The Instabilities and Multiscale Energetics Underlying the Mean–Interannual–Eddy Interactions in the Kuroshio Extension Region (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.
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.
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.
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...
Submesoscale Eddy and Frontal Instabilities in the Kuroshio Interacting With a Cape South of Taiwan
Journal of Geophysical Research: Oceans
The processes underlying the strong Kuroshio encountering a cape at the southernmost tip of Taiwan are examined with satellite-derived chlorophyll and temperature maps, a drifter trajectory, and realistic model simulations. The interaction spurs the formation of submesoscale cyclonic eddies that trap cold and high-chlorophyll water and the formation of frontal waves between the free stream and the wake flow. An observed train of eddies, which have relative vorticity about one to four times the planetary vorticity (f), is shed from the recirculation that occurs in the immediate lee of the cape as a result of flow separation. These propagate downstream at a speed of 0.5-0.6 m s −1. Farther downstream, the corotation and merging of two or three adjacent eddies are common owing to the topography-induced slowdown of eddy propagation farther downstream. It is found that the relative vorticity of a corotating system (1.2f) is 70% weaker than that of a single eddy due to the increase of eddy diameter from~16 to~33 km, in agreement with Kelvin's circulation theorem. The shedding period of the submesoscale eddies is strongly modulated by either diurnal or semidiurnal tidal flows, which typically reach 0.2-0.5 m s −1 , whereas its intrinsic shedding period is insignificant. The frontal waves predominate in the horizontal free shear layer emitted from the cape, as well as a density front. Energetics analysis suggests that the wavy features result primarily from the growth of barotropic instability in the free shear layer, which may play a secondary process in the headland wake.
Roles of Mesoscale Eddies in the Kuroshio Paths
Journal of Physical Oceanography, 2004
A high-resolution ocean general circulation model is developed to simulate connections between the Kuroshio path variations and mesoscale eddy activities as realistically as possible. The climatological mean of the modeled Kuroshio takes a nearshore nonlarge meander path. It is found that the model is capable of simulating two types of nonlarge meander state and a possible version of the large meander state. The offshore nonlarge meander is generated through interaction between the Kuroshio and an anticyclonic eddy. The large meander occurs just after significant intensification of the anticyclonic Kuroshio recirculation; successive intrusion of anticyclonic eddies from the upstream region is responsible for this process. Those anticyclonic eddies are advected by the Kuroshio from the region northeast of Luzon Island and increase the upstream Kuroshio volume transport on an interannual time scale. The cyclonic eddies propagating from the Kuroshio Extension region, on the other hand,...
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.
Statistical features of eddies approaching the Kuroshio east of Taiwan Island and Luzon Island
Journal of Oceanography
18°N-19°N and 45 ± 17 days at 22°N-23°N) than at other latitudes. These two eddy-intrusion bands are associated with the northern and southern Subtropical Countercurrents (STCCs). These STCCs have a vertically reversed sign of the meridional potential vorticity gradient, thus providing a key energy source for eddy generation. In addition, when westward-propagating eddies approach the Ryukyu Islands, the southwestward recirculation flow east of the island chain as well as topographic effects cause some eddies to head southwestward to the east of Taiwan and intrude into the Kuroshio at 22°N-23°N, rather than to dissipate directly. Therefore, we suggest that the STCCs play a key role in inducing the eddies to frequently intrude into the Kuroshio at 18°N-19°N and 22°N-23°N. In addition, the Ryukyu Islands are responsible for concentrating the eddies within 22°N-23°N.
On the eddy-Kuroshio interaction: Meander formation process
Journal of Geophysical Research, 2003
1] The Kuroshio flows along the southern coast of Japan during its non-largemeandering state, then separates from the coast near the Kii Peninsula and attaches at the north of the Izu Ridge. The amplitude of the offshore displacement of the Kuroshio changes as a result of mesoscale perturbations. Satellite SSH and SST observations (TOPEX/Poseidon: T/P and NOAA AVHRR) have suggested that the short-term Kuroshio meander formation is triggered by anticyclonic eddies originating in the Kuroshio Extension. To simulate such an event, a numerical experiment using a highresolution regional GCM was conducted by initializing the eddy with the observed T/P sea level anomaly. An anticyclonic eddy was injected to the south of the Kuroshio (140°E, 30°N) by means of sequential data assimilation of the T/P data of October 1992 for 30 days. The volume transport of the model Kuroshio was kept constant at 25 Sv, a condition which would not cause large meander of the Kuroshio. The simulation successfully reproduced the four phases of the interaction: (1) westward propagation of the eddy; (2) advection of the eddy by the Kuroshio; (3) meander formation; and (4) detachment of the eddy from the Kuroshio and their repetition. The analyses revealed that the inshore high potential vorticity (PV) water is generated at the sharp coastal topography (the Kii peninsula). The cyclonic eddies shed will eventually coalesce with the existing inshore cyclonic circulation and the meander grows. During the growth of the meander, the necessary barotropic kinetic energy is produced through the shallowing of the thermocline of the anticyclonic eddy as it elongates and splits. The growth of the meander ceases when the split anticyclonic eddies merge, the thermocline deepens, and the eddy detaches itself from the Kuroshio as a result of its own westward thrust. Simultaneously, the accumulated high PV inshore is released to the Kuroshio Extension region as cyclonic eddies. This study provides evidence for the active role of the anticyclonic eddy in causing the variability of the Kuroshio path and suggests a mechanism of rapid discharge/recharge of the available potential energy of the eddy and production/release of high PV inshore Kuroshio to cause the short-term Kuroshio meander.