Isopycnal Eddy Diffusivities and Critical Layers in the Kuroshio Extension from an Eddying Ocean Model

Abstract High spatial resolution isopycnal diffusivities are estimated in the Kuroshio Extension (KE) region (28°–40°N, 120°–190°E) from a global ° Parallel Ocean Program (POP) simulation. The numerical float tracks are binned using a clustering approach. The number of tracks in each bin is thus roughly the same leading to diffusivity estimates that converge better than those in bins defined by a regular geographic grid. Cross-stream diffusivities are elevated in the southern recirculation gyre region, near topographic obstacles and downstream in the KE jet, where the flow has weakened. Along-stream diffusivities, which are much larger than cross-stream diffusivities, correlate well with the magnitudes of eddy velocity. The KE jet suppresses cross-stream mixing only in some longitude ranges. This study estimates the critical layer depth both from linear local baroclinic instability analysis and from eddy phase speeds in the POP model using the Radon transform. The latter is a better predictor of large mixing length in the cross-stream direction. Critical layer theory is most applicable in the intense jet regions away from topography.

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Observations of the Subtropical Mode Water Evolution from the Kuroshio Extension System Study

Journal of Physical Oceanography ◽

10.1175/jpo2849.1 ◽

2006 ◽

Vol 36 (3) ◽

pp. 457-473 ◽

Cited By ~ 69

Author(s):

Bo Qiu ◽

Peter Hacker ◽

Shuiming Chen ◽

Kathleen A. Donohue ◽

D. Randolph Watts ◽

...

Keyword(s):

North Pacific Ocean ◽

Kuroshio Extension ◽

Mesoscale Eddy ◽

Lower Boundary ◽

Late Winter ◽

Mode Water ◽

Subtropical Mode Water ◽

Budget Analysis ◽

Recirculation Gyre ◽

The Kuroshio

Abstract Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 m near the center of the gyre. The lower boundary of STMW, corresponding to σθ ≃ 25.5 kg m−3, was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated from a budget analysis of potential vorticity to be in the range of ∼2–5 × 10−4 m2 s−1. The latitudinal extent of the STMW formation was narrow, extending from 30°N to the Kuroshio Extension jet near 35°N. South of 30°N, STMW did not form locally but was transported from the recirculation gyre by lateral induction.

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Meridional shift of the Kuroshio Extension induced by response of recirculation gyre to decadal wind variations

Deep Sea Research Part II Topical Studies in Oceanography ◽

10.1016/j.dsr2.2009.12.002 ◽

2010 ◽

Vol 57 (13-14) ◽

pp. 1111-1126 ◽

Cited By ~ 10

Author(s):

Hideyuki Nakano ◽

Ichiro Ishikawa

Keyword(s):

Kuroshio Extension ◽

Recirculation Gyre ◽

The Kuroshio

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A nonlinear critical layer generated by the interaction of free Rossby waves

Journal of Fluid Mechanics ◽

10.1017/s0022112098002237 ◽

1998 ◽

Vol 371 ◽

pp. 319-344 ◽

Cited By ~ 7

Author(s):

JACQUES VANNESTE

Keyword(s):

Continuous Spectrum ◽

Rossby Waves ◽

Resonance Condition ◽

Inviscid Flow ◽

Critical Layer ◽

Weakly Nonlinear ◽

Critical Layers ◽

Flow Domain ◽

Parallel Shear Flow ◽

Stream Direction

Two free waves propagating in a parallel shear flow generate a critical layer when their nonlinear interaction induces a perturbation whose phase velocity matches the basic-state velocity somewhere in the flow domain. The condition necessary for this to occur may be interpreted as a resonance condition for a triad formed by the two waves and a (singular) mode of the continuous spectrum associated with the shear. The formation of the critical layer is investigated in the case of freely propagating Rossby waves in a two-dimensional inviscid flow in a β-channel.A weakly nonlinear analysis based on a normal-mode expansion in terms of Rossby waves and modes of the continuous spectrum is developed; it leads to a system of amplitude equations describing the evolution of the two Rossby waves and of the modes of the continuous spectrum excited during the interaction. The assumption of weak nonlinearity is not however self-consistent: it breaks down because nonlinearity always becomes strong within the critical layer, however small the initial amplitudes of the Rossby waves. This demonstrates the relevance of nonlinear critical layers to monotonic, stable, unforced shear flows which sustain wave propagation.A nonlinear critical-layer theory is developed that is analogous to the well-known theory for forced critical layers. Differences arise because of the presence of the Rossby waves: the vorticity in the critical layer is advected in the cross-stream direction by the oscillatory velocity field due to the Rossby waves. An equation is derived which governs the modification of the Rossby waves that results from their interaction; it indicates that the two Rossby waves are undisturbed at leading order. An analogue of the Stewartson–Warn–Warn analytical solution is also considered.

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On the Seasonal Eddy Variability in the Kuroshio Extension

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0058.1 ◽

2018 ◽

Vol 48 (8) ◽

pp. 1675-1689 ◽

Cited By ~ 10

Author(s):

Yang Yang ◽

X. San Liang

Keyword(s):

Energy Transfer ◽

Annual Cycle ◽

Kuroshio Extension ◽

Baroclinic Instability ◽

Primary Energy ◽

Seasonal Cycles ◽

External Forcing ◽

Energy Transfers ◽

The Kuroshio ◽

Multiscale Window Transform

AbstractUsing a recently developed tool, multiscale window transform (MWT), and the MWT-based canonical energy transfer theory, this study investigates the seasonal eddy variability in the Kuroshio Extension. Distinct seasonal cycles of eddy kinetic energy (EKE) are observed in the upstream and downstream regions of the Kuroshio Extension. In the upstream Kuroshio Extension, the EKE peaks in summer and reaches its minimum in winter over an annual cycle. By diagnosing the spatiotemporal structures of the canonical barotropic and baroclinic energy transfers, we found that internal processes due to mixed instabilities (i.e., both barotropic and baroclinic instabilities) are responsible for the seasonal eddy variability in this region. In the downstream Kuroshio Extension, the EKE exhibits a different annual cycle, peaking in spring and gradually decaying from summer to winter. Significant inverse barotropic energy transfer is found in this region throughout the year, leaving baroclinic instability the primary energy source for the regional seasonal eddy variability. Besides the internal redistribution, it is also evident that the external forcing may influence the Kuroshio Extension EKE seasonality—the EKE is found to be more damped by winds during winter than summer.

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Effects of Eddy Vorticity Forcing on the Mean State of the Kuroshio Extension

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0259.1 ◽

2015 ◽

Vol 45 (5) ◽

pp. 1356-1375 ◽

Cited By ~ 19

Author(s):

Andrew S. Delman ◽

Julie L. McClean ◽

Janet Sprintall ◽

Lynne D. Talley ◽

Elena Yulaeva ◽

...

Keyword(s):

Reference Frames ◽

Model Simulation ◽

Kuroshio Extension ◽

Relative Vorticity ◽

Ocean Model ◽

Instability Criterion ◽

Mean Flow ◽

Vorticity Budget ◽

The Mean ◽

The Kuroshio

AbstractEddy–mean flow interactions along the Kuroshio Extension (KE) jet are investigated using a vorticity budget of a high-resolution ocean model simulation, averaged over a 13-yr period. The simulation explicitly resolves mesoscale eddies in the KE and is forced with air–sea fluxes representing the years 1995–2007. A mean-eddy decomposition in a jet-following coordinate system removes the variability of the jet path from the eddy components of velocity; thus, eddy kinetic energy in the jet reference frame is substantially lower than in geographic coordinates and exhibits a cross-jet asymmetry that is consistent with the baroclinic instability criterion of the long-term mean field. The vorticity budget is computed in both geographic (i.e., Eulerian) and jet reference frames; the jet frame budget reveals several patterns of eddy forcing that are largely attributed to varicose modes of variability. Eddies tend to diffuse the relative vorticity minima/maxima that flank the jet, removing momentum from the fast-moving jet core and reinforcing the quasi-permanent meridional meanders in the mean jet. A pattern associated with the vertical stretching of relative vorticity in eddies indicates a deceleration (acceleration) of the jet coincident with northward (southward) quasi-permanent meanders. Eddy relative vorticity advection outside of the eastward jet core is balanced mostly by vertical stretching of the mean flow, which through baroclinic adjustment helps to drive the flanking recirculation gyres. The jet frame vorticity budget presents a well-defined picture of eddy activity, illustrating along-jet variations in eddy–mean flow interaction that may have implications for the jet’s dynamics and cross-frontal tracer fluxes.

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Rapid Eddy-Induced Modification of Subtropical Mode Water during the Kuroshio Extension System Study

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0191.1 ◽

2014 ◽

Vol 44 (7) ◽

pp. 1941-1953 ◽

Cited By ~ 2

Author(s):

Stuart P. Bishop ◽

D. Randolph Watts

Keyword(s):

Kuroshio Extension ◽

Unstable State ◽

System Study ◽

Mode Water ◽

Unstable Regime ◽

Subtropical Mode Water ◽

Eddy Diffusivities ◽

The North ◽

Extension System ◽

The Kuroshio

Abstract From 2004 to 2006 an observational array of current- and pressure-recording inverted echo sounders (CPIES) were deployed as part of the Kuroshio Extension (KEx) System Study (KESS). KESS observed a transition from a weakly meandering (“stable”) to strongly meandering (“unstable”) state (Qiu and Chen). As the KEx made this transition, potential vorticity (PV) observed within the southern recirculation gyre (SRG) rapidly increased from January to July 2005. In this study, the authors diagnose eddy PV fluxes (EPVFs) in isentropic coordinates within the subtropical mode water (STMW) layer from the CPIES data to determine the role of mesoscale eddies in this rapid increase of PV. The rapid increase in PV within the SRG coincided with enhanced cross-front EPVFs and eddy PV flux convergence upstream of a mean trough in the KEx path and adjacent to the SRG. The enhanced cross-front EPVFs were the result of the formation of a cold-core ring (CCR) and the interaction of the jet with a preexisting CCR. Eddy diffusivities are diagnosed for the unstable regime with values that range from 100 to 2000 m2 s−1. The high eddy diffusivities during the unstable regime reflect the nature of mesoscale CCR formation and CCR–jet interaction as efficient mechanisms for stirring and mixing high PV waters from the north side of the KEx into the low PV waters of the SRG where STMW resides. This mechanism for cross-frontal exchange can explain observed increases in the STMW PV in the SRG over the 16 months of KESS observations.

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