Keisuke Taira

Velocity fluctuations of the Kuroshio near the Izu Ridge and their relationship to current path

Abstract Major velocity fluctuations in the Kuroshio around the Izu Ridge were examined by direct current measurements in the upper layer (at 250-m depth in the Oshima-West Channel) from March 1977 to May 1979 and in the lower layer (at 1670-m depth west of Hachijojima Island) from May 1978 to February 1979. Mean observations for 286 days in the upper layer and 271 days in the lower layer showed velocity fluctuations in two period bands, one near 33 days and the other near 100 days. The fluctuation with the 33-day period was dominant in both layers with an amplitude of about 20 cm s −1 . The fluctuation with the 100-day period was dominant in the upper layer and was also seen in the east-west shift of the path of the Kuroshio near Hachijojima. More than 50% of the variance of the north component of velocity with a period greater than 2 h was concentrated at frequencies lower than 0.1 cycle per day. The Kuroshio was meandering around a stationary cold water mass that appeared south of Honshu in 1975. The current velocity in the upper layer increased when the meandering path of the Kuroshio shifted eastward by about 100 km in April 1978.

Water masses and properties at 165°E in the western Pacific

Full-depth conductivity-temperature-depth-oxygen profiler (CTDO 2 ) data at 165°E in 5°S-40°N in summer 1991 (World Ocean Circulation Experiment (WOCE) Hydrographic Program P13C) and 27°-48°N in spring 1993 (P13J) are analyzed. Distributions of salinity and temperature are characterized by the fronts associated with ocean currents, such as the Kuroshio Extension front at 34°-35°N, the Kuroshio Bifurcation front at 38°-40°N, the subarctic front at 40°-42°N, other salinity fronts in the tropical waters, and some oxygen fronts. Dissolved oxygen increases upward sharply, called oxycline, just above the intermediate oxygen minimum layer. The processes of water-mass extension are clarified mainly from oxygen and potential vorticity (PV). The North Pacific Tropical Water (NPTW) forms at the sea surface in 18°-31°N from winter to summer, and the winter-formed NPTW of 23.0-25.0σ (where σ θ is potential density) extends to 12°N by the subtropical ocean gyre, so that it occupies a layer of 23.0-25.0σ θ in 12°-31°N. The Subtropical Mode Water (STMW) forms at the surface in 29°-34°N in winter and extends to 25°N by the ocean circulation; it occupies a layer around 25.4σ θ between the main and seasonal pycnoclines in 25°-34°N. The North Pacific Intermediate Water (NPIW) extends to 34°-38°N on 26.83σ θ by the Kuroshio Extension and its northern branch and extends to 27°-33°N on 26.77σ θ and to 15°-22°N on 26.61σ θ by westward currents; it occupies 15°-38°N in the intermediate subtropical gyre. The NPIW is located in the oxycline with similar PV to the STMW. The boundary of the subtropical and tropical gyres is concluded to be around 12°N (14°-16°N) in a surface (intermediate) layer 26.0σ θ ). Bottom waters are also examined.

Seasonal variability of bottom water properties off Adélie Land, Antarctica

The region off Adelie Land is considered as one of the sources of Antarctic Bottom Water. Hydrographic observations were carried out during two cruises in December 1994 and January 1995 and January and February 1996 in this region. Vertical sections along 140°E show that bottom water is colder and fresher than the water above. This bottom water also has higher dissolved oxygen and lower silicate concentrations. The saline bottom water that originated from Ross Sea is not found in these hydrographic data obtained west of 142°E. Current meter moorings were also carried out at three locations on the continental slope in this region. At one of these moorings (139°59′E, 65°10′S, 2665 m deep), data were successfully obtained from January 1995 to March 1996. Three current meters were deployed at 1075, 1778, and 2632 m deep in this mooring. The data show that the average current speed at the lower current meter is 16.2 cm s−1, and it is about 3 times larger than those at the upper two current meters. Also, variability of speed and temperature is largest at the lower current meter. In addition, seasonal variability of speed and temperature is evident only at this current meter. From August to December, speed is larger by 5.7 cm s−1 and temperature is lower by 0.27°C. Also, their variability is larger during the same period. This seasonal variability observed near the bottom suggests seasonal variability of bottom water formation in this region.

Abyssal current along the northern periphery of Shikoku Basin

AbstractAbyssal currents along the northern periphery of the Shikoku Basin south of Japan were measured by current meters moored off Cape Daio-zaki, Cape Shiono-misaki and Cape Ashizuri-misaki and on the eastern foot of the northernmost part of the Kyushu-Palau Ridge. Total length of observation off Cape Shiono-misaki was about five years including the periods of the Kuroshio large meander and no meander. Analyses of current data show:1)Mean currents with a magnitude of 5–10 cm sec−1 were observed during the whole observation period at all of current meters which were set 400 m above the sea bottom that was deeper than 4,500 m. The mean current for each current meter was parallel to the local bottom contour arond each station and was toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left.2)At each station located above the shelf toe off Cape Daio-zaki and off Cape Shiono-misaki and on the foot of the Kyushu-Palau Ridge, the mean current increases with depth (a bottomward intensification of the mean current), and the vertical extent of the mean current is estimated to be about 2,000 m above the sea bottom.3)At a station located at 2,600 m depth on the continental slope off Cape Shiono-misaki, no bottom-ward intensification of currents was observed. These results strongly suggest that a steady abyssal flow exists in the depths deeper than about 3,000 m along the northern and northwestern peripheries of the Shikoku Basin. Existence of the abyssal circulation system is also suggested, at least, in the north of the Shikoku Basin.

Deep current structure above the Izu‐Ogasawara Trench

Above the Izu-Ogasawara Trench south of Japan, direct current measurements were made at 34°N from 1987 to 1996, and hydrographic observations were carried out at 34° and 30°N in 1995. The geostrophic shears calculated from the conductivity-temperature-depth data were consistent with the shears calculated from the current data. It is found that there are opposing currents that flow southward on the western flank and northward on the eastern flank along the isobaths. In the cross-trench direction the magnitude of the mean velocity tended to increase with the distance from the deepest point of the trench and exceeded 10 cm s−1 on the eastern flank. In the vertical direction the mean velocity increased with depth, but it decreased just above the bottom, probably because of friction. The southward transport above the western flank was estimated to be 5–8 Sv both at 34° and 30°N. However, the northward transport above the eastern flank increased from 5 Sv at 30°N to 22 Sv at 34°N, suggesting a large inflow from the west.

Deep hydrographic structure along 12°N and 13°N in the Philippine Sea

Hydrographic casts down to the bottom along two zonal sections at 12°N and 13°N (from 144°E to 127°E) were made with a CTD. Their analysis verified the existence of cold and saline abyssal water between the Mariana Ridge and the Kyushu-Palau Ridge. This result provides evidence of flow into the Philippine Sea through the deep gap called the Yap-Mariana Junction. The properties of deep water are variable in the West Mariana basin but quite homogeneous in the Philippine Basin, indicating the transitional nature in the West Mariana Basin and the existence of older bottom water in the Philippine Basin. A close examination suggests that the bottom water is slightly colder in the western part of the Philippine Basin than in the eastern part of the basin. This slightly colder deep water with a hundred kilometer scale in the western Philippine Basin might be related to a broad western boundary current flowing equatorward along the eastern rise of the Philippine Trench.

Southward intrusion of the intermediate Oyashio water along the east coast of the Boso Peninsula, Japan II. Intrusion events into Sagami Bay

In the previous paper (Yanget al., 1993), it was shown that there always exists the coastal salinity-minimum-layer (SML) water just off the Boso Peninsula. The coastal SML water is bounded by the current zone of the Kuroshio, and a relatively high salinity domain separates it from the offshore SML water which would be a continum of the North Pacific Intermediate Water. We suggested that the coastal SML water region indicates the pathway along which the Intermediate Oyashio Water intrudes into Sagami Bay. In this paper, by selecting seven cases where we found the coastal SML water having abnormally high oxygen content and low salinity, we try to follow the intrusion manner of the Intermediate Oyashio Water into Sagami Bay by using available hydrographic data taken routinely by various organizations in the period from 1973 to 1986. Some of these water can be traced from the observation line near the cape of Inubo to the central part of Sagami Bay, and its propagation speed along the coast is shown to be of order of 1 cm/s. The intruding intermediate Oyashio water usually has a complicated layered structure in it, and its time scale of persistence is shown to be only a few months.

Path and volume transport of the Kuroshio current in Sagami Bay and their relationship to cold water masses near Izu Peninsula.

The path of the Kuroshio in Sagami Bay was surveyed through drifter tracking from Oshima-West Channel to Oshima-East Channel. A subsurface drifter with a drogue at 300 m depth flowed around Oshima from Oshima-West Channel to Oshima-East Channel. A difference in flow directions between the upper and lower layers was apparent in the northwest of Oshima. Flow directions there were shown to change from north in the surface layer to east in the bottom layer, and this was confirmed with moored currentmeters.A profile of northward current velocity was estimated from measurements in six layers with currentmeters deployed in the Oshima-West Channel. The profile shows a core of northward flow along the eastern bottom slope and a weak southward flow along the western bottom slope. Volume transport of the Kuroshio into Sagami Bay was estimated to be 1.8×106m3sec−1 from the profile.Long-term current measurement showed that southward flows were observed in Oshima-West Channel in July 1977, May 1978 and April 1979. Cold or warm water masses appearing south of the Izu Peninsula are suggested to have caused the changes.Displacement of the cold water mass in July 1977 is discussed on the basis of current measurements and offshore oceanographic conditions.

Southward intrusion of the intermediate Oyashio water along the east coast of the Boso Peninsula I. Coastal salinity-minimum-layer water off the Boso Peninsula

Index species of zooplankton of the Oyashio water are found in and beneath the salinity minimum layer in Sagami Bay. In order to clarify the intrusion path of the intermediate Oyashio Water (or the water of the Mixed Water Region), the oceanographic conditions off the Boso Peninsula are studied by using available hydrographic data obtained mainly by Japan Meteorological Agency. The cross-sectional salinity distribution along KJ line which extends southeastward from off the tip of the peninsula always indicates the existence of a low salinity patch just off the coast in the salinity minimum layer. This water is well separated from the offshore low salinity water which is considered as the water in the western margin of the so-called North Pacific Intermediate Water. We refer to the former water as the coastal salinity-minimum-layer (SML) water and to the latter as the offshore SML water. The coastal SML water is usually bounded by the current zone of the Kuroshio. The existence of the coastal SML water seems to indicate the possible pathway of the intermediate Oyashio water along the Boso Peninsula into Sagami Bay. The detailed water type analysis is made in T-S plane, S-Δst plane, and O2-Δst plane. There is no significant difference in distribution ranges of the water types between the coastal SML water and the offshore SML water. However, the water types of the coastal SML water is not uniformly distributed, and the water can be classified into two groups: group A with relatively high oxygen content and relatively low salinity value and group B with relatively low oxygen content and relatively high salinity value. Group A is thought to be associated with strong event-like intrusions, the details of which will be discussed in Part II.

Observation of temperature and velocity from a surface buoy moored in the Shikoku Basin (OMLET-88) —An oceanic response to a Typhoon

A surface buoy was moored from 20 April to 2 November 1988 at 28°48′ N and 135°01′ E where the water depth was 4900 m to measure temperature and velocity in the upper 150 m. The Typhoon 8824 passed at 0300 (JST) on 8 October about 50 km north to the mooring station with a maximum wind speed of 43.5 m s−1. The buoy was shifted about 30 km to southwest, and the instruments were damaged. The records of temperature at 0.5 m and velocity at 50 m were obtained. The inertial oscillation caused by the typhoon is described using the current record. The oscillation endured for about 20 days. Deep mixing and vertical, heart transport by the typhoon are discussed based on the data from the Ocean Data Buoy of the Japan Meteorological Agency moored at 29°N and 135°E.