Ocean modelling and altimeter data reveal the possible occurrence of intrinsic low-frequency variability of the Kuroshio Extension (original) (raw)
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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...
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.
Kuroshio Extension dynamics from satellite altimetry and a model simulation
Journal of Geophysical Research, 1996
Altimeter data from the Geosat Exact Repeat Mission (ERM) are analyzed with the aid of a simulation from an eddy-resolving primitive equation model of the North Pacific basin in the region of the Kuroshio and Kuroshio Extension. The model domain covers the Pacific Ocean north of 20øS and has a resolution of 0.125 ø latitude and 0.176 ø longitude. The model is synoptically driven by daily 1000-mbar winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) which encompass the Geosat time period. Model output is sampled along Geosat ground tracks for the period of the ERM. Additionally, the model and the Geosat data are compared with climatological hydrography and satellite IR frontal position analyses. Analyses compared include maps of sea surface height (SSH) mean and variability, eddy kinetic energy (EKE), seasonal transport anomaly, and time-longitude plots of SSH anomaly. The model simulation provides annual mean SSH fields for 1987 and 1988 which reproduce the four quasipermanent meanders seen in hydrographic climatology (cyclonic at 138øE and anticyclonic at 144øE, 150øE, and 160øE). These are linked to the bottom topography. In the model simulation, Geosat altimeter data, and climatology, we observe four peaks in SSH variability associated with meander activity and two peaks in EKE, with the strongest about 3200 cm 2 s -2 along the mean Kuroshio path in the Geosat data. The local maxima in SSH variability tend to occur where relatively strong, topographically steered meridional abyssal currents intersect the zonally oriented Kuroshio Extension. Westward propagation of SSH anomalies at phase speeds of 2 to 3 cm s -• in the region east of 155øE is observed in the model simulation and Geosat observations. A late summer maximum in the upper ocean transport anomaly of the Kuroshio Extension is inferred from changes in the crossstream differential in SSH from the simulation and Geosat observations.
Journal of Oceanography, 2000
A finite-difference quasigeostrophic (QG) model of an open ocean region has been employed to produce a dynamically constrained synthesis of acoustic tomography and satellite altimetry data with in situ observations. The assimilation algorithm is based upon the 4D variational data interpolation scheme controlled by the model's initial and boundary conditions. The data sets analyzed include direct and differential travel times measured at the array of five acoustic transceivers deployed by JAMSTEC in the region of the Kuroshio Extension in 1997, Topex/Poseidon altimetry, CTD soundings, and ADCP velocity profiles. The region monitored is located within the area 27.5°-36.5°N, 143°-155°E. The results of assimilation show that mesoscale variability can be effectively reconstructed by five transceivers measuring direct and reciprocal travel times supported by relatively sparse in situ measurements. The misfits between model and data lie within the observational error bars for all the data types used in assimilation. We have compared the results of assimilation with the statistical inversion of travel time data and analyzed energy balances of the optimized model solution. Energy exchange between the depth-averaged and shear components of the observed currents reveals a weak decay of the barotropic mode at the rate of 0.2 ± 0.7·10 -5 cm 2 /s 3 due to topographic interaction. Mean currents in the region are unstable with an estimate of the available potential energy flux from the mean current to the eddies of 4.7 ± 2.3·10 -5 cm 2 /s 3 . Kinetic energy transition has the same sign and is estimated as 2.8 ± 2.5·10 -5 cm 2 /s 3 . Potential enstrophy is transferred to the mesoscale at a rate of 5.5 ± 2.7·10 -18 s -3 . These figures provide observational evidence of the properties of free geostrophic turbulence which were predicted by theory and observed in numerical experiments.
Inter-decadal modulations in the dynamic state of the Kuroshio Extension system: 1905-2016
Japan Geoscience Union, 2017
The Kuroshio Extension is an eastward-flowing, inertial jet in the subtropical western North Pacific after the Kuroshio separates from the coast of Japan. Being the extension of a wind-driven western boundary current, the KE has long been recognized as a turbulent current system rich in large-amplitude meanders and energetic mesoscale eddies. An important feature emerging from the past 25-yr satellite altimeter measurements is that the KE system exhibits clearly-defined decadal modulations between a stable and an unstable dynamic state. The decadally-modulating KE dynamic state exerts a great impact on the regional sea surface temperature, heat content and water mass properties. By clarifying the relationship and physical processes between the basin-scale wind forcing and the KE dynamic state over the altimeter era, we hindcast the KE dynamic state going back to 1905 with the use of ECMWF reanalysis wind stress product. It is found that the low-frequency KE variability modulated in ...
A Coupled Decadal Prediction of the Dynamic State of the Kuroshio Extension System
Journal of Climate, 2014
Being the extension of a wind-driven western boundary current, the Kuroshio Extension (KE) has long been recognized as a turbulent current system rich in large-amplitude meanders and energetic pinched-off eddies. An important feature emerging from recent satellite altimeter measurements and eddy-resolving ocean model simulations is that the KE system exhibits well-defined decadal modulations between a stable and an unstable dynamic state. Here the authors show that the decadally modulating KE dynamic state can be effectively defined by the sea surface height (SSH) anomalies in the 31°–36°N, 140°–165°E region. By utilizing the SSH-based KE index from 1977 to 2012, they demonstrate that the time-varying KE dynamic state can be predicted at lead times of up to ~6 yr. This long-term predictability rests on two dynamic processes: 1) the oceanic adjustment is via baroclinic Rossby waves that carry interior wind-forced anomalies westward into the KE region and 2) the low-frequency KE varia...
Journal of Geophysical Research, 1995
The Geosat radar altimeter data during the first year of its exact repeat mission are analyzed to investigate the behavior of individual anomalies of sea surface dynamic topography (SSDT) and their statistical properties in the Kuroshio Extension region. The SSDT anomalies are compared with mesoscale anomalies of sea surface temperature (SST) derived from satellite radiometer data. Remarkable baroclinic anomalies are detected in the time series of both SSDT and SST, and their westward propagation is clearly traced. The pinching-off of a Kuroshio Extension meander at 147øE is clearly seen as the coalescence of two cyclonic anomalies which had moved fairly constantly westward without changing their relative positions. Westward propagation is statistically dominant in the SSDT and SST anomaly field. Approximate agreement between SSDT and SST anomalies suggests the baroclinic nature of the anomalies. The westward phase speed of SSDT anomalies is faster than the theoretical phase speed of the baroclinic first-mode long Rossby wave in the upstream region of the Kuroshio Extension. In the downstream region it is consistent with the theoretical value but is somewhat faster around 35øN. In the area between 170øE and 180 ø, SSDT anomalies have a structure like plane waves with major axis oriented NW-SE in the northern part and NE-SW in the s•,uthern part. The distribution of the Reynolds stress suggests the tendency of acceleration of the mean current by those anomalies. This strong acceleration tendency at 170øE-180 ø may be related to bathymetry. 839 their physical aspects and their influence of fisheries [Kawai, 1972; Kitano, 1975; Tomosada, 1978]. For instance, Yasuda et al. [1992] thoroughly described the process of interaction between warm core rings. However, only a little is known about the detailed behavior of Kuroshio cold core rings [Cheney, !977; Mizuno and White, 1983]. This is partly because their locations are outside the area of routine observations by Japanese agencies and therefore data coverage is insufficient to describe their precise developments. It is important to estimate statistically the eddy energy and propagation characteristics of anomalies, in order to understand the mechanism of their generation and spread. The eddy kinetic energy is remarkably high in the Kuroshio Extension region [Wyrtki et al., 1976]. Numerical models have been used to investigate various mechanisms for the generation of anomalies and the energy flows intrinsic to each mechanism. In reality, the eddy kinetic energy is not homogeneously dis-840 AOKI ET AL.: BAROCLINIC DISTURBANCES IN KUROSHIO EXTENSION 140E 150E 160E 170E 180 LON(;I TUDE 40N 50N 170W Figure 1. Distribution of geopotential anomalies at the sea surface referred to 1000 dbar in the Kuroshio Extension region [Teague et al., 1990]. Contour interval is 0.1 dynamic meter. tributed in this area because the different mechanisms combine. Therefore it is important to observe its detailed distribution in order to understand the actual mechanism of generation of the anomalies. Bernstein and White [1977] reported from temperature sections a striking decrease of baroclinic eddy energy at wavelengths of 500-1000 km east of 170øW. Nishida and White [1982] showed that the baroclinic eddy kinetic energy is larger to the west of the Shatsky Rise (Figure 2) than to the east. Propagation properties of anomalies are also different; the phase speeds of temperature anomalies at 300 m depth are different to the west and to the east of 155øE [Mizuno and White, 1983]. There are some conjectures that these regional differences in the properties of anomalies may have some relationship with major bathymetric features. The subsurface temperature fronts in the Kuroshio Extension region seem to be related with major bathymetric features [Levine and White, 1983]. However, there are too few studies of the eddy energy and propagation characteristics to describe fully, with well-distributed data coverage, their differences among regions. Mesoscale anomalies are said to have an important role in maintaining the general ocean circulation. In connection with momentum transfer, "negative viscosity," which means the tendency to transfer momentum from anomalies to the mean current, has been observed in the Gulf Stream [Webster, 1961; Schmitz, 1977]. In the Kuroshio Extension, similar results were suggested from observations of expendable bathythermograph (XBT) [Nishida and White, 1982], but the detailed distribution of momentum transfer by anomalies can not be described thoroughly from conventional ship observation data.
Interannual Modulation of Kuroshio in the East China Sea Over the Past Three Decades
Frontiers in Marine Science
Previous studies have suggested that westward-migrating mesoscale eddies are a dominant factor that modulate the interannual Kuroshio intensity in the East China Sea (ECS), indicating a close positive correlation between them. According to the extended record of altimetry-based sea level anomalies (SLAs) until 2020, however, the interannual variation of the Kuroshio intensity no longer has a strong positive correlation with eddy activity in the subtropical countercurrent (STCC) region since the early 2000s. Our observational analyses showed that the Kuroshio intensity in the ECS can be modulated by the combined effect of westward-migrating mesoscale eddies and westward-propagating oceanic planetary waves from the east. Until the early 2000s, the interannual variability of Kuroshio was mainly affected by eddy migration from the STCC region, associated with oceanic instability driven by large-scale wind patterns over the western North Pacific. Since then, oceanic planetary waves propa...