Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front

Nagai, Takeyoshi; Tandon, Amit; Yamazaki, Hidekatsu; Doubell, Mark; Gallager, Scott M.

doi:10.1029/2011jc007228

Cited by 72 publications

(62 citation statements)

References 65 publications

(119 reference statements)

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“…3a) of similar magnitudes as observed during 2008 (Nagai et al 2009) and previous studies in the Kuroshio (Rainville and Pinkel 2004) and Gulf Stream (Winkel et al 2002;Inoue et al 2010). Horizontal and vertical wavelengths are l h ; O(10) km and l z ; O(100) m, respectively.…”

Section: B Near-inertial Shearsupporting

confidence: 84%

“…A freefall towyo CTD (Underway CTD) was used to measure upper-500-m temperature and salinity every 14.8 km in the 2012 survey. During 2008, five Falmouth CTD profiles of temperature and salinity to 500-m and TurboMAP-II microstructure profiles to 300-m depth were collected every 28 km (Nagai et al 2009). During 17-24 October 2009, five north-south transects were sampled across the Kuroshio to measure CTD and microstructure at 5-8 stations with 9-km resolution in each section (Nagai et al 2012).…”

Section: A Kuroshio Surveysmentioning

confidence: 99%

“…We use the three-dimensional nonhydrostatic equation process study ocean model (PSOM) (Mahadevan et al 1996a,b) (Nagai et al 2009) is used. To suppress initial unbalanced disturbances in the simulation, temperature and salinity at each vertical level across the front are fitted with high-order polynomial functions.…”

Section: A Model Setupmentioning

confidence: 99%

“…During 2008, the Kuroshio was relatively unstable, exhibiting 100-km-scale meanders upstream of the transect (Fig. 1 of Nagai et al 2009). …”

mentioning

confidence: 99%

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Spontaneous Generation of Near-Inertial Waves by the Kuroshio Front

Nagai

Tandon

Kunze

et al. 2015

Journal of Physical Oceanography

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While near-inertial waves are known to be generated by atmospheric storms, recent observations in the Kuroshio Front find intense near-inertial internal-wave shear along sloping isopycnals, even during calm weather. Recent literature suggests that spontaneous generation of near-inertial waves by frontal instabilities could represent a major sink for the subinertial quasigeostrophic circulation. An unforced three-dimensional 1-km-resolution model, initialized with the observed cross-Kuroshio structure, is used to explore this mechanism. After several weeks, the model exhibits growth of 10-100-km-scale frontal meanders, accompanied by O(10) mW m 22 spontaneous generation of near-inertial waves associated with readjustment of submesoscale fronts forced out of balance by mesoscale confluent flows. These waves have properties resembling those in the observations. However, they are reabsorbed into the model Kuroshio Front with no more than 15% dissipating or radiating away. Thus, spontaneous generation of near-inertial waves represents a redistribution of quasigeostrophic energy rather than a significant sink.

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Section: B Near-inertial Shearsupporting

confidence: 84%

Section: A Kuroshio Surveysmentioning

confidence: 99%

Section: A Model Setupmentioning

confidence: 99%

“…During 2008, the Kuroshio was relatively unstable, exhibiting 100-km-scale meanders upstream of the transect (Fig. 1 of Nagai et al 2009). …”

mentioning

confidence: 99%

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Spontaneous Generation of Near-Inertial Waves by the Kuroshio Front

Nagai

Tandon

Kunze

et al. 2015

Journal of Physical Oceanography

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“…Among these mechanisms, particular interest has been focused on forced buoyancy loss at the ocean surface. Convective heat loss is known to intensify frontal ageostrophic circulations within submesoscale filaments [ Yoshikawa et al ., ], but more recently the buoyancy flux achieved by down‐front winds through the cross‐front Ekman transport of dense water over light has emerged as a potentially more prominent generation mechanism at strong fronts [ Thomas and Lee , ; Lee et al ., ; Thomas and Joyce , ; Nagai et al ., ]. Recent efforts have attempted to diagnose the relative influence of cooling versus wind‐driven buoyancy loss by quantifying the reduction in surface Ertel potential vorticity (EPV),

E P V = ω_{a} \cdot \nabla b

, where

ω_{a} = f + ζ

is the absolute vorticity, due to each process [ Yoshikawa et al ., ].…”

Section: Introductionmentioning

confidence: 99%

Wind-driven submesoscale subduction at the north Pacific subtropical front

Hosegood

Gregg

Alford

2013

J. Geophys. Res. Oceans

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[1] Upper ocean observations from the north Pacific subtropical front during late winter demonstrate the generation of submesoscale intrusions by buoyancy loss. Prior to generation, a sharp thermohaline front was intensified by confluent flow of 1-2 Â 10 À5 s À1 . Relative vertical vorticity, , across a surface-intensified, along-front jet on the warm side of a frontal trough was 0.5 f. During the storm, buoyancy loss arose due to cooling of $650 W m À2 and down-front wind stress <0.5 N m À2 that generated a southward, cross-front Ekman transport of dense water over light. The resulting wind-driven buoyancy flux was concentrated at the front where it exceeded that due to convection by an order of magnitude. The intrusions appeared immediately following the storm both within the surface mixed layer and beneath the seasonal pycnocline. They were approximately 20 m thick and horizontally elongated in the cross-frontal direction. The near-surface intrusions had cool and fresh properties characteristic of the water underlying the seasonal pycnocline, whereas the subsurface intrusions were composed of warm and saline water from the surface. The apparent vertical exchange was constrained within a thin filament of 2 km zonal extent that was characterized by O(1) Rossby and Richardson numbers, pronounced cyclonic veering in the horizontal velocity throughout the surface mixed layer, and sloping isopycnals. The intrusion properties, background environmental context, and forcing history are consistent with prior numerical modeling results for the generation of ageostrophic vertical circulations by frontogenesis intensified by buoyancy loss, possibly resulting in symmetric instability.Citation: Hosegood, P. J., M. C. Gregg, and M. H. Alford (2013), Wind-driven submesoscale subduction at the north Pacific subtropical front,

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Enhancement in vertical fluxes at a front by mesoscale‐submesoscale coupling

2014

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Oceanic frontal instabilities are of importance for the vertical exchange of properties in the ocean. Submesoscale, O(1) Rossby number, dynamics are particularly relevant for inducing the vertical (and lateral) flux of buoyancy and tracers in the mixed layer, but how these couple with the stratified pycnocline is less clear. Observations show surface fronts often persist beneath the mixed layer. Here we use idealized, three-dimensional model simulations to show how surface fronts that extend deeper into the pycnocline invoke enhanced vertical fluxes through the coupling of submesoscale and mesoscale instabilities. We contrast simulations in which the front is restricted to the mixed layer with those in which it extends deeper. For the deeper fronts, we examine the effect of density stratification on the vertical coupling. Our results show deep fronts can dynamically couple the mixed layer and pycnocline on time scales that increase with the peak stratification beneath the mixed layer. Eddies in the interior generate skew fluxes of buoyancy and tracer oriented along isopycnals, thus providing an adiabatic pathway for the interior to interact with the mixed layer at fronts. The vertical enhancement of tracer fluxes through the mesoscale-submesoscale coupling described here is thus relevant to the vertical supply of nutrients for phytoplankton in the ocean. A further implication for wind-forced fronts is that the vertical structure of the stream function characterizing the exchange between the interior and the mixed layer exhibits significant qualitative differences compared to a linear combination of existing parameterizations of submesoscale eddies in the mixed layer and mesoscale eddies in the interior. The discrepancies are most severe within the mixed layer suggesting a potential role for Ekman-layer dynamics absent in existing submesoscale parameterizations.

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Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front

Cited by 72 publications

References 65 publications

Spontaneous Generation of Near-Inertial Waves by the Kuroshio Front

Spontaneous Generation of Near-Inertial Waves by the Kuroshio Front

Wind-driven submesoscale subduction at the north Pacific subtropical front

Enhancement in vertical fluxes at a front by mesoscale‐submesoscale coupling

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