Yasushi Takatsuki

Water property and current distributions along the WHP‐P9 section (137°–142°) in the western North Pacific

The Japan Meteorological Agency carried out full-depth, high-resolution hydrographic measurements in July-August 1994 along the 137°-142°E line as a contribution to the World Ocean Circulation Experiment. The line spans the subtropical and tropical regions of the western North Pacific with a full set of water mass and current distributions. At 28°N to the south of the Kuroshio recirculation, there exists an eastward current distinct from the currents associated with the subtropical fronts. The eastward current is characterized by the meridional thermal gradient extending down to 1500-m depth. Deep silicate has a meridional boundary at 25°N, forming two cores of the maximum to the north and south of the boundary. The northern core centered at 28°N is meridionally coincident with other property extrema, indicating an inflow from the Northwest Pacific Basin down to 2000-m depth. The southern core centered at 16°N suggests another westward inflow from the East Mariana Basin to the West Mariana Basin. A broad isopycnal depression at middepths and bottom water properties over the Shikoku and West Mariana Basins indicate a near-bottom circulation along which the inflow water through the Yap-Mariana Junction at 12°N fills the West Mariana Basin and then spreads to the Shikoku Basin and Philippine Basin successively. A part of the bottom water from the South Pacific bifurcates around the Yap-Mariana Junction and enters the West Caroline Basin via the Yap Trench. This supports another path of deep water inflow to the Philippine Basin from the southernmost channel at 4000-m depth.

Water masses labeled with global fallout 137Cs formed by subduction in the North Pacific

[1] We provide a first cross section of the 137 Cs concentration along 165°E longitude in the western North Pacific. The 137 Cs profile is characterized by several subsurface cores with high 137 Cs, including two 137 Cs concentration maxima at 20°N, one at 250 m (δ θ ≈ 25.5) and one at 400-500 m (σ θ ≈ 26.0) depths. The shallower maximum is in the density range of North Pacific Subtropical Mode Water (NPSTMW) and the deeper one is in the density range of Lighter Central Mode Water (LCMW). The main 137 Cs cores, therefore, were formed by movements of NPSTMW and LCMW in the interior ocean during the past four decades. The 137 Cs has been transported from subarctic region to subtropics and tropics as a result of subduction.

Drift Characteristics of a Moored Conductivity–Temperature–Depth Sensor and Correction of Salinity Data

Abstract The temperature and conductivity drift (time change of the characteristics) of moored SBE37IM conductivity and temperature (CT) sensors was investigated by pre- and postdeployment calibration of the Triangle TransOcean Buoy Network (TRITON). This buoy network comprises the western portion of the basinwide (Tropical Atmosphere Ocean) TAO/TRITON buoy array, which monitors phenomena such as El Nino and contributes to forecasting climate change. Over the time of deployment the drift of the temperature sensors was very small, within 3 mK of the postdeployment calibration data. The drift of the conductivity sensors was more significant. After 1 yr of mooring, conductivity drift observed in the shallowest layer (1.5–100 m) was positive and 0.010 S m−1 [equivalent to 0.065 (PSS-78) at 30°C and 6 S m−1; here, 1 S is 1 Ω−1] at 6 S m −1 on average. Drift observed in the thermocline layer (125–200 m) was also positive and 0.0053 S m−1 [0.034 (PSS-78)] at 6 S m−1 on average. Conversely, the drift of conductiv...

Circulation of Intermediate and Deep Waters in the Philippine Sea

The circulation of intermediate and deep waters in the Philippine Sea west of the Izu-Ogasawara-Mariana-Yap Ridge is estimated with use of an inverse model applied to the World Ocean Circulation Experiment (WOCE) Hydrographic Program data set. Above 1500 m depth, the subtropical gyre is dominant, but the circulation is split in small cells below the thermocline, causing multiple zonal inflows of intermediate waters toward the western boundary. The inflows along 20°N and 26°N carry the North Pacific Intermediate Water (NPIW) of 11 × 109 kg s−1 in total, at the density range of 26.5σθ–36.7σ2 (approximately 500–1500 m depths), 8 × 109 kg s−1 of the NPIW circulate within the subtropical gyre, whereas the rest is conveyed to the tropics and the South China Sea. The inflow south of 15°N carries the Tropical Salinity Minimum water of 35 × 109 kg s−1, nearly half of which return to the east through a narrow undercurrent at 15–17°N, and the rest is transported into the lower part of the North Equatorial Countercurrent. Below 1500 m depth, the deep circulation regime is anti-cyclonic. At the density range of 36.7σ2, – 45.845σ4 (approximately 1500–3500 m depths), deep waters of 17 × 109 kg s−1 flow northward, and three quarters of them return to the east at 16–24°N. The remainder flows further north of 24°N, then turns eastward out of the Philippine Sea, together with a small amount of subarctic-origin North Pacific Deep Water (NPDW) which enters the Philippine Sea through the gap between the Izu Ridge and Ogasawara Ridge. The full-depth structure and transportation of the Kuroshio in total and net are also examined. It is suggested that low potential vorticity of the Subtropical Mode Water is useful for distinguishing the net Kuroshio flow from recirculation flows.

The Kuroshio Structure and Transport Estimated by the Inverse Method

Abstract Structure and transport of the Kuroshio are examined by the inverse method, based on a dataset around the Tokara Strait. Subsurface velocities measured by a shipborne acoustic Doppler current profiler are used together with CTD sections across the Kuroshio to estimate the absolute flow fields. The dataset was obtained along a closed line south off the Kyushu by the R/V Chofu Maru in September 1987. The Kuroshio transport through the Tokara Strait was 28 × 109 kg s−1 at the time of observations. The transport of warm water eddy south of the Kuroshio is estimated at larger than 39 × 109 kg s−1, but its precise value is difficult to determine because of strong transport dependency on the solution rank. The result also suggests a southward flow below the Kuroshio off Cape Toimisaki and in the Tokara Strait. In the section east of Amamioshima Island, a broad northward flow is suggested on the continental slope.

Standard seawater comparison up to P129

Batch to batch differences of IAPSO standard seawater (SSW) batches P103 to P129 were examined. Several comparison experiments, in which 2–23 batches of SSW were compared with each other, were carried out during the period from 1991 to 1997. Batch to batch differences for the KCl-labeled batches, P91 through P129 except for P113, P115, P117, P125 and P126, ranged from −2.2×10−3 to 1.2×10−3 in salinity with an average of −0.1×10−3 in salinity. Within-batch differences were observed to be <0.3×10−3 in salinity for recent batches except a few for which the differences were up to 0.6–0.9×10−3 in salinity. The KCl-labeled batches generally agreed to within 1.0×10−3 in salinity with the assumed reference. Applications of batch to batch differences of SSW for World Ocean Circulation Experiment (WOCE) expedition salinity data led to a significant reduction in salinity variance at crossovers both at the regional scale of the Philippine Sea and the basin scale of the Pacific Ocean and Atlantic Ocean. To obtain accuracy better than 2×10−3 in salinity, within-batch differences were also considered before the standardization for several ampoules of SSW. It is prudent that we continue the monitoring of batch to batch differences of SSW by comparing new with previous batches, so that one may correct both future salinity data and the historical salinity data.

Four-dimensional variational ocean reanalysis: a 30-year high-resolution dataset in the western North Pacific (FORA-WNP30)

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.

Standard Seawater Comparison of Some Recent Batches

Abstract The comparison experiment results of the standard seawater (SSW) were presented for batches P90, P100, P104, P106, P108 P111, and P112. This work shows that the SSW batch-to-batch agreement was recently improved. The adjusted mean differences relative to the mean of SSW batches P91 to P102 (Mantyla 1987) ranged from −0.6×10−3 to 0.8×10−3 for the SSW batches of P100, P104, P106, P108, P111, and P112.

Pre-Japan-ARGO: Experimental Observation of Upper and Middle Layers South of the Kuroshio Extension Region Using Profiling Floats

We deployed two profiling floats in the region south of the Kuroshio Extension in March 2000. Temperature and salinity profiles from a depth of 1500 × 104 Pa to the surface are reported every two and four weeks, respectively. The floats performed very well for first four months after deployment. Later they failed in surfacing for a few months when the sea surface temperature in the region was high. The salinity sensors seemed to suffer from some damage during their failure-in-surfacing period. Despite this trouble, the results clearly demonstrate that the profiling float is a very useful and cost-effective tool for physical oceanographic observation in the open sea.