Chemical evidence for the origin of the cold water belt along the northeastern coast of Hokkaido

In the southwestern Okhotsk Sea, the cold water belt (CWB) is frequently observed on satellite images offshore of the Soya Warm Current flowing along the northeastern coast of Hokkaido, Japan, during summertime. It has been speculated that the CWB is upwelling cold water that originates from either subsurface water of the Japan Sea off Sakhalin or bottom water of the Okhotsk Sea. Hydrographic and chemical observations (nutrients, humic-type fluorescence intensity, and iron) were conducted in the northern Japan Sea and southwestern Okhotsk Sea in early summer 2011 to clarify the origin of the CWB. Temperature–salinity relationships, vertical distributions of chemical components, profiles of chemical components against density, and the (NO₃ + NO₂)/PO₄ relationship confirm that water in the CWB predominantly originates from Japan Sea subsurface water.     

Ba, Si, U, Al, Sc, La, Th, C and C/C in a sediment core in the western subarctic Pacific as proxies of past biological production

A sediment core covering the last 145 kyrs was collected in the western subarctic Pacific (WSAP), and analyzed for Ba, U, Al, Sc, La, Yb, Th, biogenic opal (Opal) and organic carbon (C_org) as well as its isotopic ratio (δ¹³C). This study examined the change of past biological production in WSAP with multiple proxies, together with understanding the relation between Loess from the Asian continent and the biological production. The Loess content was estimated from the metal components, Al, Sc, La, Yb and Th. In this high latitude core (50°N), the Loess content was generally high during the glacial periods, but it was also high even in some interglacial periods. The excess amount of Ba relative to the detrital material composition, Ba_ex, showed the best correlation with the Vostok δD (r = 0.72, p < 0.001), indicating that the biological production was lower in the glacial periods than in the interglacials. This corroborates the pervasive correlation between Ba_ex in the polar region, WSAP and the Antarctic Sea, and Antarctic temperature, combined with previous research. This correlation might be explained by the stratification caused by cooling. In addition, the time variations of Ba_ex in WSAP were similar to those of Ba_ex in the Okhotsk Sea and of other proxies (C_org and Opal) in both the Okhotsk and the Bering Sea, indicating the spatial homogeneity of Ba_ex in WSAP including proximal marginal seas. The Opal content was more weakly correlated with the Vostok δD (r = 0.46, p < 0.001) than Ba_ex, reflecting that Opal in WSAP including proximal marginal seas was spatially heterogeneous compared to Ba_ex. While both the C_org content and U_ex, the excess amount of U relative to the detritus composition, were not positively correlated with the Vostok δD, they behaved similarly in the sediments. The positive correlation between δ¹³C and the Vostok δD (r = 0.42, p < 0.001), between δ¹³C and Ba_ex (r = 0.60, p < 0.001) and between δ¹³C and Opal (r = 0.36, p < 0.01) indicates that δ¹³C in WSAP may give some information on the phytoplankton growth rate. There was not a significant correlation between the spatially homogeneous Ba_ex in WSAP and Loess (r = − 0.16, p > 0.01), suggesting that the increase of biological production with the increase of Loess supply during the glacial periods did not occur.     

Interannual variations of the occurrence of epipelagic fish in the diets of the seabirds breeding on Teuri Island, northern Hokkaido, Japan

The diets of breeding seabirds can be a good monitor of marine environmental changes. From 1984 to 2001 we monitored the diets of black-tailed gulls (Larus crassirostris) (“surface foragers”), rhinoceros auklets (Cerorhinca monocerata) (“epipelagic divers”), and Japanese cormorants (Phalacrocorax filamentotus) (“bottom divers”) that breed on Teuri Island at the northern boundary of the Tsushima Warm current in the Sea of Japan/East Sea. Between 1984 and 1987, both the gulls and the auklets foraged on the sardine (Sardinops melanostictus), but after 1992, they switched to the anchovy (Engraulis japonica). This change might reflect the collapse of the sardine stock in the late 1980s. In the 1990s, the year-to-year variations of the percentage of anchovy in the diets of the three seabird species showed similar trends: High in 1994 and 1998–2001; and low in 1992–1993 and 1995–1997. The estimated stock size of the anchovy population in the Tsushima Current area was positively correlated with the percentage of mass of anchovy in the seabirds’ diets. Thus, the short-term annual changes of the total anchovy availability, which might reflect SST or the volume transport of Tsushima Current, possibly affected the seabirds diets on this island.     

Seasonal variations of shelf circulation in Hidaka Bay,Hokkaido, Japan,with an interpretation of the migration route of juvenile walleye pollock

The outer shelf of Funka Bay, located at the bay head of Hidaka Bay, is a recognised main winter spawning ground for walleye pollock (Theragra chalcogramma), whose newly hatched juveniles migrate eastward along the Hidaka shelf to the nursery ground located in the Doto area. To examine the seasonal change of the coastal current along this migration route, four current moorings were deployed along the shelf in Hidaka Bay from April, 2004 to June, 2006. Since these mooring sites were close to the coast, the circulation was estimated after removal of the wind-driven component. It was found that the winter coastal flow, forced by a north-westerly monsoon wind, is clockwise along the Hidaka shelf. However, this flow is weak due to the superimposition of the opposite Coastal Oyashio flow trapped on the shelf. In summer, a bifurcation of the coastal current occurs along the north-eastern Hidaka shelf with a south-eastward flow, associated with the Tsugaru Gyre, and a north-westward flow, consisting of a branch from this gyre. Our results provide a new understanding of the migration of juvenile walleye pollock: (1) very slow transport of juveniles along the Hidaka shelf from winter to spring, and (2) selection of earlier spawning survivors due to the bifurcated flow in early summer.     

239,240Pu and137Cs in the East China and the Yellow Seas

The distribution and inventory of artificial radionuclides,^239,240Pu and¹³⁷Cs were determined in the East China and the Yellow Seas in 1987. Almost all of^239,240Pu and 50 to 80% of¹³⁷Cs in the continental shelf area are retained in the sediment column.^239,240Pu sediment inventory in the sea area is larger than the fallout input and tends to increase southwardly. This excess^239,240Pu and the lateral distribution are attributable to the supply of^239,240Pu by the Yangtze River discharge. On the contrary,¹³⁷Cs sediment inventory shows a decrease to the south, and the fact can be accounted for by the southward dispersion of fine silt particles discharged from the Yellow River. Total¹³⁷Cs inventory is smaller than the estimated fallout input, and the fact seems to indicate dispersion of¹³⁷Cs out of the shelf region. Vertical profiles of^239,240Pu and¹³⁷Cs contents in sediments differ from that of natural²¹⁰Pb, implying the effect of varied accumulation rates of the artificial radionuclides over the sediment particle mixing by benthic organisms. Apparent maximum sediment particle mixing coefficient (D _B ) calculated from the excess²¹⁰Pb profiles in stations located between the inner and outer shelves ranged from 1.4 to 8.3 cm²y^–1. ThisD _B value is higher than that in the Okinawa Trough (1.0 cm²y^–1), but lower than previously estimatedD _B value (26 cm²y^–1) in the outer shelf mud.     

Numerical study of the behavior of heated water discharged into the ocean

Matsuno and Nagata (1987) showed numerically that the spreading characteristics of the discharged heated water from power plants of existing scale is significantly influenced by the earth's rotation effect. Although the effect of the enhancement of the Coriolis parameter on the spreading characteristics of the formed warm water mass was discussed in order to demonstrate the rotation effect, other parameters such as the density difference between the discharged heated water and ambient water and the vertical eddy viscosity and diffusivity were fixed. In this paper, the dependence of the spreading characteristics on these parameters is examined. Then, it is shown that the overall shape of the formed warm water mass and density and velocity structure strongly depend on these parameters. Also, it is indicated that the behavior of the warm water mass under the rotation effects is too complicated to be described with a few parameters. For example, the internal radius of deformation seems to be one of the determinative parameters, and the increase of the density difference between discharged water and ambient water has a similar effect on the shape of the formed warm water mass as a decrease of the Coriolis parameter. However, a change of the two values has different effects on the detailed density structure and current structure of the warm water mass. The Prandtl number seems to determine some aspects of the veloczty field such as velocity magnitude and width of the southward flowing current zone. However, other features such as the thickness of the warm water mass are not determined by the Prandtl number.     

Tritium in the northwestern North Pacific

Vertical profiles of tritium in seawater were determined for samples collected during the period from 1988 to 1990 at fourteen stations in the northwestern North Pacific (the Oyashio region) including the Okhotsk Sea and the Bering Sea. The profiles usually had a maximum in the surface layer and decreased gradually with depth down to 1,000 m. The water column inventory of tritium averaged 63% of the total atmospheric input in this region.The horizontal distribution of tritium showed a maximum in the region facing the Okhotsk Sea near 45°N for every isopycnal surface of ₀ ranging from 26.60 to 27.40. The ages of the intermediate water were calculated for the respective isopycnal surfaces in the maximum region. This calculation assumed that the intermediate water was formed by the isopycnal mixing of two water masses—the Okhotsk Sea and the Bering Sea Component Waters, which had been produced in wintertime by the diapycnal mixing of the surface and the deep waters in the respective marginal seas. The results show that the intermediate water in this region was formed in the late 1980's for the water which has ₀ of 26.60 to 26.80 and about 1970 for the water which has ₀ of 27.00 to 27.40. Although we have estimated the mean ages of the intermediate water, the horizontal profile of dissolved oxygen suggests that the Okhotsk Sea Component Water is younger than the mean age.     

Bispectra of spike-array type time series and their application to the analysis of oceanic microstructures

Bispectral analysis is applied to records of the vertical profile of the vertical temperature gradient in the oceanic thermocline in the San Diego Trough. The bispectra exhibit three notable features; (1) bispectral peaks at the points (0.2 m^–1, 0.2 m^–1) and (0.2 m^–1, 0.1 m^–1), (2) bispectral ridges along the lines ( ₁= ₀, ₂= ₀ and ₁+ ₂= ₀ corresponding to peak wavenumbers ₀ in power spectra, and (3) array of bispectral peaks of interval of 0.2 m^–1 The results are compared with the bispectra of several modeled time series of spike-array type. The periodicity of 5 m found in the records seems to have two meanings: spacing of predominant spikes and wavelength of predominant sinusoidal wave. If this indicates the existence of internal waves having a vertical wavelength the same as the scale of homogeneous layers, it would suggest the possible importance of internal waves in the formation and maintenance mechanisms of oceanic microstructure.     

Cooling-Induced Current in the Upper Ocean of the Japan Sea

The most plausible scenarios for seasonal to interannual variabilities and their possible causes are investigated for the Tsushima Current system passing through the Japan Sea. The study is based on the north and south two-box model across the polar front in an idealized upper ocean of the Japan Sea. The boxes are connected by lateral diffusive heat transport and cooled by atmospheric forcing at the annual mean state. The south box, i.e. the Tsushima Current region, only interacts with the outside warmer box in the East China Sea and has an eastward thermal-driven current originating in the outside box. The magnitude of this current depends on the strength of the thermal gradient between the north and south boxes; the inflow of warm waters can therefore be maintained by net heat loss through the sea-surface. I call such a thermal-driven inflow process a "Cooling-Induced Current" system in the present study. Under periodical heat forcing, the perturbation response of the model to water temperature fields and inflow transport were examined. It is shown that the lateral diffusion time across the polar front (over a period of 10 years) is crucial to the interannual modeled response. An analysis of the seasonal heat budget suggests that the heat transported into the Japan Sea from the East China Sea in summer is stored mainly within the Tsushima Current region and contributes to heat loss by the sea-surface cooling in winter.     

Transport of Oyashio Water across the Subarctic Front into the Mixed Water Region and Formation of NPIW

Oyashio water flowing into the Mixed Water Region (MWR) and the Kuroshio Extension region that forms North Pacific Intermediate Water (NPIW) has been examined, based on four Conductivity-Temperature-Depth profiler (CTD)/Lowered Acoustic Doppler Current Profiler (L-ADCP) surveys of water masses and ocean currents. There are two processes by which the Oyashio water intrudes across the Subarctic Front (SAF): one is a direct cross-nearshore-SAF transport near Hokkaido along the western boundary, and the other is a cross-offshore-SAF process. Seasonal variations were observed in the former process, and the transport of the Oyashio water across SAF near Hokkaido in the density range of 26.6–27.4σ_θ was 5–10 Sv in spring 1998 and 2001, and 0–4 Sv in autumn 2000, mainly corresponding to the change of the southwestward Oyashio transport. Through the latter process, 5–6 Sv of the Oyashio water was entrained across the offshore SAF from south of Hokkaido to 150° in both spring 2001 and autumn 2000. The total cross-SAF Oyashio water transport contributing to NPIW formation is more than 10 Sv, which is larger than previously reported values. Most of the Oyashio water formed through the former process was transported southeastward through the Kuroshio Extension. It is suggested that the Oyashio intrusion via the latter process feeds NPIW in the northern part of the MWR, mainly along the Subarctic Boundary and SAF. This revised version was published online in July 2006 with corrections to the Cover Date.