Ikuo Kaneko

Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific Intermediate Water

Recently obtained World Ocean Circulation Experiment (WOCE) sections and pre-WOCE hydrography are used to study the water-mass structure and formation and transformation of North Pacific Intermediate Water (NPIW). Five neutral density surfaces are selected and mapped, encompassing NPIW from 400 to 900 m in the subtropical latitudes with a distance of ∼100 m between a pair of surfaces. NPIW is defined as a subtropical gyre salinity minimum which is well followed by a neutral density surface σN = 26.9. Formation and transformation of NPIW is examined by the mapped Turner angle on neutral density surfaces. Apparent diffusive double diffusion is found in the Alaskan gyre on σN = 26.5 neutral surface, in the northwest subpolar gyre and the Okhotsk Sea on σN = 26.9 neutral surface, and mainly in the Okhotsk Sea on the two deep neutral surfaces σN =27.2 and σN = 27.4. These diffusive regions indicate transformation sources for NPIW. Along with additional information of potential vorticity and stream function, it is found that there are two different NPIW formation sources: one in the Gulf of Alaska characterized by high potential vorticity and the other in the Okhotsk Sea characterized by low potential vorticity. The former lies shallower at σN =26.2–26.5, but its effect deepens to NPIW core density level at σN = 26.8 on the basis of potential vorticity distribution. The latter includes the influence of the northwest subpolar gyre and extends much deeper to σN = 27.4. We call them Gulf of Alaska Intermediate Water (GAIW) and Okhotsk Intermediate Water (OIW), respectively. GAIW contributes to NPIW in the eastern part of the subtropical gyre east of date line, whilst OIW dominates in the west and entire lower part of NPIW. Seasonal flow stream function mapped on neutral surfaces shows that the contribution of GAIW to NPIW occurs mainly in the wintertime, because in winter a significant northward shift of zero wind stress curl makes the Gulf of Alaska an additional source for NPIW.

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.

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.

Mid‐depth freshening in the North Pacific subtropical gyre observed along the JMA repeat and WOCE hydrographic sections

[1] We report a freshening at mid-depth in the North Pacific subtropical gyre by using long-term repeat hydrographic data along the 137°E section and one-time hydrographic data along the World Ocean Circulation Experiment Hydrographic Program (WHP) P2 and P3 sections. North of 15°N along the 137°E section, we estimated a linear freshening trend of 0.0015/yr between the main thermocline and the salinity minimum layer of the North Pacific Intermediate Water, mainly caused by isopycnal surface deepening due to warming, and by westward shifts of the salinity-minimum tongue due to strengthening of the subtropical gyre. Furthermore, along the WHP-P2 section, the linear freshening trend could be classified into several groups according to longitude. Such spatial differences in the freshening trend seem to reflect differences in the formation processes and mid-depth pathways of the salinity minimum waters.