutrient entrainment events and thereby sustain more phytoplankton at fronts in nutrient‐limited oceans. Low‐frequency (i.e., synoptic to planetary scale) along‐front wind drives oscillatory cross‐front Ekman transport, which induces intermittent deeper mixing layers on the less dense side of fronts. High‐frequency wind with variance near the Coriolis frequency resonantly excites inertial oscillations, which also induce deeper mixing layers on the less dense side of fronts. Moreover, we show that low‐frequency and high‐frequency winds have a synergistic effect and larger impact on the deepest mixing layers, nutrient entrainment, and phytoplankton growth on the less dense side of fronts than either high‐frequency winds or low‐frequency winds acting alone. These theoretical results are supported by two‐dimensional numerical simulations of fronts in an idealized nutrient‐limited open‐ocean region forced by low‐frequency and high‐frequency along‐front winds. In these model experiments, higher‐amplitude low‐frequency wind strongly modulates and enhances the impact of the lower‐amplitude high‐frequency wind on phytoplankton at a front. Moreover, sensitivity studies emphasize that the synergistic phytoplankton response to low‐frequency and high‐frequency wind relies on the high‐frequency wind just below the Coriolis frequency.

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“…Here, a two‐dimensional PV section (e.g., Ramachandran et al, ; Thomas et al, ) is evaluated from the objective field, assuming that along‐stream variations of quantities are negligible. However, these gradients could be significant, refuting the approximation of two‐dimensional PV (see details in Thomas et al, ; Ramachandran et al, ). The Ertel PV is given by

q = false(f \overset{k}{true} + \nabla \times boldu false) \cdot \nabla b,

where

f

is the planetary vorticity,

\overset{k}{true}

is the unit vector, and

boldu = false(u, v, w false)

is the 3‐D velocity vector.…”

Section: The Mesoscale Fieldmentioning

confidence: 99%

“…According to Thomas et al (), is expressed by a sum of two constituents that emphasizes the contrasting roles of vertical vorticity/stratification and baroclinicity. The scaled PV,

q \approx \frac{(][, ()(, f + \frac{\partial v}{\partial x}) N^{2} - \frac{\partial v}{\partial z} \frac{\partial b}{\partial x}) f}{\overset{f^{2} N^{2}}{true}},

where the overbar denotes spatial averaging, is outlined in Figure .…”

Section: The Mesoscale Fieldmentioning

confidence: 99%

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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Symmetric Instability, Inertial Oscillations, and Turbulence at the Gulf Stream Front

Cited by 91 publications

References 28 publications

Mesoscale and Submesoscale Processes in the Southeast Atlantic and Their Impact on the Regional Thermohaline Structure

Mesoscale and Submesoscale Processes in the Southeast Atlantic and Their Impact on the Regional Thermohaline Structure

Low-frequency and high-frequency oscillatory winds synergistically enhance nutrient entrainment and phytoplankton at fronts

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

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