Okhotsk Sea Intermediate Water Research Articles

Sea-ice distribution and atmospheric pressure patterns in southwestern Okhotsk Sea since the Last Glacial Maximum

Sea-ice diatom taxa ( Fragilariopsis cylindrus and Fragilariopsis oceanica) and their relative abundance in the Okhotsk Sea were used to reconstruct the history of sea-ice distribution and atmospheric pressure patterns since the Last Glacial Maximum (LGM). The temporal state of sea-ice distribution and atmospheric pressure patterns since the LGM can be divided into three modes: northern Aleutian Low mode, southern Aleutian Low mode, and strong Siberian High mode. The Southern Aleutian Low mode was dominant before 15 ka and after 6.5 ka, respectively, showing expanded sea-ice distribution into the central and southern Okhotsk Sea. During the deglaciation period (15 ka to 10 ka), sea-ice retreated from the southern Okhotsk Sea because of the pronounced westerly winds under the strong Siberian High mode. However, sea-ice distribution expanded in the northern Okhotsk Sea, which favors the development of extensive polynyas on the northern continental shelf. Occurrences of northern Aleutian Low mode were frequent between 10 and 6.5 ka, while sea-ice distribution expanded into the eastern Okhotsk Sea. Formation of the Okhotsk Sea Intermediate Water, inferred from radiolarian species Cycladophora davisiana, intensified under both northern Aleutian Low mode and strong Siberian High mode.

Characteristics of the cold‐water belt formed off Soya Warm Current

We examined the data obtained by acoustic Doppler current profiler, conductivity‐temperature‐depth profiler, and expendable bathythermograph observations, which were collected in the summers of 2000, 2001, and 2002, to clarify the characteristics of the cold‐water belt (CWB), i.e., lower‐temperature water than the surrounding water extending from the southwest coast of Sakhalin along the offshore side of Soya Warm Current (SWC) and to confirm one of the formation mechanisms of the CWB as suggested by our previous study, i.e., the upwelling due to the convergence of bottom Ekman transport off the SWC region. The CWB was observed at about 30 km off the coast, having a thickness of 14 m and a minimum temperature of 12°C at the sea surface. The CWB does not have the specific water mass, but is constituted of three representative water types off the northeast coast of Hokkaido in summer, i.e., SWC water, Fresh Surface Okhotsk Sea Water, and Okhotsk Sea Intermediate Water. In a comparison of the horizontal distributions of current and temperature, the CWB region is found to be advected to the southeast at an average of 40 ± 29% of the maximum current velocity of the SWC. The pumping speed due to the convergence of the bottom Ekman transport is estimated as (1.5–3.0) × 10−4 m s−1. We examined the mixing ratio of the CWB, and the results implied that the water mass of the CWB is advected southeastward and mixes with a water mass upwelling in a different region off SWC.

Iron supply to the western subarctic Pacific: Importance of iron export from the Sea of Okhotsk

Iron is an essential nutrient and plays an important role in the control of phytoplankton growth (Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991; Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface‐mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron‐rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea.

Warming of intermediate water in the sea of Okhotsk since the 1950s

Okhotsk Sea Intermediate Water (OSIW), the source water for ventilation of North Pacific Intermediate Water, exhibits a multidecadal warming trend. Historical data show that OSIW temperatures increased by 0.28, 0.57, 0.31 and 0.10°C during 1955 to 2003 at potential densities of 26.8, 27.0, 27.2 and 27.4σθ, at depths of approximately 250, 500, 700 and 900 m, respectively. This rate of warming is much faster than that of the global ocean. This OSIW warming is likely linked to the reduced ventilation of cold Dense Shelf Water associated with brine rejection during sea ice formation.

Distribution of N ⁎ in the Sea of Okhotsk and its use as a biogeochemical tracer of the Okhotsk Sea Intermediate Water formation process

Nutrient data collected from the Sea of Okhotsk in July 1998, September 1999, and June 2000 were used to calculate N ⁎, the deviation from the stoichiometric relationship between nitrogenous nutrients and phosphate. Intermediate water from east of Sakhalin to the Bussol Strait had low N ⁎ values (< − 5 μmol kg − 1 ), which led to very low N ⁎ values of approximately − 11 μmol kg − 1 in the northwestern shelf bottom water. The low N ⁎ distribution is consistent with distributions of low temperature and low salinity at densities around 26.8 σ θ , which are characteristic of the Okhotsk Sea Intermediate Water (OSIW) and the Dense Shelf Water (DSW) that is a source water of the OSIW. To assess N ⁎ as a new tracer of OSIW formation and to determine whether the low N ⁎ signature is created by in situ biogeochemical processes or reflects a conservative property of the water mass, we analyzed the nitrogen isotopic ratio of nitrate, δ 15NO 3 −. Although low N ⁎ usually suggests the occurrence of denitrification in a water column with large isotopic fractionation, neither the intermediate nor shelf bottom waters had high δ 15NO 3 − values, indicating that denitrification in the water column did not cause these low N ⁎ values. Sedimentary denitrification with little isotopic fractionation and phosphate dissolution from sediment likely caused the extremely low N ⁎ of the shelf bottom water. The low N ⁎ of the intermediate water must thus be unaffected by in situ biogeochemical processes and instead reflect the intrusion of shelf bottom water into the offshore intermediate layer. Our results show that N ⁎ acts as a conservative tracer of the OSIW formation process. Because the water masses characterized by low N ⁎ are created at the shelf bottom, N ⁎ is a valuable tracer of DSW which originates from continental shelf areas.

Middle to late Holocene changes of the Okhotsk Sea Intermediate Water and their relation to atmospheric circulation

High abundance of the radiolarian Cycladophora davisiana in the Sea of Okhotsk has been observed at the Okhotsk Sea Intermediate Water (OSIW). This water is formed by brine rejection in the polynya and is characterized by cold, oxygenated conditions, with high a microbial content. The C. davisiana abundance recorded in three sediment cores from the southwestern part of the sea implies ventilation changes of the OSIW during the last 7.5 kyrs. The ventilation had significantly increased during the interval from 7.5 to 3.5 ka corresponding to a warm period, but declined during 3 to 2 ka and at 0.3–0.4 ka coincident with cold periods such as the Neoglaciation and Little Ice Age, respectively. The declined ventilation might be caused by reduction of the polynyas with the wind vector change from a northerly to an easterly direction relating to the southward shift of the Aleutian Low during the strong winter.

Chlorofluorocarbons in the Sea of Okhotsk: Ventilation of the intermediate water

Ventilation of the intermediate layer in the Sea of Okhotsk is studied with regard to the ventilation of North Pacific Intermediate Water (NPIW). Measurements of chlorofluorocarbons (CFCs) from 1998 to 2000 reveal that the Okhotsk intermediate water is ventilated in two ways. The first consists of dense water formation in the polynyas that form on the northern shelves during winter. Measurements show that on the northwestern shelf, because of atmospheric cooling and brine rejection, the cold water that forms is dense enough to enter the intermediate layer. The CFC concentration in this dense shelf water (DSW) is high and almost saturated with respect to atmospheric CFCs, indicating that the DSW experiences active air‐water gas exchange during its formation. Away from the shelves, the CFC distribution shows that the DSW directly ventilates the upper level of the Okhotsk Sea Intermediate Water (OSIW). The second ventilation process is driven by the strong tidal mixing around the Bussol' Strait. The observed distribution of CFCs at the 27.4 σθ level suggest that diapycnal mixing around the strait transports CFCs from the surface to this deeper layer, ventilating the OSIW. Combination of both processes means that the OSIW is more ventilated than Pacific water at the same density levels.

Distribution and formation of Okhotsk Sea Intermediate Water: An analysis of isopycnal climatological data

In order to clarify the distribution and formation of Okhotsk Sea Intermediate Water (OSIW) an isopycnal climatological data set based on all of the available historical observations is developed and examined. The isopycnal maps clearly show that there are two direct ventilation sources for OSIW: Dense Shelf Water (DSW) with cold and fresh properties influenced by sea ice formation in the northwest shelf region and Forerunner of Soya Warm Current Water (FSCW), which has warm and saline properties originating in the Japan Sea. The cold and fresh water extends southward from the northwest shelf to the shelf slope off Sakhalin Island. It suggests that DSW is transported southward by the East Sakhalin Current. The DSW enters the Kuril Basin and mixes with FSCW and Western Subarctic Water (WSAW) originating from the North Pacific to form OSIW. The annual mean production rate of DSW estimated from the total volume of DSW over the shelf in spring and summer is 0.67 Sv (1 Sv = 106 m3/s). The annual mean volume transport of FSCW is estimated to be 0.08 Sv on the basis of its cross‐section area and the current speed. Assumption of isopycnal mixing yields a mixing ratio of 1:1:0.1 among DSW, WSAW, and FSCW to form OSIW. We estimate the annual production rate of OSIW to be 1.4 Sv; the corresponding OSIW's renewal time is 7 years. Continued assumption of isopycnal mixing yields a mixing ratio of 0.6:0.4 between WSAW and OSIW to form Oyashio Intermediate Water (OYIW).

Influence of Okhotsk Sea Intermediate Water on the Oyashio and North Pacific Intermediate Water

The volume transport between the Okhotsk Sea and western subarctic North Pacific is estimated on the basis of direct current measurements made by a lowered acoustic Doppler current profiler (LADCP) and hydrographic data taken in August and September 1999 near the Kuril Islands. The estimated southwestward Oyashio transport was 11–12 Sv (=106 m3/s) in the density range of 26.7–27.4 σθ. The net transport of the Okhotsk Sea water (OSW) was 3.4 Sv, of which 1.3 Sv was coming out through the Kruzenshterna Strait and 2.1 Sv was coming out through the Bussol' Strait. A bidirectional flow was observed in the Bussol' Strait: inflow in the northern part of the strait and outflow in the southern part; 0.6 Sv of OSW recirculated along the western subarctic gyre. The remaining 2.8 Sv of OSW could contribute to the formation of North Pacific Intermediate Water as a cross‐Subarctic Front transport along the coast (1.4 Sv) and across the offshore SAF (1.4 Sv). The production rate of the Dense Shelf Water (DSW) through sea ice formation was estimated to be at least 1.1 Sv. When the intermediate OSW is formed through the mixing between cold and fresh DSW and warm and salty waters from the Pacific and Japan Sea, 16.8 TW (=1012 W) of heat, 0.007 Sv of freshwater, and 2.3 × 105 kg/s of salt are exchanged. The net flux of oxygen and anthropogenic CO2 (ex‐CO2) from the Okhotsk Sea into the Pacific in 26.7–27.4 σθ is estimated to be 10.5 and 2.1 Tmol/yr (0.025 GtC/yr), respectively. The ex‐CO2 transported by OSW accounts for about 10% of the accumulation of ex‐CO2 in the subarctic and subtropical Pacific.

An extremely turbid intermediate water in the Sea of Okhotsk: Implication for the transport of particulate organic matter in a seasonally ice‐bound sea

We found a water mass characterized with enormous turbidity and very low temperature in the intermediate layer of the Sea of Okhotsk in September 1999. In June 2000 and September 1999, the cold and turbid water mass was also located in bottom boundary layer on the northwestern shelf region, where sea ice had rejected large amounts of brine waters in winter. Cold brine waters settled on the bottom of shelf must have entrained sedimentary particles, including newly produced organic matter, due to strong tidal mixing, and finally flowed out into Okhotsk Sea Intermediate Water (OSIW) by a western boundary current intensified in autumn. Sediment traps moored in the deep basin recorded the outburst of particles in autumn. This mechanism is likely to export the particulate organic matter produced on the shelf efficiently to pelagic intermediate waters.

Effects of sea ice formation and diapycnal mixing on the Okhotsk Sea intermediate water clarified with oxygen isotopes

Processes relating to the formation of dense shelf water and intermediate water in the Okhotsk Sea were studied by examining oxygen isotope ratios ( δ 18O), salinity, and temperature. The salinity and δ 18O of the cold dense shelf water on the northern continental shelf showed peculiar relationship. The relationship indicates that 3% of the mixed-layer water, having salinity of 32.6, froze and the remaining 97% became dense shelf water of salinities of more than 33.2 ( σ θ >26.7) during the sea ice formation. The salinity– δ 18O relationship also shows that 20% of the Okhotsk Sea Intermediate Water at the σ θ =26.8 level was derived from the dense shelf water. The remaining 80% came from the Western Subarctic Pacific water modified by diapycnal mixing of water affected by the surface cooling and freshening within the Okhotsk Sea. The mixing with dense shelf water contributes to only 26% of the temperature difference or 8% of the salinity difference between the original Pacific water and the Okhotsk Sea Intermediate Water at σ θ =26.8. This result suggests that the cold and less saline properties of the Okhotsk Sea Intermediate Water are produced mainly by diapycnal mixing, rather than by mixing of the Pacific water with the dense shelf water.

Okhotsk Sea intermediate water formation deduced from oxygen isotope systematics

To clarify the mechanisms of the Okhotsk Sea intermediate water formation, water samples collected in the Okhotsk Sea and neighboring western North Pacific were analyzed for δ18O as well as salinity and other routinely measured components. The δ18O‐salinity relation in the Okhotsk Sea was markedly different from that in the western subarctic Pacific feeding the Okhotsk Sea. The intermediate water of the Okhotsk Sea was less saline than that of the western subarctic water at the same density but was more saline at the same δ18O value, especially in the density range 26.5&lt;σθ&lt;27.0. This could be due to mixing of dense shelf water formed during sea‐ice formation with neighboring water. The amount of fresh water removed by the sea‐ice formation was estimated from the salinity anomaly of the Okhotsk Sea intermediate water. The calculated amounts of fresh water removed from the Okhotsk Sea intermediate water corresponded to the amount of sea ice formed for 0.6–3.8 years in the Okhotsk Sea. This suggests that the residence time of the Okhotsk Sea intermediate water is a few years.