Southern Coast Of Japan Research Articles

Roles of Air–Sea Interactions in the Predictability of Typhoon Mawar and Remote Heavy-Rainfall Events after Five Days

This study investigated the relationship between the predicted track of Typhoon Mawar (2023) and the quasi-stationary front along the southern coast of Japan where heavy rainfall occurred. Also, the role of ocean coupling was explored by using global model predictions and numerical simulations conducted by a regional atmosphere–wave–ocean coupled model. The track predictions by four major global models showed that the prediction errors became significantly larger after the recurvature. One of the global models could reasonably predict both the track and the location of the front, even after five days. The results of numerical simulations of which the initial and boundary conditions were based on the successful predictions suggest that ocean coupling contributes to the improvement of central pressure simulations compared with fixed oceanic conditions. More northward translation of Mawar after the recurvature simulated by the coupled model could be explained by the separation of the inner-core vortex into two parts in the upper and lower troposphere. However, the predictability of the Subtropical High was more important in determining not only the track but also environmental southerly flow over the moisture road formed between Mawar and the Subtropical High and in accurately predicting the location of the front.

Growth rate estimation by 210Pb chronology in precious corals collected off the southern coast of Japan

The Lead 210 (210Pb) concentrations were determined to estimate the skeletal growth rates in the four species of precious corals including Japanese red coral (Corallium japonicum), pink coral (Pleurocorallium elatius), white coral (P. konojoi), and a deep-sea coral (Coralliidae sp.), and in a bamboo coral (Keratoisis sp.). Colonies were collected from the southern coast of Japan, western North Pacific margin. The 210Pb concentrations typically showed the exponential decrease with distance from the edge to the center of the skeleton axis in a cross section of main stem of colonies. The mean radial growth rates for colonies were estimated from the slopes of the best fit curves for exponential decrease in 210Pb concentration along the multiple transects. The obtained mean and standard deviation of radial growth rates in the diameter (the number of transects, n) for three colonies of C. japonicum were 0.31 mm/y (n = 1), 0.21 ± 0.05 mm/y (n = 2), and 0.36 ± 0.13 mm/y (n = 2), respectively, which were broadly comparable to the previously reported growth rates obtained by growth ring counting. Those obtained for three white coral colonies were 0.52 mm/y (n = 1), 0.60 mm/y (n = 1), and 0.36 ± 0.25 mm/y (n = 2). The radial growth rate for the bamboo coral colony was 0.13 mm/y in radius. The linear growth of the main steam of some colonies was analyzed from several cross sections. The linear growth rates of colonies could be estimated from the slope of the best fit curve (1) for increasing mean 210Pb concentration along the cross transects of the axis and (2) for increasing center 210Pb concentration of the skeleton axis. The linear growth rates calculated from the mean and center concentrations for two C. japonicum colonies were 8.5 and 6.1 mm/y, and 1.8 and 1.4 mm/y, respectively. Those for a P. elatius colony were 4.7 and 3.4 mm/y. The difference between the two estimates may reflect the morphological structure of the skeleton. The growth rates measured for the radial increase and linear extension of the main stem of the colonies could provide basic information for the demographic study of these coral species thereby fostering a rational exploitation of their populations.

Mesoscale movement patterns of common bottlenose dolphins along Kuroshio Current and its bifurcation

AbstractIn the waters off Japan, the patterns of the Kuroshio Current (KC) and its bifurcations are highly variable, which significantly affect the physical and biological environments. Knowledge about species' habitats of marine apex predators such as cetaceans and their variability is fundamental to understand not only their population structures but also their ecosystem structures related through bottom‐up and top‐down trophic linkages. In dynamic ocean environments, however, such crucial information has not been provided adequately by snapshot‐based conventional approaches, such as sighting surveys. Spatiotemporal habitat utilization patterns of the common bottlenose dolphin (Tursiops truncatus) were investigated using fin‐mounted Advanced Research and Global Observation Satellite (ARGOS) tags. Behavioral states at each tracking location were categorized into transiting‐ or area‐restricted searching (ARS) modes based on Bayesian state‐space model. Twelve satellite‐tracked dolphins showed different movement patterns. Their habitat ranges were estimated by minimum convex polygons (MCPs), and movement patterns were classified into three types (four animals each of eastward, offshore, and westward movements) based on MCP overlapping among tagged dolphins. The animals that restricted their habitat to the coastward side of the KC (CSKC) tended to show ARS modes more frequently. Estimated locations of these animals corresponded well with cold coastal waters. On the other hand, offshore movements corresponded well with clockwise flow generated by the bifurcation of the KC, and those dolphins were likely straying into less productive waters. These patterns suggest that common bottlenose dolphins off the southern coast of Japan mainly utilize productive CSKC waters but occasionally move to offshore waters.

Elevated Nutrient Supply Caused by the Approaching Kuroshio to the Southern Coast of Japan

Although the Kuroshio Current has been well-known for being nutrient poor in its upper layers, it sustains a great biodiversity within its region, including the Kuroshio Extension. Previous studies reported that the Kuroshio carries a large amount of nutrients in its dark subsurface layers, yet, the mechanisms of how these nutrients are brought to sunlit layers remain unclear. In a previous observational study, it is shown that when the Kuroshio flows near the shelf break, very strong turbulence with dissipation rates of O (10 -7 W kg -1), caused by trapped near-inertial internal waves, leads to the generation of large diffusive nitrate fluxes of 1-10 mmol N m -2 day -1. Nevertheless, since these results were obtained from just one transect survey, questions remain, such as how long the enhanced turbulent diffusive nitrate flux extends downstream, and how the Kuroshio path modulations affect this nutrient injection. In this study, observed features were reproduced by using a high-resolution nested simulation coupled with a N2PZD2 ecosystem model. The results show that when the model Kuroshio flows closely to the coast, it hits a small bump in the upstream region of the Hyuganada Sea producing negative potential vorticity. At the same time, trapped near-inertial internal waves in a streak of lowered lowest internal wave frequencies are reproduced near the area with low PV values, generating strong vertical mixing similar to the observations. Since more nitrate is brought upward on the continental shelf when the Kuroshio approaches to the coast, the enhanced vertical mixing effectively diffuses up the nitrate from subsurface to surface layers at a rate of ~O(1 mmol N m -2 day -1) toward 50-100 km downstream along the Kuroshio. Further analyses on how the distance between the model Kuroshio and the southeastern coast of Kyushu affects the nutrient supply, suggest that when the Kuroshio flows closer to the coast, the internal wave kinetic energy, the nitrate concentration, its vertical gradients and vertical diffusive flux increase.

Rapid Increase of Explosive Cyclone Activity over the Midwinter North Pacific in the Late 1980s

Abstract Long-term changes in the activity of explosively developing “bomb” cyclones over the wintertime North Pacific are investigated by using a particular version of a global atmospheric reanalysis dataset into which only conventional observations have been assimilated. Bomb cyclones in January are found to increase rapidly around 1987 in the midlatitude central North Pacific. Some of the increased bomb cyclones formed over the East China Sea and then moved along the southern coast of Japan before developing explosively in the central North Pacific. The enhanced cyclone activity is found to be concomitant with rapid warming and moistening over the subtropical western Pacific and the South and East China Seas under the weakened monsoonal northerlies, leading to the enhancement of the lower-tropospheric Eady growth rate and equivalent potential temperature gradient, setting a condition favorable for cyclone formation in the area upstream of the North Pacific storm track. Along the storm track, poleward moisture transport in the warm sector of a cyclone and associated precipitation along the warm and cold fronts tended to increase and thereby enhance its explosive development. After the transition around 1987, a bomb cyclone has become more likely to develop without a strong upper-level cyclonic vortex propagating from Eurasia than in the earlier period. The increased bomb cyclone activity in January is found to contribute to the diminished midwinter minimum of the North Pacific storm track activity after the mid-1980s.

Revisit of the Occurrence of the Kuroshio Large Meander South of Japan

AbstractA unique characteristic by the Kuroshio off the southern coast of Japan is its bimodal path variations. In contrast to its straight path that follows the coastline, the Kuroshio takes a large meander (LM) path when its axis detours southward by as much as 300 km. Since 1950, eight Kuroshio LM events took place and their occurrences appeared random. By synthesizing available in situ/satellite observations and atmospheric reanalysis product, this study seeks to elucidate processes conducive for the LM occurrence. We find neither changes in the inflow Kuroshio transport from the East China Sea nor in the downstream Kuroshio Extension dynamic state are determinant factors. Instead, intense anticyclonic eddies with transport > 20 Sv (1 Sv ≡ 106 m3 s−1) emanated from the Subtropical Countercurrent (STCC) are found to play critical roles in interacting with Kuroshio path perturbations southeast of Kyushu that generate positive relative vorticities along the coast and lead the nascent path perturbation to form a LM. Occurrence of this intense cyclonic–anticyclonic eddy interaction is favored when surface wind forcing over the STCC is anticyclonic during the positive phasing of Pacific decadal oscillations (PDOs). Such wind forcing strengthens the meridional Ekman flux convergence and enhances eddy generation by the STCC, and seven of the past eight LM events are found to be preceded by 1–2 years by the persistent anticyclonic wind forcings over the STCC. Rather than a fully random phenomenon, we posit that the LM occurrence is regulated by regional wind forcing with a positive PDO imprint.

Seasonal occurrence of Japanese pygmy octopus Octopus parvus in the intertidal zone

The Japanese pygmy octopus, Octopus parvus, is a small octopus that commonly occurs along the coast of southern Japan; however, the ecological information is limited. This study investigated the annual seasonal occurrence of O. parvus in the intertidal zone of Fukue Island, Nagasaki, Japan, and examined its growth, maturation, and feeding habits. We found 140 individuals in the intertidal zone from August to January, and zero from March to July. During the emergence period, they appear to move from the high to low intertidal zone, and subsequently migrate to the subtidal zone. During low tide at night in the intertidal zone, we observed that the octopuses actively captured and fed on prey, such as fish, crabs, hermit crabs, shrimp, gastropods, and bivalves. The body size of collected individuals increased significantly from summer to winter. Both males and females were found to be characterized by similar body sizes and growth, although gonadal development occurred earlier in males. However, no mating or brooding behaviors were observed during the course of the study. Our observations thus indicate that O. parvus uses intertidal zones as feeding grounds for rapid growth and maturation, and may thereafter move to the subtidal zone for spawning.

Comprehensive Observational Features for the Kuroshio Transport Decreasing Trend During a Recent Global Warming Hiatus

AbstractLinear trends in Kuroshio transport during a recent global warming hiatus (1998–2013) were evaluated using long‐term ferryboat ADCP (acoustic Doppler current profiler) data and tidal gauge data in the Tokara Strait south of Japan. The Kuroshio exhibited a remarkable weakening trend of approximately 0.05 Sv year−1 (1 Sv = 106 m3 s−1). The pycnocline in the weakened Kuroshio was relaxed and displayed shoaling at the offshore edge, which was attributed to vertical thermocline displacement rather than to water mass modification. Importantly, Kuroshio transport trends in the Tokara Strait were affected by sea surface height anomalies, which were driven by the combined effects of the clockwise baroclinic‐mode coastal trapped wave propagation along the southern coast of Japan and downstream Kuroshio advection in the East China Sea. Both features were induced by wind stress curl changes related to the global warming hiatus over the North Pacific.

DNA/RNA metabarcoding and morphological analysis of epipelagic copepod communities in the Izu Ridge off the southern coast of Japan

AbstractThe Izu Ridge off the southern coast of Japan is composed of many seamounts and knolls and is an important fishery ground and hatchery for commercially important fish. As pelagic copepods link lower and higher trophic levels, we investigated the epipelagic copepods in the Izu Ridge using 28S DNA/RNA metabarcoding and microscopic approaches. In comparison with off-ridge regions, a low water temperature and high chlorophyll a concentration were observed in the Izu Ridge, possibly attributed to high levels of nutrients supplied by strong turbulences around the complex topography. We observed unique copepod communities with a large total biomass in the Izu Ridge, owing to the increased dominance of large copepod species. rRNA:rDNA (28S ratio) of these large species mainly showed positive correlations with the chlorophyll a concentration, suggesting a rapid response to the food supply. Migrating copepods were less dominant around the Izu Ridge with a shallow water depth. Higher species diversity and more rare species were detected in the Izu Ridge, which might be maintained by complex environment conditions including temporal and spatial variations in the concentration of chlorophyll a. Thus, the combined approach characterized copepod community structure and diversity supporting the rich pelagic ecosystems in the Izu Ridge.

Chemical speciation of iron in the euphotic zone along the Kuroshio Current

The distributions of dissolved iron (DFe) and organic iron-binding ligands (LFe) in the Kuroshio Current and its neighboring waters were elucidated from four cross-sectional surveys from the eastern coast of Taiwan to Sagami Bay. The Kuroshio surface water was characterized by a high salinity, low concentration of DFe, high concentration of LFe, and low fluorescence of humic-like substances, in comparison to the neighboring waters. The neighboring waters of the Kuroshio can be categorized into two types: the East China Sea water and coastal water. The East China Sea water was found near the Okinawa Trough, and characterized by a steep vertical gradient of DFe concentration, almost as low as that in the Kuroshio within the surface mixed layer, but reached ~0.5 nM below the mixed layer. Coastal water was observed on the southern coast of Japan, and its DFe concentration was high throughout the upper 200 m. The concentration of LFe in the Kuroshio water was significantly higher at 10-m depth than in the deeper layers. This suggests that humic-like substances, which are a major component of LFe in estuarine and deep waters, play a relatively minor role in the Kuroshio surface waters. Mixing with coastal waters can increase the concentration of bioavailable inorganic ferric ions in the Kuroshio surface water in two ways, by increasing the DFe concentration and decreasing the LFe concentration.

Seafloor Pressure Change Excited at the Northwest Corner of the Shikoku Basin by the Formation of the Kuroshio Large-Meander in September 2017

The Kuroshio takes a greatly southward displaced path called a large-meander (LM) path off the southern coast of Japan on interannual to decadal time scales. The transition of the current path from a non-large-meander path to an LM path is the most salient ocean current variation south of Japan. The change in pressure on the seafloor due to the formation of the LM path in September 2017 is of critical importance to understand the dynamics of the LM path and to distinguish the change due to the Kuroshio path variation from changes due to crustal deformation. Hence, we examined the seafloor pressure across the continental slope off the eastern coast of Kyushu for the period March 2014 to April 2019. The pressure and its cross-slope gradient over the continental slope shallower than 3,000 m beneath near the Kuroshio are invariable. As a mesoscale current path disturbance, called a small meander, passed over the observation stations, the pressure decreased by approximately 0.1 dbar on the continental slope deeper than 3000 m and was kept low until the end of the observation period (April 2019). The pressure decrease is consistent with the changes in sea surface height and subsurface water density and is caused by the baroclinic enhancement of the Shikoku Basin local recirculation. This seafloor pressure change implies a strengthening of the deep southwestward current, possibly as a part of a deep cyclonic circulation in the Shikoku Basin. The present study demonstrated that, in addition to altimetric sea surface height data, hydrographic data are useful to distinguish the ocean variation in seafloor pressure from the variation due to crustal deformation, and vice versa.

How the Kuroshio Current Delivers Nutrients to Sunlit Layers on the Continental Shelves With Aid of Near‐Inertial Waves and Turbulence

AbstractThe Kuroshio Current carries a large amount of nutrients in dark subsurface layer along the southern coast of Japan. However, due to lack of sufficient multidisciplinary high‐resolution observations, it has been unclear whether and how the subsurface nutrients are injected to sunlit layers on the continental shelf. In this study, using a state‐of‐the‐art tow‐yo microstructure profiler and nitrate measurements, we show that the Kuroshio flowing on the shelfbreak induces very strong turbulent dissipation rate ∼O(10−7W·kg−1) and diffusivity ∼O(10−3 m2·s−1) over an across frontal scale of 20–30 km. This strong turbulence could provide a diffusive nitrate flux to euphotic zone of >O(1 mmol N·m−2·day−1). As the Kuroshio steadily runs on the continental shelves along the path, the nutrient injection found in this study is a very important supply for biological production on the continental margin along the southern coast of Japan.

Development of an Active Learning Program for a Community Utilizing a Scenario Participating as Simulated Rescuers and Sufferers in an Earthquake Prone Area of Japan

Introduction:Major earthquakes with a magnitude of 8-9 are anticipated to occur in the next 30 years at a 60 percent chance on the southern coast of Japan. Since the most part of our Prefecture is likely to be damaged by tsunami and landslides, residents are expected to take a self-reliant approach on the initial several days after the earthquakes and tsunami.Aim:To improve the resilience of the local communities we have developed and applied an educational program of disaster response.Methods:An active learning program was designed on roles of rescuers and sufferers, and conducted two-hour sessions for high school students using a scenario in which they encountered an earthquake during a field trip. Half of the participants were assigned to play students on a field trip and asked to discuss options as a small group to survive and secure their safety in an isolated situation after an earthquake. They exchanged ideas to stay alive, cooperate with local residents and request disaster assistance using very short radio messages to the appropriate counterpart. The other half of the participants were assigned to be school administrations and asked to estimate the situation of sufferers. Their task as a small group was to organize assistance based on the best assumption from the limited information of the isolated students and local villagers.Results:After the sessions, the participants expressed their discovery in the discrepancy of situational recognition between the two groups and they learned about assumption-based planning as well as good information sharing.Discussion:Through this program, the participants experienced simulated situations and learned perspectives from both sides; providing relief as rescuers and receiving aid as sufferers. The participants were motivated to share and utilize their knowledge and skills to make their community resilient to disasters.

First Geographical Record of Corymorpha bigelowi (Cnidaria: Hydrozoa, Corymorphidae) in the Northern Red Sea Coast of Egypt, Based on Morphological Description

The Anthomedusae species, Corymorpha bigelowi Maas, 1905 (Cnidaria: Hydrozoa, Corymorphidae), was recorded in the northern Red Sea fauna, representing it "first invasion" in Egypt. A fact proven by past studies that this species is endemic in the Indo-Pacific region, which is mainly located in southern coast of Japan and Indian Ocean. The specimens were collected from an area located off Hurghada city, and between latitudes 27° 14.427ˊ and 27° 10.816ˊ N, and longitudes 33° 51.085ˊ and 33° 51.603ˊ E using plankton net (350 µm mesh). In total, six mature medausae of C. bigelowi were caught in December 2014 and February 2015 (4 and 2 medusae, respectively). Following this, the captured species medusa of C. bigelowi was photographed and morphological characteristics described in details. As well as, further discussions of biogeographical distribution and morphological speciation of the present species were provided. This work makes a noteworthy contribution to clarify the migration route of C. bigelowi from southern Red Sea to the Mediterranean Sea.

Zenopsis filamentosa (Zeidae), a new mirror dory from the western Pacific Ocean, with redescription of Zenopsis nebulosa

A new mirror dory, Zenopsis filamentosa, is described from specimens collected off the Pacific coast of southern Japan (Mie and Tosa Bay), the Kyushu-Palau Ridge, Fiji, and Tonga. Within Zenopsis, the new species is most similar to Zenopsis nebulosa, widely distributed in the Pacific Ocean, in having nine dorsal-fin spines and similar buckler numbers along the anal-fin base. However, it can be distinguished from the latter in having 24–27 (modally 26) dorsal-fin soft rays (vs. 25–28, modally 27 in Z. nebulosa), 24–25 (usually 25) anal-fin rays (vs. 25–27, usually 26), filamentous membranes extending from the spinous portion of the dorsal fin, of length more than twice SL (vs. moderately elongated, not exceeding the caudal-fin posterior margin when adpressed); 6–9 (modally 7) bucklers between the pelvic and anus (vs. 7–10, modally 8), no bucklers above the anal-fin spines (vs. a small buckler present), and the pelvic fin with three to five dark stripes of all sizes (vs. lacking distinct markings in specimens >ca. 100 mm SL). Sequence variations on the mitochondrial DNA COI region demonstrated the validity of each species. Zenopsis nebulosa is redescribed, on the basis of western and Central Pacific Ocean specimens.

The Effects of Orography on the Extratropical Transition of Tropical Cyclones: A Case Study of Typhoon Sinlaku (2008)

Abstract Extratropical transition (ET) can cause high-impact weather in midlatitude regions and therefore constitutes an ongoing threat at the end of a tropical cyclone’s (TC) life cycle. Most of the ET events occur over the ocean, but some TCs recurve and undergo ET along coastal regions; however, the latter category is less investigated. Typhoon Sinlaku (2008), for example, underwent ET along the southern coast of Japan. It was one of the typhoons that occurred during the T-PARC field campaign, providing unprecedented high-resolution observational data. Sinlaku is therefore an excellent case to investigate the impact of a coastal region, and in particular orography, on the evolution of ET. Here, observations from T-PARC are employed to verify high-resolution simulations of Sinlaku. In addition, a sensitivity simulation is performed with the orography of Japan removed. The presence of orography causes blocking of low-level, cool midlatitude air north of Japan. Without this inflow of cool air, ET is delayed. Only once Sinlaku moves away from the orographic barrier does the cool, dry environmental air penetrate equatorward, and ET continues. On a local scale, evaporatively cooled air from below Sinlaku’s asymmetric precipitation field could be advected toward the cyclone center when orography was favorable for it. Changes in the vortex structure, as known from mature TCs interacting with orography, were only minor due to the high translation speed during ET. This study corroborates that orography can impact ET by modulating both the synoptic-scale environmental conditions and the mesoscale cyclone structure during ET.

Observed Features of Langmuir Turbulence Forced by Misaligned Wind and Waves under Destabilizing Buoyancy Flux

AbstractSeveral features of Langmuir turbulence remain unquantified despite its potentially large impacts on ocean surface mixing. For example, its vertical velocity variance, expected to be proportional to based on numerical simulations, was proportional to in recent field observations, where is the friction velocity and is surface Stokes velocity. To investigate unquantified features of Langmuir turbulence, we conducted a field experiment around a marine observation tower in a shallow sea off the southern coast of Japan in early winter when winds and waves (often swells) were often misaligned. Coherent structures similar to Langmuir cells were successfully identified in the horizontal and vertical structures of turbulent flows measured with upward- and horizontally looking acoustic Doppler current profilers (ADCPs). ADCPs and several anemometers attached at the tower showed that turbulent vertical velocity variance was large when the Langmuir number and Hoenikker number (; where B is surface buoyancy flux and H is the water depth) were both small and that the orientation of the cells was generally aligned in the direction of Lagrangian current shear. These results agree well with the previous numerical results. As in the previous observations, however, the vertical velocity variance appeared to be proportional to . In our experiment, this curious feature was explained by compensatory effects between waves and convection. Misaligned wind with waves also seems to characterize the observed Langmuir turbulence, though further quantitative analysis is required to confirm this result.

Bottom pressure change associated with the 2004–2005 large meander of the Kuroshio south of Japan

A large meander of the Kuroshio was generated in the region off the southern coast of Japan in August 2004 and continued until approximately July 2005. The formation and decay of the large-meander (LM) path was observed by bottom pressure (BP) sensors installed on inverted echo sounders (PIESs) and a seismic observing system off Shikoku. The variation in BP was examined focusing on the development, persistence, and decay of the LM path. The BP was found to be depressed associated with a Kuroshio path disturbance, called a small meander, and this BP depression led the sea surface height (SSH) depression by up to approximately two months. The temporal phase shift between the sea surface and deep disturbances was significantly greater than those of other small meanders that did not develop into large meanders. After the formation of the LM path, the BP beneath the Kuroshio increased with a lag of approximately two months behind the SSH elevation along with the upward displacement of the main thermocline. The increase in BP is associated with that of the positive southward BP gradient anomaly, i.e., the eastward deep Kuroshio current anomaly, which suggests an enhancement of the topographic steering and stability of the LM path. This is consistent with the fact that no small meanders occurred in the early LM period from late July 2004 to late January 2005.

Spatial and temporal variability in the trans-Pacific migration of Pacific bluefin tuna (Thunnus orientalis) revealed by archival tags

Archival electronic tags were internally implanted in 713 age-0 Pacific bluefin tuna (PBF) caught in their nursery waters off the southern coast of Japan and in the East China Sea over an extended study period (1995–2015) to clarify the spatial and temporal variability of their trans-Pacific migration. Two hundred twenty-five of these tagged tuna were recaptured by fisheries (31.6%), and we successfully retrieved tag data from 14 of 21 individuals recovered in the Eastern pacific. Furthermore, one archival tag recovered in the Western Pacific revealed that the individual had performed a trans-Pacific migration, so in total 21 tagged PBF were shown to have migrated to the Eastern Pacific (2.9% of the total tags released). We successfully downloaded data from 15 of these 21 archival tags, which revealed that some age-1 PBF migrate rapidly (123.9 ± 82.8 km day−1) and directly from waters offshore of Japan to the eastern Pacific (160.0°E to 130.0°W), a journey that takes an average of 2.5 months (ranging from 1.2 to 5.5 months) through relatively cool waters (14.7 ± 2.0 °C). All juvenile PBF began their trans-Pacific migration shortly after exposure to cooler water temperatures (<14 °C), suggesting that sustained residence in lower water temperatures presents a physiological challenge for this age class. Three patterns were identified in the timing of the departure of juvenile PBF from the western Pacific: departing 12–14 months post-hatch (N = 7) in early summer (May-July), departing 17–19 months post-hatch (N = 7) in late autumn (October-December), and departing 21 months post-hatch (N = 1) in late winter (February). The PBF tagged along the southern coast of Japan (SCJ) arrived in the eastern Pacific earlier than those tagged in the East China Sea (ECS), most likely due to the shorter travel distance. Additionally, the PBF that began their trans-Pacific migration in the earlier period remained in an offshore foraging zone (the Kuroshio-Oyashio transition region) for shorter periods (2.8 months on average) and at lower latitudes (35.0°N) during the spring, while the PBF that delayed their migration spent more time (6.7 months on average) in the productive waters between 35.0 and 45.0°N during the spring-autumn months. The variability in the departure timing of the trans-Pacific migration of age-1 PBF may be related to geographic differences between nursery areas in addition to oceanographic conditions and foraging opportunities encountered by the tuna in the offshore waters of Japan during their first year.

Long-Term Changes in the Extreme Significant Wave Heights on the Western North Pacific: Impacts of Tropical Cyclone Activity and ENSO

Seasonal extreme wave statistics were reproduced by using the 25-km-grid global wave model of WAVEWATCH-III. The results showed that the simulated wave dataset for the present climate (1979-2009) was similar to Climate Forecast System Reanalysis (CFSR) wave data. Statistics such as the root mean squared error (RMSE) and correlation coefficient (CC) over the western North Pacific (WNP) basin were 0.5 m and 0.69 over the analysis domain. The largest trends and standard deviation were around the southern coast of Japan and western edge of the WNP. Linear regression analysis was employed to identify the relationship between the leading principal components (PCs) of significant wave heights (SWHs) in the peak season of July to September and sea surface temperature (SST) anomalies in the equatorial Pacific. The results indicated that the inter-annual variability of SWH can be associated with the El Nino-Southern Oscillation in the peak season. The CC between the first PC of the SWH and anomalies in the Nino 3.4 SST index was also significant at a 99% confidence level. Significant variations in the SWH are affected by tropical cyclones (TCs) caused by increased SST anomalies. The genesis and development of simulated TCs can be important to the variation in SWHs for the WNP in the peak season. Therefore, we can project the variability of SWHs through TC activity based on changes in SST conditions for the equatorial Pacific in the future.