Two‐dimensional ageostrophic secondary circulation at ocean fronts due to vertical mixing and large‐scale deformation

Nagai, Takeyoshi; Tandon, Amit; Rudnick, Daniel L.

doi:10.1029/2005jc002964

Cited by 76 publications

(97 citation statements)

References 43 publications

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“…They used realistic and idealized numerical model calculations, along with an analytic theory, to describe the frontogenesis resulting from spatial variability in turbulent mixing, a process they call turbulent thermal wind (TTW). This drives frontogenesis and downwelling in the vicinity of dense filaments as is found here, a phenomenon that has also been derived in two-dimensional theoretical calculations (e.g., Garrett and Loder 1981;Thompson 2000;Nagai et al 2006).…”

Section: Dynamics and Water Mass Transformationmentioning

confidence: 60%

Downfront Winds over Buoyant Coastal Plumes

Spall

Thomas

2016

Journal of Physical Oceanography

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Downfront, or downwelling favorable, winds are commonly found over buoyant coastal plumes. It is known that these winds can result in mixing of the plume with the ambient water and that the winds influence the transport, spatial extent, and stability of the plumes. In the present study, the interaction of the Ekman velocity in the surface layer and baroclinic instability supported by the strong horizontal density gradient of the plume is explored with the objective of understanding the potential vorticity and buoyancy budgets. The approach makes use of an idealized numerical model and scaling theory. It is shown that when winds are present the weak stratification resulting from vertical mixing and the strong baroclinicity of the front results in near-zero average potential vorticity q. For weak to moderate winds, the reduction of q by diapycnal mixing is balanced by the generation of q through the geostrophic stress term in the regions of strong horizontal density gradients and stable stratification. However, for very strong winds the wind stress overwhelms the geostrophic stress and leads to a reduction in q, which is balanced by the vertical mixing term. In the absence of winds, the geostrophic stress dominates mixing and the flow rapidly restratifies. Nonlinearity, extremes of relative vorticity and vertical velocity, and mixing are all enhanced by the presence of a coast. Scaling estimates developed for the eddy buoyancy flux, the surface potential vorticity flux, and the diapycnal mixing rate compare well with results diagnosed from a series of numerical model calculations.

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Section: Dynamics and Water Mass Transformationmentioning

confidence: 60%

Downfront Winds over Buoyant Coastal Plumes

Spall

Thomas

2016

Journal of Physical Oceanography

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“…Other processes-such as wind-induced frictional forces, diabatic processes, time-dependent near-and super-inertial motions, or higher-order corrections to the quasigeostrophic omega equation-could also generate vertical motion (Viú dez et al 1996;Giordani et al 2006;Nagai et al 2006;Thomas et al 2010). For example, a spatially uniform wind blowing over a current with lateral variations in vertical vorticity will generate Ekman pumping/suction owing to the modification of the Ekman transport by the vorticity (Stern 1965;Niiler 1969).…”

Section: February 2010 N O T E S a N D C O R R E S P O N D E N C Ementioning

confidence: 99%

Subduction on the Northern and Southern Flanks of the Gulf Stream

Thomas

Joyce

2010

Journal of Physical Oceanography

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Sections of temperature, salinity, dissolved oxygen, and velocity were made crossing the Gulf Stream in late January 2006 to investigate the role of frontal processes in the formation of Eighteen Degree Water (EDW), the Subtropical Mode Water of the North Atlantic. The sections were nominally perpendicular to the stream and measured in a Lagrangian frame by following a floating spar buoy drifting in the Gulf Stream's warm core. During the survey, EDW was isolated from the mixed layer by the stratified seasonal pycnocline, suggesting that EDW was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted (648-708W). However, in two of the sections, the seasonal pycnocline in the core of the Gulf Stream was broken by an intrusion of cold, fresh, weakly stratified water, nearly saturated in oxygen, that appears to have been subducted from the surface mixed layer north of the stream. The intrusion was identified in three of the sections in profiles with a nearly identical temperature-salinity relation. From the western-toeasternmost sections, where the intrusion was observed, the depth of the intrusion's salinity minimum descended by ;90 m in the 71 h it took to complete this part of the survey. This apparent subduction occurred primarily on the upstream side of a meander trough, where the cross-stream velocity was confluent and frontogenetic. Using a variant of the omega equation, the vertical velocity driven by the confluent flow was inferred and yielded downwelling in the vicinity of the intrusion spanning 10-40 m day 21, a range of values consistent with the intrusion's observed descent, suggesting that frontal subduction was responsible for the formation of the intrusion. In the easternmost section located downstream of the meander trough, the flow was diffluent, driving an inferred vertical circulation that was of the opposite sense to that in the section upstream of the trough. In transiting the two sides of the trough, the intrusion was observed to move toward the center of the stream between the downwelling branches of the opposing vertical circulations, resulting in a downward Lagrangian mean vertical velocity and net subduction. Hydrographic evidence of the subduction of weakly stratified surface waters was seen in the southern flank of the Gulf Stream as well. The solution of the omega equation suggests that this subduction was associated with a relatively shallow vertical circulation confined to the upper 200 m of the water column in the proximity of the front marking the southern edge of the warm core.

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“…[11] Enhanced departures from geostrophic shear (Figure 4c Nagai et al, 2006] illustrate that the along frontal ageostrophic shear can be forced by the internal geostrophic stress, i.e.,…”

Section: Source Of the Turbulencementioning

confidence: 99%

“…Several studies have found that turbulent mixing [Garrett and Loder, 1981;Thompson, 2000;Nagai et al, 2006Nagai et al, , 2008 and the alongfront component of the wind forcing [Thomas, 2005;A. Mahadevan et al, Rapid changes in mixed layer stratification driven by submesoscale instabilities and winds, submitted to Journal of Geophysical Research, 2009] also provide generation mechanisms for the ASCs. Furthermore, recent theoretical studies using high-resolution numerical models have illustrated that surface trapped forward energy cascade due to turbulent mixing near fronts, associated with unbalanced submesoscale flows, provides a direct route for kinetic and potential energy to be dissipated, [Capet et al, 2008;McWilliams, 2008; M. J. Molemaker et al, Balanced and unbalanced routes to dissipation in an equilibrated Eady flow, submitted to Journal of Fluid Mechanics, 2009] addressing the conundrum of how energy goes from larger to smaller scales.…”

Section: Introductionmentioning

confidence: 99%

Evidence of enhanced turbulent dissipation in the frontogenetic Kuroshio Front thermocline

Nagai

Tandon

Yamazaki

et al. 2009

Geophysical Research Letters

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Enhanced turbulent dissipation, O(10−8–10−7) Wkg−1 in the thermocline on the cyclonic side of the Kuroshio Front was observed during a period of frontogenesis, using a microstructure profiler, XBT and ADCP along 143°E across the Kuroshio Front in August 2008. The eddy diffusivity corresponding to the mixing below the central jet is estimated to be O(10−4–10−3) m2 s−1. The strong turbulent mixing we observed in the Kuroshio is in sharp contrast to previous field measurements which found that small scale diapycnal mixing in western boundary currents remains at levels typically measured in the open ocean thermocline. The turbulence in the Kuroshio is attributed to frontogenesis arising out of an estimated confluence rate of O(10−5) s−1 based on satellite altimeters, suggesting the ubiquity of a forward energy cascade from mesoscale to microscale turbulence near ocean fronts as indicated by recent theoretical studies.

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Two‐dimensional ageostrophic secondary circulation at ocean fronts due to vertical mixing and large‐scale deformation

Cited by 76 publications

References 43 publications

Downfront Winds over Buoyant Coastal Plumes

Downfront Winds over Buoyant Coastal Plumes

Subduction on the Northern and Southern Flanks of the Gulf Stream

Evidence of enhanced turbulent dissipation in the frontogenetic Kuroshio Front thermocline

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