On the Decadal Variability of the Eddy Kinetic Energy in the Kuroshio Extension (original) (raw)
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Using a recently developed energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), this study investigates the intricate nonlinear mutual interactions among the decadally modulating mean flow, the interannual fluctuations, and the transient eddies in the Kuroshio Extension region. It is found that the mean kinetic energy maximizes immediately east of the Izu–Ogasawara Ridge, while the transient eddy kinetic energy does not peak until 400 km away downstream. The interannual variabilities, which are dominated by a jet-trapped Rossby wave mode, provide an energy reservoir comparable to the other counterparts. In the upstream, strong localized barotropic and baroclinic transfers from the mean flow to the eddies are observed, whereas those from the interannual variabilities are not significant. Besides fueling the eddies, the unstable mean jet also releases energy to the interannual-scale processes. Between 1448 and 1548E, both transfers from the mean flow and the interannual variabilities are important for the eddy development. Farther downstream, eddies are found to drive the mean flow on both the kinetic energy (KE) and available potential energy (APE) maps. They also provide KE to the interannual vari-abilities but obtain APE from the latter. The gained eddy APE is then converted to eddy KE through buoyancy conversion. Upscale energy transfers are observed in the northern and southern recirculation gyre (RG) regions. In these regions, the interannual–eddy interaction exhibits different scenarios: the eddies lose KE to the interannual processes in the northern RG region, while gaining KE in the southern RG region.
Effects of Eddy Vorticity Forcing on the Mean State of the Kuroshio Extension
Journal of Physical Oceanography, 2015
Eddy–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 mom...
Decadal variability of the Kuroshio Extension: mesoscale eddies and recirculations
Ocean Dynamics, 2010
An eddy-resolving multidecadal ocean model hindcast simulation is analyzed to investigate timevarying signals of the two recirculation gyres present respectively to the north and south of the Kuroshio Extension (KE) jet. The northern recirculation gyre (NRG), which has been detected at middepth recently by profiling float and moored current meter observations, is a major focus of the present study. Lowfrequency variations in the intensity of the recirculation gyres are overall highly correlated with decadal variations of the KE jet induced by the basin-wide wind change. Modulation of the simulated mesoscale eddies and its relationship with the time-varying recirculation gyres are also evaluated. The simulated eddy kinetic energy in the upstream KE region is inversely correlated with the intensity of the NRG, consistent with previous observational studies. Eddy influence on the Responsible Editor: Yukio Masumoto low-frequency modulation of the NRG intensity at middepth is further examined by a composite analysis of turbulent Sverdrup balance, assuming a potential vorticity balance between the mean advection and the convergent eddy fluxes during the different states of the recirculation gyre. The change in the NRG intensity is adequately explained by that inferred by the turbulent Sverdrup balance, suggesting that the eddy feedback triggers the low-frequency modulation of the NRG intensity at middepth.
The distribution of eddy kinetic and potential energies in the global ocean
Tellus A, 2010
A B S T R A C T Understanding of the major sources, sinks, and reservoirs of energy in the ocean is briefly updated in a diagram. The nature of the dominant kinetic energy reservoir, that of the balanced variablity, is then found to be indistinguishable in the observations from a sum of barotropic and first baroclinic ordinary quasi-geostrophic modes. Little supporting evidence is available to partition the spectra among forced motions and turbulent cascades, along with significant energy more consistent with weakly non-linear wave dynamics. Linear-response wind-forced motions appear to dominate the high frequency (but subinertial) mooring frequency spectra. Turbulent cascades appear to fill the high wavenumber spectra in altimetric data and numerical simulations. Progress on these issues is hindered by the difficulty in connecting the comparatively easily available frequency spectra with the variety of theoretically predicted wavenumber spectra.
Spatio-Temporal Variability of the Eddy Kinetic Energy in the South Atlantic Ocean
IEEE Geoscience and Remote Sensing Letters, 2014
The spatio-temporal variability of the eddy kinetic energy (EKE) in the South Atlantic Ocean (SAO) is investigated using 19 years of satellite altimetry observations. The EKE in this region presents different significant frequency modes. The interannual to intrannual cycles dominate the EKE variability spectrum. Spatial patterns of the EKE variance were also determined and associated with the propagation of the Agulhas Current eddies across the SAO. At the annual frequency, EKE anomalies are generated in the Agulhas leakage (AL) region and propagate westward. The interannual signal was associated with the Antarctic oscillation and displays a stationary spatial oscillation pattern in the Agulhas eddy corridor (AEC). This is the first time that a full spectral analysis of the EKE variability in the SAO is produced. Results show that the AEC is an important feature in the SAO, with low (high) frequencies associated to the west (east) part of the basin. The AL is a significant source of mesoscale variability to the South Atlantic subtropical gyre. Index Terms-Agulhas eddy corridor (AEC), Agulhas leakage (AL), Eddy kinetic energy (EKE), South Atlantic Ocean (SAO).
Baroclinic Instability: An Oceanic Wavemaker for Interdecadal Variability
Journal of Physical Oceanography, 1999
Numerical simulations of coarse-resolution, idealized ocean basins under constant surface heat flux are analyzed to show that the interdecadal oscillations that emerge naturally in such configurations are driven by baroclinic instability of the mean state and damped by horizontal diffusion. When the surface heat fluxes are diagnosed from a spinup in which surface temperatures are strongly restored to apparent atmospheric temperatures, the most unstable regions diagnosed by large downgradient eddy heat fluxes are located in the basin northwest corner where the surface heat losses are largest. The long-wave limit of the baroclinic instability of idealized mean flows in a three-layer model with vertical shears as observed in the GCMs demonstrates that growth rates of order one cycle per year can be produced locally, large enough to amplify thermal anomalies in the face of lateral diffusion. The proposed instability mechanism that favors surface-intensified perturbations also explains the lack of oscillations if the restoring to a surface climatology is too strong. To assess whether this instability process of oceanic origin is robust enough to cause interdecadal variability of coupled ocean-atmosphere models, a four-box ocean-atmosphere model is constructed. Given the large heat capacity of the ocean as compared to the atmosphere, the dynamical system that governs the model evolution is reduced to only two degrees of freedom, the oceanic overturning thermohaline circulation and the interior north-south temperature gradient. The authors show that, when the baroclinic instability growth rate exceeds the overall dissipation caused by turbulent eddy diffusion in the atmosphere and ocean and infrared back radiation, the dynamical system undergoes a Hopf bifurcation, and interdecadal oscillations emerge through a limit cycle.
Warming of Western North Pacific Ocean and Energetics of Transient Eddy Activity
Monthly Weather Review, 2012
This study examines the increase in the sea surface temperature (SST) in the western North Pacific Ocean (WNPO) during December-February for the period 1959-2008. The relationship of this SST increase with significant interdecadal changes in the baroclinicity and the energetics of transient eddy activity is also examined. These results show that the interannual variations of the WNPO SST and atmospheric conditions including baroclinicity and turbulent heat flux are responses to the upstream atmospheric forcings associated with the East Asian winter monsoon (EAWM). For the interdecadal variations, the intensified baroclinicity downstream of the Pacific storm-track activity is responsible for an increase in the baroclinic energy conversion (BCEC) from the mean available potential energy (MAPE) to the eddy available potential energy (EAPE) to the east of 1808. This in turn increases the BCEC from the EAPE to the eddy kinetic energy (EKE) over this region. The BCEC and generation of EAPE by diabatic heating are enhanced to the east of 1808 as a result of the intensified baroclinicity. This could be responsible for the development of transient eddy activity downstream of the Pacific storm track over the North Pacific.
The assumption that local baroclinic instability dominates eddy-mean flow interactions is tested on a global scale using a dynamically consistent eddy-permitting state estimate. Interactions are divided into local and nonlocal. If all the energy released from the mean flow through eddy-mean flow interaction is used to support eddy growth in the same region, or if all the energy released from eddies through eddy-mean flow interaction is used to feed back to the mean flow in the same region, eddy-mean flow interaction is local; otherwise, it is nonlocal. Different regions have different characters: in the subtropical region studied in detail, interactions are dominantly local. In the Southern Ocean and Kuroshio and Gulf Stream Extension regions, they are mainly nonlocal. Geographical variability of dominant eddy-eddy and eddy-mean flow processes is a dominant factor in understanding ocean energetics.
Temporal Changes in Ocean Eddy Transports
Journal of Physical Oceanography, 2006
New estimates from 11 yr of altimetric data are made of the global time-average variability kinetic energy and its decadal-scale variability. Making the approximation that the variability reflects primarily eddy motions, a time-mean, but spatially varying, eddy mixing coefficient is then estimated along with its changes over the last decade. With a record length more than 2 times that previously available, the time-mean variability kinetic energy KE is statistically more reliable and smoother in its spatial pattern. Minimum values of KE are present in the subpolar North Pacific Ocean and in the eastern South Pacific (both less than 100 cm2 s−2). In contrast to the North Pacific, the subpolar North Atlantic Ocean shows relatively enhanced KE. Eddy kinetic energy and eddy mixing appear to have declined during the last decade over large parts of the western Pacific Ocean, in some regions by as much as 50% of the time-mean value. Increased eddy variability can be found in the Kuroshio a...
Scientific Reports
Coherent oceanic mesoscale eddies with unique dynamical structures have great impacts on ocean transports and global climate. Eddy kinetic energy (EKE), derived from time-dependent circulation, is commonly used to study mesoscale eddies. However, there are three deficiencies of EKE when focusing on the analysis of coherent mesoscale eddies. Here, we propose a comprehensive concept—Lagrangian EKE (LEKE) as an additional metric which is a combination of gridded EKE calculated in Eulerian framework and tracked coherent mesoscale eddies in Lagrangian framework. Evidences suggest that LEKE can make up these deficiencies as an effective supplement. In this study, regional application over Northwestern Pacific Ocean is taken as an example. It clearly demonstrates that LEKE reveals more accurate and detailed characteristics of both cyclonic and anticyclonic eddies than EKE when coherent mesoscale eddies are the specific focus, such as the variation rates of kinetic energy during the eddy pr...