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The Kuroshio Extension (KE) flows eastward at the northern boundary of the North Pacific subtropical gyre. By transporting large amounts of seawater with heat, the KE contributes significantly to the formation of sea surface temperature (SST) fields. Recently, poleward shifts of major ocean gyres in the world ocean, including the North Pacific subtropical gyre, have been highlighted based on basin-scale changes in SST and sea surface height (SSH) distributions. However, a detailed investigation of the long-term meridional KE movement has not been presented. Investigation of KE path changes helps provide insights into long-term changes in the physical fields in the western North Pacific. In this study, we identified the KE path from satellite-derived SSH and surface current velocity data using a front identification method and showed that the KE migrated northward by approximately 210 km during 1993–2021. We further explored the cause of the northward KE shift based on atmospheric reanalysis data and numerical experiments using a high-resolution ocean general circulation model. It was revealed that the northward KE shift is mostly caused by the trend of wind stress curl in the North Pacific during 1993–2021. [ABSTRACT FROM AUTHOR]

Decadal variability of the North Pacific subtropical mode water (STMW) and its influence on the upper thermocline and mixed layer are examined in a four-dimensional variational ocean re-analysis for the Western North Pacific over 30 years (FORA–WNP30). The STMW that forms south of the Kuroshio Extension becomes thick/cold and thin/warm on decadal timescales. These variations are subducted and advected to the south, where thick (thin) STMW causes the upper thermocline to heave up (down) above the STMW, producing cold (warm) temperature anomalies at subsurface depths, with especially large anomalies at the depths of the seasonal thermocline. Temperature anomalies also appear in the mixed layer from March to November, except in September. These anomalies have the same sign as the temperature anomalies of the STMW, although they are due not to the reemergence of the STMW at the surface but to the heaving of the upper thermocline. In the FORA–WNP30, because the formation of the mixed layer temperature anomalies owes much to the increment introduced by data assimilation, the mechanism remains unclear. A heat budget analysis of the mixed layer, however, suggests the importance of entrainment and/or vertical diffusion at the base of the mixed layer for conveying temperature anomalies from the upper thermocline to the mixed layer. The STMW also affects the mixed layer depth. A thick (thin) STMW shoals (deepens) the seasonal thermocline, enhancing (weakening) stratification at depths below the mixed layer and thus hindering (favoring) mixed layer development from July to September. [ABSTRACT FROM AUTHOR]

Sea level variability along the Japanese coast and its relation to the Kuroshio-Kuroshio Extension (KE) are investigated using ocean reanalysis data. The first mode of an empirical-orthogonal-function for the coastal sea-level represents a simultaneous sea-level change along the whole Japanese coast, which is synchronized with sea levels in the KE region , the Japan Sea and the East China Sea. The second mode is characterized by an east–west dipole pattern at the south coast. The first mode is correlated with the Kuroshio variations around the Izu–Ogasawara Ridge (IOR) and tends to be in a positive phase when the Kuroshio takes a nearshore path around IOR. The Kuroshio's position around IOR is closely related to the KE dynamic state. When the KE jet is in a stable (unstable) state, a nearshore (meandering) Kuroshio path is formed around IOR. A composite analysis suggests that the sea level along the Japanese coast becomes high due to propagation of coastal trapped waves when the Kuroshio takes a nearshore path around IOR. That is why the first mode is synchronized with the KE decadal variability. The second mode has a close relation with the Kuroshio Large Meander (LM). The eastern positive anomaly at the coast between the Izu and Kii Peninsulas is formed by warm Kuroshio water brought by a westward branch flow along the coast. The western negative anomaly is attributed to a southward shift in the Kuroshio south of the Kii Peninsula associated with the LM. [ABSTRACT FROM AUTHOR]

Unusually high sea level (UHSL) in early September 1971, which caused coastal flooding around the southern coasts of Japan despite no severe weather conditions, is examined using a coastal assimilation system with 2-km resolution. The observed duration of high sea-level anomalies (SLAs) for the UHSL was successfully represented by assimilation results. Through sensitivity experiments, we investigated the contribution of two factors, the wind and the Kuroshio effects, as suggested by previous research. The northeasterly winds associated with Typhoon 7123 induced coastal-trapped waves (CTWs) along the Kashima-nada Sea. The CTWs propagated clockwise along the coast at a speed of about 2.3 m/s and caused the sea level to increase by about 15 cm in Sagami and Tokyo Bays. A cold eddy associated with a Kuroshio meander was formed, and as a result, the warm Kuroshio water intruded into the Enshu-nada Sea due to the northward flow in the eastern flank of the eddy. The warm water intrusion significantly contributed to the duration of SLAs of about 30 cm around the Enshu-nada and Kumano-nada Seas. It was also shown that the high SLAs for the UHSL were quantitatively explained by the sum of the two factors. Thus, we conclude that the UHSL event was caused by the superposition of the wind-induced CTWs and the Kuroshio-induced warm water intrusions. [ABSTRACT FROM AUTHOR]

Using a coastal assimilation model, generation mechanism of unusually high sea level (UHSL) at the south coast of Japan in September 2011 is investigated. Both model results and tide gauge observations indicate that sea level rise associated with the UHSL event occurred twice in the middle and end of September. The first one, which is localized around the eastern part of the Seto Inland Sea, is caused by a cyclonic circulation in the Kii Channel formed as a result of northward migration of the Kuroshio axis toward Cape Shionomisaki. The second sea level rise, which is the main contributor to this UHSL event, is observed in wide areas not only at the south coast of Japan, but also at the coast of the Japan Sea. It is brought about by a coastal trapped wave (CTW) induced as a result of a fluctuation of the Kuroshio path to the south of the Boso Peninsula. The CTW with positive SSH anomalies propagates westward along the south coast of Japan, and then goes into the coast of the Japan Sea. Sensitivity experiments and a modal characteristic analysis indicate that the CTW is mainly characterized by the first mode baroclinic Kelvin wave. The phase speed for the first mode is calculated at 2.96 m s - 1 , which compares well with that estimated by tide gauge observations. [ABSTRACT FROM AUTHOR]

We developed a new system to monitor and forecast coastal and open-ocean states around Japan for operational use by the Japan Meteorological Agency. The system consists of an eddy-resolving analysis model based on four-dimensional variational assimilation and a high (2-km) resolution forecast model covering Japanese coastal areas that incorporates an initialization scheme with temporal and spatial filtering. Assimilation and forecast experiments were performed for 2008 to 2017, and the results were validated against various observation datasets. The assimilation results captured well the observed variability in sea surface temperature, coastal sea level, volume transport, and sea ice. Furthermore, the volume budget for the Japan Sea was significantly improved by the use of the 2-km resolution forecast model compared with the 10-km resolution analysis model. The forecast results indicate that this system has a predictive limit longer than 1 month in many areas, including in the Kuroshio current area south of Japan and the southern Japan Sea. In the forecast results of case studies, the 2017 Kuroshio large meander was well predicted, and warm water intrusions accompanying Kuroshio path variations south of Japan were also successfully reproduced. Sea ice forecasts for the Sea of Okhotsk largely captured the evolution of sea ice in late winter, but sea ice in early winter included relatively large errors. This system has high potential to meet operational requirements for monitoring and forecasting ocean phenomena at both meso- and coastal scales. [ABSTRACT FROM AUTHOR]

In order to expand the coastal ocean monitoring and forecasting system of the Japan Meteorological Agency from the Seto Inland Sea to the entire coastal seas of Japan, a 2-km resolution model has been newly developed. We investigated the model performance based on a 4-year hindcast experiment without data assimilation. The model realistically reproduced basic distributions and seasonal variations of sea surface temperature, salinity, sea level, etc. Small-scale features in the coastal seas, such as fronts, were also well simulated due to the high resolution. Daily sea level variations on the coasts followed the tide gauge observation with an error of 7.1 cm on average. Transports through main Japanese straits were also consistent with previous research. In addition to expanding the domain, some dynamical elements are newly introduced. One of the major updates is implementing of explicit tides with high precision, as the amplitude error was only 9.2% and the phase error was 10.2∘ for the M2 tide. The second is coupling of a sea ice model reproducing the seasonal development of sea ice in the Sea of Okhotsk. The third is introduction of approximately 4000 river inflows across Japan to improve salinity in coastal seas. Introduction of the inverse barometer effect and high-resolution atmospheric forcings also contributed to coastal sea level variations. The Japan Meteorological Agency plans to widely utilize the model with a new data assimilation system for coastal disaster prevention and oceanic condition monitoring, and our verification results support the utility of the model. [ABSTRACT FROM AUTHOR]

We produced a four-dimensional variational ocean re-analysis for the Western North Pacific over 30 years (FORA-WNP30). It is the first-ever dataset covering the western North Pacific over 3 decades at eddy-resolving resolution. The four-dimensional variational analysis scheme version of the Meteorological Research Institute Multivariate Ocean Variational Estimation system (MOVE-4DVAR) is employed to conduct a long-term reanalysis experiment during 1982-2012. After evaluating the basic performance of FORA-WNP30, the interannual to decadal variability is analyzed. Overall, FORA-WNP30 reproduces basic features in the western North Pacific well. One of outstanding features in FORA-WNP30 is that anomalous events such as the Kuroshio large meander and anomalous intrusion of the Oyashio in the 1980s, when there were no altimeter data, are successfully reproduced. FORA-WNP30 is therefore a valuable dataset for a variety of oceanographic research topics and potentially for related fields such as climate study, meteorology and fisheries. [ABSTRACT FROM AUTHOR]

The effects of the El Niño Southern Oscillation (ENSO)-mediated environmental changes to marine resources were long recognized; however, species-specific responses were also reported to vary, possibly more so, under the emerging event-to-event diversity of the ENSO conditions. Hence, the objective of this study is to characterize the potential impacts of the ENSO-regulated environmental variability to squid habitat in the central North Pacific under the different ENSO flavors, using habitat models developed from at least a decade of fishery-dependent dataset and environmental parameters. Our findings revealed that the potential squid habitats were largely influenced by ENSO-forced environmental changes during the squid's spawning and nursery periods, resulting in substantial reduction/enhancement of available habitats in the succeeding summers of Central Pacific El Niño/La Niña, where the latter led to an expansion of favorable spawning and nursery grounds. However, the autumn-winter periods of weaker and short-lived Eastern Pacific El Niño showed elevated potential habitats due to minimal sea surface temperature drop and close proximity of spawning and nursery grounds to optimal feeding environments. The quick, albeit variable, squids' responses to ENSO flavors accentuate their promising potential as ecological beacons under climate changes, aiding the development of adaptive management strategies for commercially exploited fisheries. [ABSTRACT FROM AUTHOR]

A multivariate data assimilation experiment was conducted in order to improve the global representation of both the ocean and sea ice fields through the inclusion of sea ice concentration (SIC) data. Our method corrects the surface forcing and ocean temperature fields (as well as the SIC field) through the use of three-dimensional variational analysis. The adjustments to surface air temperatures resulting from the SIC assimilation are estimated on the basis of two constraints. First, we assume that the interfacial temperature difference between the surface air and the average value at the 'top' of the grid (which represents a weighted mean according to the relative coverage of sea ice to open water within the grid) is maintained at the pre-assimilation value. Similarly, the vertical temperature structure for each of the five sea ice categories considered here remains unchanged throughout the assimilation. In making the necessary adjustments to upper-layer ocean temperatures, we again adopt a weighting procedure based on the condition that ice-free water temperature must remain the same. Thus, areas containing sea ice are allotted the freezing-point temperature such that the weighted mean value across the grid can be derived. The reproduction of the SIC field in both hemispheres is improved by incorporating the resulting corrections to the surface forcing and ocean temperature values, indicating that these boundary conditions produce results that are more consistent with the corrected SIC field in the sea ice model. The enhanced ocean-sea ice fields provide initial conditions that are better suited for coupled atmosphere-ocean-sea ice prediction experiments. [ABSTRACT FROM AUTHOR]

We have developed a coastal model of the Seto Inland Sea, Japan, for a monitoring and forecasting system operated by the Japan Meteorological Agency (JMA). We executed a hindcast experiment using reanalysis datasets for the atmospheric and lateral boundaries without ocean initialization by data assimilation. The seasonal variability is verified to be realistic by comparing sea surface temperature and salinity of the hindcast experiment with observations. With a horizontal resolution of approximately 2 km, the model represents explicitly various coastal phenomena with a scale of 10-100 km, such as the Kuroshio water intrusion into Japanese coasts. This leads to good representation of intramonthly variations. For example, intensity of the sea level undulations with a period shorter than 23 days shows 1.6-fold improvement, as compared to the present model of JMA with the horizontal resolution of approximately 10 km. In addition to the increased resolution, the model is optimized for coastal modeling as follows. Incorporation of a tidal mixing parameterization reduces a high temperature bias in the Bungo Channel (a western channel of the Seto Inland Sea) and contributes to formation of a frontal structure. An accurate dataset of the river discharges is used for runoff, which has a strong impact on salinity. Enhancement of coastal friction improves surface currents. Owing to the increased resolution and these optimizations, the model shows realistic variability in a wide temporal range from several days to seasons. Root-mean-square errors of sea surface temperature and heights are evaluated as 1-2 K and 7-10 cm, respectively, without data assimilation. In the eastern part, however, the predictability is relatively low, which might be related to representation of an eastward mean flow in the Seto Inland Sea. [ABSTRACT FROM AUTHOR]

The volume transports measured at the Korea/Tsushima, Tsugaru, and Soya/La Perouse Straits remain quantitatively inconsistent. However, data assimilation models at least provide a self-consistent budget despite subtle differences among the models. This study examined the seasonal variation of the volume transport using the multiple linear regression and ridge regression of multi-model ensemble (MME) methods to estimate more accurately transport at these straits by using four different data assimilation models. The MME outperformed all of the single models by reducing uncertainties, especially the multicollinearity problem with the ridge regression. However, the regression constants turned out to be inconsistent with each other if the MME was applied separately for each strait. The MME for a connected system was thus performed to find common constants for these straits. The estimation of this MME was found to be similar to the MME result of sea level difference (SLD). The estimated mean transport (2.43 Sv) was smaller than the measurement data at the Korea/Tsushima Strait, but the calibrated transport of the Tsugaru Strait (1.63 Sv) was larger than the observed data. The MME results of transport and SLD also suggested that the standard deviation (STD) of the Korea/Tsushima Strait is larger than the STD of the observation, whereas the estimated results were almost identical to that observed for the Tsugaru and Soya/La Perouse Straits. The similarity between MME results enhances the reliability of the present MME estimation. [ABSTRACT FROM AUTHOR]

A consistency between seasonal fluctuation of actual sea surface height (SSH) and those caused by mass and density variations in gyre-scale regions is examined. The SSH obtained from satellite altimetry (altimetric SSH) is adopted as the actual SSH. SSH caused by mass variation (mass-related SSH) is simulated using a barotropic global ocean model forced by water flux, wind stress and surface pressure. SSH caused by density variation (steric SSH) is calculated from water density profile, i.e. temperature and salinity profiles. The model SSH well represents mass-related SSH for gyre-scale regional means, and seasonal fluctuation of the altimetric SSH corrected for the model SSH is similar to that of steric SSH above a pressure level larger than 300 dbar. The results indicate that the mass-related SSH does not much respond to the baroclinic adjustment to the seasonally varying wind stress curl. The mass-related SSH forced by wind stress and surface pressure should be accounted for regional evaluation, though it is not necessary for global mean evaluation. Detection of steric SSH from altimetric SSH would be useful for assimilation approaches in which the altimetric SSH is treated as the variable reflecting subsurface temperature and salinity. [ABSTRACT FROM AUTHOR]

This study investigates the long-term variability of the Kuroshio path south of Japan. Sensitivity experiments using a data-assimilative model suggest that the duration of the large meander (LM) strongly depends on the Kuroshio transport; specifically, low transport leads to a long duration of the LM. Actually, we find a good correlation between the duration of the past LMs and the Sverdrup transport estimated by a wind-driven linear baroclinic vorticity model. Then we explore favorable conditions for the LM and find a close relationship between the Kuroshio Extension (KE) state and the LM. That is, a precondition for the LM that the Kuroshio path on the Izu Ridge is fixed at a deep channel located around 34°N is achieved during a stable KE state. In addition, westward propagating signals with negative anomalies in the Kuroshio region and high sea-surface height (SSH) state east of Taiwan are key for generation of a small meander southeast of Kyushu that triggers a subsequent LM. The signals related to the above conditions change the upstream Kuroshio transport and velocity, which are consistent with features indicated by the former observational studies. Using reanalysis data, we construct long-time series of indices for the three conditions, which explain well the past LMs. The indices suggest that long-term non-LM states around 1970 and in the 1990s were attributed to a low-SSH state east of Taiwan and an unstable KE state, respectively. [ABSTRACT FROM AUTHOR]

The effects of large-scale wind forcing on the bimodality of the Kuroshio path south of Japan, the large meander (LM) and non-large meander (NLM), were studied by using a historical simulation (1948-2007) with a high-resolution Ocean general circulation models (OGCM). The Kuroshio in this simulation spent much time in the NLM state, and reproduced several aspects of its long-term path variability for the first time in historical OGCM simulation, presumably because the eddy kinetic energy was kept at a moderate level. By using the simulated fields, the relationships between wind forcing (or Kuroshio transport) and path variation proposed by past studies were tested, and specific roles of eddies in those variations were investigated. The long-term variation of the simulated net Kuroshio transport south of Japan was largely explained by the linear baroclinic Rossby wave adjustment to wind forcing. In the simulated LM events, a triggering meander originated from the interaction of a wind-induced positive sea surface height (SSH) anomaly with the upstream Kuroshio and was enlarged by cyclonic eddies from the recirculation gyre. The cyclonic eddy of the trigger meander was followed by a sizable anticyclonic eddy on the upstream side. Subsequently, a weak (strong) Kuroshio favored the LM (NLM). The LM tended to be maintained when the Kuroshio transport off southern Japan was small, and increasing Kuroshio transport promoted decay of an existing LM. The supply of disturbances from upstream, which is related to the wind-induced SSH variability at low latitudes, contributed to the maintenance of an existing LM. [ABSTRACT FROM AUTHOR]

We assess validity of a Gaussian error assumption, the basic assumption in data assimilation theory, and propose two kinds of constraints regarding non-Gaussian statistics. In the mixed water region (MWR) off the east coast of Japan exhibiting complicated frontal structures, a probability density function (PDF) of subsurface temperature shows double peaks corresponding to the Kuroshio and Oyashio waters. The complicated frontal structures characterized by the temperature PDF sometimes cause large innovations, bringing about a non-Gaussianity of errors. It is also revealed that assimilated results with a standard three-dimensional variational (3DVAR) scheme have some issues in MWR, arising from the non-Gaussianity of errors. The Oyashio water sometimes becomes unrealistically cold. The double peaks seen in the observed temperature PDF are too smoothed. To improve the assimilated field in MWR, we introduce two kinds of constraints, J and J, which model the observed temperature PDF. The constraint J prevents the unrealistically cold Oyashio water, and J intends to reproduce the double peaks. The assimilated fields are significantly improved by using these constraints. The constraint J effectively reduces the unrealistically cold Oyashio water. The double peaks in the observed temperature PDF are successfully reproduced by J. In addition, not only subsurface temperature but also whole level temperature and salinity (T-S) fields are improved by adopting J and J to a multivariate 3DVAR scheme with vertical coupled T-S empirical orthogonal function modes. [ABSTRACT FROM AUTHOR]

Assimilation and prediction experiments of the Kuroshio path variability south of Japan were conducted to investigate the predictability of the Kuroshio path. The assimilation and prediction system is composed of an eddy-resolving model and a three-dimensional variational analysis scheme with vertical coupled temperature–salinity empirical orthogonal function modes. The sea surface height (SSH) variability and the variations of the Kuroshio path of the assimilation fields are in good agreement with those observed. The results of the assimilation are then used as the initial conditions for 138 cases of 90-day prediction experiments conducted from 1993 to 2004. The predictive limit of our system is assessed by the SSH anomaly in the assimilation field and is found to be around 40–60 days, which is much longer than that of the persistence. The prediction results show good performance in the transition stage from a straight to a meandering path. For example, a large meandering event that occurred in August 2004 is successfully predicted in a 2-month forecast. Two types of failure cases are investigated. One is a case where the eastward propagation speed of the meander is faster than a real state. The dynamical response of the model to the assimilation revealed that an initial shock, caused by the dynamically unbalanced initial condition, induces the fast eastward propagation of the meander. The other case exhibits an unrealistic meander. In this case, a cold anomaly at an intermediate layer in the initial condition grows rapidly and results in the unrealistic meander. This implies that the Kuroshio path south of Japan has a chaotic nature. These facts revealed by the failure cases give us some insight for improving the predictive skill of the Kuroshio path variability. [ABSTRACT FROM AUTHOR]

In the original publication of the article, Fig. 18 was published incorrectly. The correct version of Fig. 18 is provided here. [ABSTRACT FROM AUTHOR]