Effects of Eddy Vorticity Forcing on the Mean State of the Kuroshio Extension (original) (raw)
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On the Decadal Variability of the Eddy Kinetic Energy in the Kuroshio Extension
Previous studies have found that the decadal variability of eddy kinetic energy (EKE) in the upstream Kuroshio Extension is negatively correlated with the jet strength, which seems counterintuitive at first glance because linear stability analysis usually suggests that a stronger jet would favor baroclinic instability and thus lead to stronger eddy activities. Using a time-varying energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), and the MS-EVA-based nonlinear instability theory, this study investigates the physical mechanism responsible for such variations with the state estimate from the Estimating the Circulation and Climate of the Ocean (ECCO), Phase II. For the first time, it is found that the decadal modulation of EKE is mainly controlled by the barotropic instability of the background flow. During the high-EKE state, violent meanderings efficiently induce strong barotropic energy transfer from mean kinetic energy (MKE) to EKE despite the rather weak jet strength. The reverse is true in the low-EKE state. Although the enhanced meander in the high-EKE state also transfers a significant portion of energy from mean available potential energy (MAPE) to eddy available potential energy (EAPE) through baroclinic instability, the EAPE is not efficiently converted to EKE as the two processes are not well correlated at low frequencies revealed in the time-varying energetics. The decadal modulation of barotropic instability is found to be in pace with the North Pacific Gyre Oscillation but with a time lag of approximately 2 years.
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.
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.
Journal of Physical Oceanography, 2014
High spatial resolution isopycnal diffusivities are estimated in the Kuroshio Extension (KE) region (288-408N, 1208-1908E) from a global 1 /108 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 crossstream 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.
On the eddy-Kuroshio interaction: Evolution of the mesoscale eddy
Journal of Geophysical Research, 2002
1] Presented are results from a numerical study of the interaction of a mesoscale eddy and the Kuroshio using a high-resolution regional general circulation model (GCM). The distinct evolution of the mesoscale eddies is not new in the literature, but most of the previous studies were conducted with simpler model configurations. To our knowledge the present study is the first to use a high-resolution GCM, proven to replicate a realistic ocean circulation. An anticyclonic eddy was injected to the south of Kuroshio (140°E, 30°N) by means of sequential data assimilation of TOPEX/Poseidon altimeter data of October 1992 for 30 days. The strength of the assimilation was varied to produce five eddies (Rossby number e $ 0.012-0.014) comparable in scale to those observed in this region. The two results (e = 0.0131, 0.0141) resemble the observed onset of shortterm meandering events in 1993 and 1998. The westward propagating anticyclonic eddy collides with the Kuroshio southeast of Kyushu, propagates downstream, and triggers the short-term Kuroshio meander (occurring between the Kii Peninsula and Izu ridge with duration of half a year). The sequence resembles the scenario hypothesized on the basis of altimeter observations and other in situ measurements during Tokyo Ogasawara Line Experiment. The description of these cases is important, but we focus here on the differentiation between the strong eddy cases (e = 0.0131, 0.0141) and the weak eddy cases (e = 0.0118, 0.0124, 0.0125) classified on the basis of their subsequent evolutions: the strong geddy (meandering case) propagates west as it elongates zonally and barotropic tripolar vortices form, and the weak eddy (nonmeandering case) makes an abrupt southward migration while the eddy core splits as a result of advection by lower layer geostrophic motions. Such distinct evolutions are conjectured a result of a competition of the effects of nonlinearity, dispersion, and barotropicity of which their relative importance varied among the simulated eddies. Because the scale variation of the injected eddies are natural, we expect that even the weak-eddy case exists in nature; in fact, such a case was observed in 1994.
Journal of Marine Research, 2012
Strong energy in the 30-60 day band was observed using 39 deep pressure and current records from the Kuroshio Extension System Study (KESS). Energy in this band accounted for 25-50% of the total deep-pressure variance and was strongest under the Kuroshio Extension jet axis. Often, deep-pressure anomalies propagated into the region from the north-northeast and locally intensified as they passed under and interacted with the Kuroshio Extension. The topographically controlled deeppressure anomalies translate nearly along lines of constant f/H . Statistically significant coherence between 30-60 day upper-and deep-ocean streamfunction anomalies demonstrated that there was strong vertical coupling in that time band. Twenty-five percent of the total upper-ocean streamfunction variance was contained within the 30-60 day band near the Kuroshio Extension. Joint CEOFs of the upper-and deep-ocean streamfunctions revealed that near the axis of the Kuroshio Extension the phases were laterally offset alongstream, with the deep ocean leading the upper ocean. This arrangement is attributed to producing joint development of upper-ocean meanders and deep-pressure anomalies.
Journal of Geophysical Research, 1996
A set of numerical simulations is used to investigate the Pacific Ocean circulation north of 20øS, with emphasis on the Kuroshio/Oyashio current system. The primitive equation models used for these simulations have a free surface and realistic geometry that includes the deep marginal seas such as the Sea of Japan. Most of the simulations have 1/8 ø resolution for each variable but range from 1/2 ø, 1.5-layer reduced gravity to 1/16 ø , six layer with realistic bottom topography. These are used to investigate the dynamics of the Kuroshio/Oyashio current system and to identify the processes that contribute most to the realism of the simulations. This is done by model-data comparisons, by using the modularity of layered ocean models to include/exclude certain dynamical processes, by varying the model geometry and bottom topography, and by varying model parameters such as horizontal grid resolution, layer structure, and eddy viscosity. In comparison with observational data the simulations show that the barotropic mode, at least one internal mode, nonlinearity, high "horizontal" resolution (1/8 ø or finer), the regional bottom topography, and the wind forcing are critical for realistic simulations. The first four are important for baroclinic instability (eddy-mean energetics actually show mixed barotropic-baroclinic instability), the wind curl pattern for the formation and basic placement of the current system, and the bottom topography for the distribution of the instability and for influences on the pathways of the mean flow. Both the Hellerman and Rosenstein (1983) (HR) monthly wind stress climatology and 1000-mbar winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) have been used to drive the model. East of about 150øE, they give a mean latitude for the Kuroshio Extension that differs by about 3 ø, approximately 34øN for HR, 37øN for ECMWF, and 35øN observed. The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and April-September mean zero wind stress curl lines (which are similar), while the Kuroshio Extension is associated with wintertime zero wind stress curl. This means that part of the flow from the Kuroshio must pass north of the Kuroshio Extension and connect with the Oyashio and subarctic front. Realistic routes for this connection are flow through the Sea of Japan, a nonlinear route separated from the east coast of Japan, and bifurcation of the Kuroshio at the Shatsky Rise. In addition, the sixlayer simulations show a 3-Sv meridional overturning cell with southward surface flow and northward return flow centered near 400 m depth. Baroclinic instability plays a critical role in coupling the shallow and abyssal layer circulations and in allowing the bottom topography to strongly influence the shallow circulation. By this means the Izu Ridge and Trench and seamounts upstream and downstream of these have profound influence on (1) the mean path of the Kuroshio and its mean meanders south and east of Japan and (2) on separating the northward flow connecting the Kuroshio and the Oyashio/subarctic front from the east coast of Japan. Without the topographic influence the models show an unrealistic northward current along the east coast of Japan. In essence, the topography regulates the location and strength of the baroclinic instability. The baroclinic instability gives eddy-driven deep mean flows that follow the f/h contours (where f is the Coriolis parameter and h is the depth of the water column) of the bottom topography. These abyssal currents then strongly influence the pathway for subtropical gyre flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan. This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (World Ocean Circulation Experiment line PCM 7). The meander path south Paper number 95JC01674. 0148-0227/96/95 J C-01674 $05.00 941 942 HURLBURT ET AL.' DYNAMICS OF THE KUROSHIO/OYASHIO CURRENT SYSTEM of Japan depends on the occurrence of baroclinic instability west of the Izu Ridge; otherwise, a straight path occurs. The pathway shows little sensitivity to the Tokara Strait transport over the range simulated (36-72 Sv in yearly means). However, interannual increases in wind forcing or Tokara Strait transport give rise to a predominant meander path, while decreases yield a predominant straight path. Resolution of 1/8 ø in an ocean model is comparable to the 2.5 ø resolution used in atmospheric forecast models in the early 1980s based on the first internal mode Rossby radius of deformation. Model comparisons at 1/8 ø and 1/16 ø resolution and comparisons with current meter data and Geosat altimeter data show that 1/16 ø resolution is needed for adequate eastward penetration of the high eddy kinetic energy associated with the Kuroshio Extension. 1. Introduction The dynamics of the Kuroshio/Oyashio current system are investigated by using several variations of an eddy-resolving numerical ocean model covering the Pacific Ocean north of 20øS and by model-data comparisons. The horizontal grid resolution of the principal simulations is 1/8 ø for each variable (1/8 ø in latitude by 45/256 ø in longitude), and the vertical structure ranges from 1.5-layer reduced gravity to six layers with realistic bottom topography. In addition, it was possible to extend one of the 1/8 ø simulations at 1/16 ø resolution and two different eddy viscosities. Simulations at 1/4 ø resolution and comparisons of 1/8 ø and 1/16 ø subtropical gyre simulations are also used. The subtropical gyre model treats that gyre in isolation by incorporating solid boundaries at 15øN, 48øN, 121øE, and 140øW. The basic model has a free surface, and all versions of the Pacific model include the deep marginal seas such as the Bering Sea, the Sea of Okhotsk, the Sea of Japan, the South China Sea, the Sulu Sea, and the Indonesian archipelago. The subtropical gyre model includes the Sea of Japan. These seas can play a significant role in the main basin dynamics. The Sea of Japan plays a role in the Kuroshio/Oyashio dynamics, which is discussed in the paper. The modularity of layered ocean models can be used to include or exclude certain features or dynamical processes. For example, a 1.5-layer reduced gravity model can allow barotropic instability but excludes baroclinic instability. This approach and comparisons with observations form the paradigm used to identify the essential model features and dynamical processes for realistic simulations. Section 2 covers the model design, and in section 3, simulations of the basic upper ocean features of the equatorial and North Pacific are discussed. Kuroshio/Oyashio current system dynamics are discussed in section 4. Topics include (1) the development of the basic current system including the roles of the wind curl, nonlinearity, bottom topography, and the Sea of Japan; (2) the essential model features and dynamics required for realistic simulation of the Kuroshio Extension; (3) how the Kuroshio feeds part of its transport north of the Kuroshio Extension and into the Oyashio/subarctic frontal region; and (4) the combined effects of baroclinic instability and bottom topography on the meandering of the Kuroshio/Kuroshio Extension mean path and in generating local maxima in variability. The dynamics of the meander path versus straight path for the Kuroshio south of Japan has been a topic of considerable debate (e.g., see Yoon and Yasuda [1987], who include a substantial review of the earlier literature, and Yamagata and Umatani [1989]). Section 5 discusses the eastward penetration of the Kuroshio Extension including model-data comparisons from the surface to 4000 m depth and the effects of model resolution (1/8 ø versus 1/16ø). Previous investigations of this topic for the Kuroshio and/or the Gulf Stream include those by Schmitz and Holland [1982, 1986], Holland and Schmitz [1985], Thompson and Schmitz [1989], Hogan et al. [1992], Marshall and Marshall [1992], and Schmitz and Thompson [1993]. Section 6 contains the summary and conclusions. Hurlburt et al. [1992] discuss an initial 1/8 ø six-layer Pacific simulation with realistic bottom topography. Hogan et al. [1992] include model-Geosat comparisons for a 1/8 ø two-layer version of the model. Jacobs et al. [this issue] and Mitchell et al. [this issue] investigate additional aspects of the dynamics and provide extensive comparisons between Geosat altimeter measurements and more recent simulations by improved versions of the 1/8 ø six-layer Pacific model run 1981-1993. These simulations are also used in model-data comparisons in section 5. 2. The Model The model is a primitive equation layered formulation where the model equations have been vertically integrated through each layer. It is a descendent of the semi-implicit free-surface model of Hurlburt and Thompson [1980] but with expanded capability [Wallcraft, 1991]. The equations for the n layer finite depth, hydrodynamic model are for layers k = looon OVa Ot --+ (V. Va + Va' V)va + fcx fVa = -ha E Gt,V(hl-Hi) + max (0, to0va+, /=1
Climate Dynamics
The Kuroshio Extension (KE) shifts between elongated and convoluted states on interannual to decadal time scales. The nature of this low frequency variability (LFV) is still under debate since it is known to be driven by intrinsic oceanic mechanisms, but it is also synchronized with the Pacific Decadal Oscillation (PDO). In this analysis we present the results from two present-climate coupled simulations performed with the CMCC-CM2 model under the CMIP6 HighResMIP protocol and differing only by their atmospheric component resolution. The impact of increased atmospheric resolution on the KE LFV is assessed inspecting several aspects: the KE bimodality, the large-scale variability and the air–sea interactions. The KE LFV and the teleconnection mechanism that connects the KE and the PDO are well captured by both configurations. However, higher atmospheric resolution favors the occurrence of the elongated state and leads to a more realistic PDO representation. Moreover, both simulations...
Journal of Physical Oceanography, 2008
Middepth, time-mean circulation in the western North Pacific Ocean (28°-45°N, 140°-165°E) is investigated using drift information from the profiling floats deployed in the Kuroshio Extension System Study (KESS) and the International Argo programs. A well-defined, cyclonic recirculation gyre (RG) is found to exist north of the Kuroshio Extension jet, confined zonally between the Japan Trench (ϳ145°E) and the Shatsky Rise (ϳ156°E), and bordered to the north by the subarctic boundary along ϳ40°N. This northern RG, which is simulated favorably in the eddy-resolving OGCM for the Earth Simulator (OFES) hindcast run model, has a maximum volume transport at 26.4 Sv across 159°E and its presence persists on the interannual and longer time scales. An examination of the time-mean x-momentum balance from the OFES hindcast run output reveals that horizontal convergence of Reynolds stresses works to accelerate both the eastward-flowing Kuroshio Extension jet and a westward mean flow north of the meandering jet. The fact that the northern RG is eddy driven is further confirmed by examining the turbulent Sverdrup balance, in which convergent eddy potential vorticity fluxes are found to induce the cyclonic RG across the background potential vorticity gradient field. For the strength of the simulated northern RG, the authors find the eddy dissipation effect to be important as well.
Response of ocean dynamics to multiple equilibria of the Kuroshio path South of Japan
Dynamics of Atmospheres and Oceans, 2019
Variability of the Kuroshio path to the south of Japan plays a central role in the local climate change and exerts tremendous influences on the local atmosphere and ocean. In this study, the response of ocean dynamics, in terms of the eddy kinetic energy (EKE), potential vorticity (PV), relative vorticity, and eddy-mean flow interaction, to the Kuroshio path change is discussed. Kuroshio path south of Japan includes the near-shore non-large meander (nNLM), the offshore non-large meander (oNLM), and the typical large meander (tLM). Analyses reveal that the distribution of EKE, PV, relative vorticity, and energy exchange between the eddy field and the mean flow respectively varies with the Kuroshio path: (1) The tLM has the maximum EKE along the path; (2) The positive and negative PV are located at the onshore and offshore side of Kuroshio axis, respevetively; (3) The distributions of anomalous relative voritcity of nNLM, oNLM, and tLM are consistent with sea surface height anomalies (SSHAs); (4) The tLM has the largest energy exchange between the eddy field and the mean flow in terms of the rate of barotropic energy conversion. On the other hand, the stability analysis of ocean currents suggests that the three Kuroshio paths south of Japan have their own intrinsic properties of the instability.