Surface velocity time series derived from satellite altimetry data in a section across the Kuroshio southwest of Kyushu

A time series of surface geostrophic velocity is developed using satellite altimetry data during 1992–2010 for a track across the Kuroshio southeast of Kyushu, Japan. The temporal mean geostrophic velocity is estimated by combining the along-track sea level anomaly and shipboard ADCP data. This approximately 6-km resolution dataset is successful in representing the Kuroshio cross-current structure and temporal variation of the Kuroshio current-axis position during 2000–2010. The authors use this dataset to examine the winter Kuroshio path destabilization phenomenon. Its seasonal features are characterized as follows: the velocity shear on the inshore side of the Kuroshio becomes stronger and the Kuroshio path state becomes unstable from the summer to winter. This evidence is consistent with the hypothetical mechanism governing the destabilization phenomenon discussed in a previous study. Furthermore, the interannual amplitude modulation of the seasonality is examined in relation to interannual variations in the winter northerly wind over the northern Okinawa Trough and the Pacific Decadal Oscillation (PDO) index. The destabilization phenomenon appears 15 times in the period 2000–2010. Ten cases are related to local wind effects, and 7 of these are also connected with the PDO index. This is probably because the winter northerly wind over the northern Okinawa Trough is regulated by the PDO signal in interannual time-scales. Only 4 cases are related to the PDO index, but their driving mechanism remains uncertain.     

Properties of winter mixed layer variability on the shelf-slope region facing the Kuroshio—study of Tosa Bay, southern Japan

To examine the properties of winter mixed layer (ML) variability in the shelf-slope waters facing the Kuroshio, we analyzed historical temperature records and the simulated results of a triply nested high-resolution numerical model. As a candidate of the shelf-slope waters, we focused on Tosa Bay, off the southern Japan. A time series of observed monthly mean ML temperatures and depths in the bay exhibits a remarkable seasonal variation. The period when the ML develops can be divided into two regimes: from September to November, when the sea surface cooling is gradually enhanced, the ML temperature and depth decreases and increases, respectively; from January to March, the ML temperature and depth are kept nearly constant, while the sea surface cooling in January reaches its annual maximum. In the latter regime, variance for the monthly mean ML depth is the largest of the year. To further study the ML properties in the latter regime corresponding to winter, we examined simulated results. It was found that the largest variance for ML depth is attributed to a dominant intramonthly variation. This is related to a submesoscale variation with typical spatial scales of 10–20 km, induced by the Kuroshio and its frontal disturbances. Simulated monthly mean heat balance within the ML showed that heat advection balances with heat flux at the sea surface and entrainment through the ML bottom. Moreover, the monthly mean heat advection is determined mainly by the intramonthly eddy heat advection, suggesting that the high-frequency intramonthly variation related to submesoscale variations contributes significantly to the low-frequency monthly variations of the ML in winter.     

Sediment distribution and depositional processes along the fluvial to marine transition zone of the Mekong River delta,Vietnam

The area of coastal rivers with a combination of fluvial, tidal and wave processes is defined as the fluvial to marine transition zone and can extend up to several hundreds of kilometres upstream of the river mouth. The aim of this study is to improve the understanding of sediment distribution and depositional processes along the fluvial to marine transition zone using a comprehensive dataset of channel bed sediment samples collected from the Mekong River delta. Six sediment types were identified and were interpreted to reflect the combined action of fluvial and marine processes. Based on sediment‐type associations, the Mekong fluvial to marine transition zone could be subdivided into an upstream tract and a downstream tract; the boundary between these two tracts is identified 80 to 100 km upstream of the river mouth. The upstream tract is characterized by gravelly sand and sand and occasional heterolithic rhythmites, suggesting bed‐load supply and deposition mainly controlled by fluvial processes with subordinate tidal influence. The downstream tract is characterized by heterolithic rhythmites with subordinate sand and mud, suggesting suspended‐load supply and deposition mainly controlled by tidal processes with subordinate fluvial influence. Sediment distributions during wet and dry seasons suggest significant seasonal changes in sediment dynamic and depositional processes along the fluvial to marine transition zone. The upstream tract shows strong fluvial depositional processes with subordinate tidal influence during the wet season and no deposition with weak fluvial and tidal processes during the dry season. The downstream tract shows strong coexisting fluvial and tidal depositional processes during the wet season and strong tidal depositional processes with negligible fluvial influence during the dry season. Turbidity maxima are present along the downstream tract of the fluvial to marine transition zone during both wet and dry seasons and are driven by a combination of fluvial, tidal and wave processes.     

Fifteen years progress of the TRITON array in the Western Pacific and Eastern Indian Oceans

The Triangle Trans‐Ocean Buoy Network (TRITON) project by the Japan Agency for Marine-Earth Science and Technology began with deployment in the western tropical Pacific Ocean in 1998 and has shifted to steady, long-term observations since 1999. After on-site inter-comparison with the Autonomous Temperature Line Acquisition System mooring system of the Tropical Atmosphere and Ocean (TAO) array by the National Oceanic and Atmospheric Administration, the TRITON array became the international TAO/TRITON array in 2000 as a key component of the Global Ocean and Climate Observing Systems. The TAO/TRITON array took over from the TAO array, which was developed during the Tropical Ocean and Global Atmosphere program (1985–1994), and replaced the western part of TAO with new additional real-time measurements of salinity and ocean currents. In 2001, two TRITON moorings were deployed in the eastern Indian Ocean for capturing the eastern pole of the Indian Ocean Dipole. From this initiative, the Indian Ocean Observing System (IndOOS) was designed, and the Indian Ocean mooring array (Research Moored Array for Africa–Asian–Australian Monsoon Analysis and Prediction) was developed as a key component of IndOOS. In this paper, 15 years of progress in the TRITON project in the western Pacific and eastern Indian Oceans is reviewed with regards to scientific outcomes, technological development, and collaborations with international and domestic partners. Future directions for sustainable observation in the Pacific and Indian Oceans are also discussed.     

Influence of interaction between small asperities on various types of slow earthquakes in a 3-D simulation for a subduction plate boundary

Recently, the occurrence of slow earthquakes such as low-frequency earthquakes and very low-frequency earthquakes have been recognized at depths of about 30 km in southwest Japan and Cascadia. These slow earthquakes occur sometimes in isolation and sometimes break into chain-reaction, producing tremor that migrates at a speed of about 5–15 km/day and suggesting a strong interaction among nearby small asperities. In this study, we formulate a 3-D subduction plate boundary model with two types of small asperities chained along the trench at the depth of 30 km. Our simulation succeeds in representing various types of slow earthquakes including low-frequency earthquakes and rapid slip velocity in the same asperity, and indicates that interaction between asperities may cause the very low-frequency earthquakes. Our simulation also shows chain reaction along trench with propagation speed that can be made consistent with observations by adjusting model parameters, which suggests that the interactions also explain the observed migration of slow earthquakes.     

Field-based description of near-surface crustal deformation in a high-strain shear zone: A case study in southern Kyushu,Japan

This study investigated geological evidence for near-surface crustal deformation in a high-strain shear zone that has been geodetically identified but which is not associated with obvious tectonic landforms. Fieldwork was conducted in the east–west-trending southern Kyushu high-strain shear zone (SKHZ), Japan, focusing mainly on occurrences of fracture zones, which are defined by a visible fracture density of >1 per 10 cm² and are commonly associated with cataclasite, fault breccia, and gouge. The area in which east–west-trending fracture zones are dominant is restricted to the east–west-trending, ~2-km-wide aftershock area of the 1997 Northwestern Kagoshima Earthquakes. Analysis of slip data from minor faults using the multiple inverse method, irrespective of whether the faults are in fracture zones, reveals that the area where the calculated main stress field is consistent with the current stress field estimated from focal-mechanism solutions of microearthquakes is restricted to the east–west-trending aftershock area. This finding for the SKHZ contrasts with the case of the Niigata–Kobe Tectonic Zone, which is a major strain-concentration zone with many exposed active faults in central Japan and for which the stress field estimated using fault-slip data is considered to be uniform and coincides with the current stress field. The cumulative amount of displacement estimated from the areal density of fracture zones in the SKHZ study area is smaller than that estimated from geodetically measured strain rates. Investigations based on slip data from minor faults and fracture-zone occurrence could help to identify concealed faults that are too small to generate tectonic landforms but which are sufficiently large to trigger major earthquakes.     

Kinetics of nitrate and ammonium uptake by the natural populations of marine phytoplankton in the surface water of the Oyashio region during spring and summer

The maximum uptake rate (_max) and affinity constant (K _s) for nitrate and ammonium were estimated in the surface water of offshore Oyashio in May (spring) and September (summer), 1990. The average _max/Chl.a for ammonium was 2.1 times larger than that of nitrate in both seasons. The average _max/Chl.a for both nitrogens were 3.5 times larger in summer than in spring. Water temperature and size composition of phytoplankton population were related to the seasonal difference in the _max/Chl.a. Phytoplankton population showed high affinity for both nitrogens in the spring and summer. In addition, the contribution of new production to total production was estimated by _max[_max–No₃/(_max–NO₃+_max–NH₄)]. The spring value was in the range of 0.26 to 0.45 (mean±SD=0.35±0.092), and the values in spring bloom were especially a little over 0.4. The summer value was in the range of 0.30 to 0.37 (0.34±0.04).     

Deep and Bottom Currents in the Challenger Deep,Mariana Trench,Measured with Super-Deep Current Meters

The bottom currents in the Challenger Deep, the deepest in the world, were measured with super-deep current meters moored at 11°22′ N and 142°35′ E, where the depth is 10915 m. Three current meters were set at 9687 m, 10489 m and 10890 m at the station in the center of the Challenger Deep for 442 days from 1 August 1995 to 16 October 1996. Although rotor revolutions in 60 minutes of recording interval were zero for 37.5% of the time, the maximum current at the deepest layer of 10890 m was 8.1 cm s⁻¹, being composed of tidal currents, inertia motion and long period variations. Two current meters were set at 6608 m and 7009 m at a station 24.9 km north of the center for 443 days from 31 July 1995 to 16 October 1996, and two current meters at 6214 m and 6615 m at a station 40.9 km south of the center for 441 days from 2 August 1995 to 16 October 1996. The mean flow at 7009 m depth at the northern station was 0.7 cm s⁻¹ to 240°T, and that at 6615 m depth at the southern station was 0.5 cm s⁻¹ to 267°T. A westward mean flow prevailed at the stations, and no cyclonic circulation with mean flows of the opposite directions was observed in the Mariana Trench at a longitude of 142°35′ E. Power spectra of daily mean currents showed three spectral peaks at periods of 100 days, 28–32 days and 14–15 days. The peak at 100 day period was common to the power spectra.