North Pacific Intermediate Water Research Articles

Synchronous bidecadal periodic changes of oxygen, phosphate and temperature between the Japan Sea deep water and the North Pacific intermediate water

From time series of dissolved oxygen (O2), phosphate (PO4) and water temperature (T) during the last forty years in the Japan Sea Deep Water and the North Pacific Intermediate Water, we found that O2, PO4 and T show a clear bidecadal oscillations of about 18 years superimposed on the linear trends of decrease of 0.47 μmol‐O2 kg−1 yr−1 and increase of 0.003 μmol‐PO4 kg−1 yr−1 and 0.005°C yr−1 in both two oceanic areas. The changes of O2, PO4 and T in the two oceanic areas were synchronized despite the water formation systems of each area being independent. Both the linear trends and the oscillations of O2, PO4 and T in the two oceanic areas also showed a strong correlation with the anomaly of the sea surface level pressure in the North Pacific that possibly affects the change of surface oceanic condition.

The pathway and circulation of North Pacific Intermediate Water

It has been speculated that the subtropical North Pacific Intermediate Water (NPIW) was formed by a shortcut of Okhotsk Sea source into the western subtropical gyre because this is the geographically shortest distance, and based on incomplete data. This mechanism is contradictory to the distribution of seawater properties, and, dynamically, is inconsistent with a basin‐wide distribution of NPIW and associated gyre circulation. A lengthy transpacific pathway has been identified for the transformed NPIW source waters that first enter the eastern subtropical gyre, which is robust in a water‐mass age distribution. Using an updated hydrography and a water‐mass mixing scheme combining property distribution with flow streamfunction the present study establishes a dynamically self‐consistent gyre circulation of NPIW and its pathway from the subpolar formation regions (source) to the Indonesian Throughflow (sink).

North Pacific Intermediate Water response to a modern climate warming shift

Oceanic observations and an isopycnal ocean model simulation are used to investigate the response of North Pacific Intermediate Water (NPIW) to atmospheric forcing associated with the well‐known 1976–1977 climate regime shift to a warm regime. The model reproduces numerous features of NPIW including distribution, depth, temperature, and salinity. Changes in NPIW associated with the climate shift in the California coastal region were strongly influenced by an anomalous poleward flow at depth (300–1100 m). This current transports old, high salinity, low oxygen intermediate waters from the northern tropics to the midlatitudes. For depths below the mixed layer, the model reproduces observed changes in salinity, nitrates, and, to some extent, oxygen, thus suggesting that advective/diffusive processes are dominant in determining their concentrations below 300 m, isolated from the surface effects of direct atmospheric forcing and biological processes. These changes are structurally similar to those induced by much larger, abrupt climate changes at the end of the last glacial episode.

Provenance of intermediate waters in the western North Pacific deduced from thermodynamic imprint on δ13C of DIC

Stable carbon isotopes of dissolved inorganic carbon (δ13CDIC), together with phosphate concentrations (PO43−) of the western North Pacific marginal area are presented. Although δ13CDIC controlled not only air‐sea exchange but also biological activities, by subtracting PO43− as a biological factor from the δ13CDIC, the thermodynamic imprint on the carbon isotopic composition (δ13Cas) is used as a conservative tracer. The linear relationship between δ13Cas and temperature with the δ13Cas/temperature slope of −0.104‰/°C is observed in the North Pacific subtropical region. This relationship can be extended to the Weddell Sea deep waters. Therefore oceanic DIC is apparently in temperature‐dependent isotopic equilibrium with atmospheric CO2. To characterize each water mass, the “isotopically labeled temperature” (Tiso), which is derived from δ13Cas‐temperature relationship obtained for the subtropical gyre water and Weddell Sea Water, is introduced. In contrast, the δ13Cas‐temperature relationship is not necessarily linear for the northern Japan Sea Water, the Kuroshio‐Oyashio Mixed Water and the Okhotsk Sea Water. This nonlinearity implies apparent air‐sea carbon isotopic disequilibria resulting from water cooling without accompanying air‐sea carbon isotope equilibrium. By using the potential temperature (Tpot) and the Tiso, the source waters constituting intermediate waters (26.8σθ) have been estimated. The results clearly indicate that (1) the intermediate water in the southern Okhotsk Sea contains up to 20% Japan Sea Water, and that (2) “new” North Pacific Intermediate Water (NPIW) consists of Okhotsk Sea Water, subarctic gyre water, and “old” subtropical NPIW.

Data-Based Meridional Overturning Streamfunctions for the Global Ocean

The meridional overturning circulation for the Atlantic, Pacific, and Indian Oceans is computed from absolute geostrophic velocity estimates based on hydrographic data and from climatological Ekman transports. The Atlantic overturn includes the expected North Atlantic Deep Water formation (including Labrador Sea Water and Nordic Sea Overflow Water), with an amplitude of about 18 Sv through most of the Atlantic and an error of the order of 3–5 Sv (1 Sv ≡ 106 m3 s−1). The Lower Circumpolar Deep Water (Antarctic Bottom Water) flows north with about 8 Sv of upwelling and a southward return in the South Atlantic, and 6 Sv extending to and upwelling in the North Atlantic. The northward flow of 8 Sv in the upper layer in the Atlantic (sea surface through the Antarctic Intermediate Water) is transformed to lower density in the Tropics before losing buoyancy in the Gulf Stream and North Atlantic Current. The Pacific overturning streamfunction includes 10 Sv of Lower Circumpolar Deep Water flowing north into the South Pacific to upwell and return southward as Pacific Deep Water, and a North Pacific Intermediate Water cell of 2 Sv. The northern North Pacific has no active deep water formation at the sea surface, but in this analysis there is downwelling from the Antarctic Intermediate Water into the Pacific Deep Water, with upwelling in the Tropics. For global Southern Hemisphere overturn across 30°S, the overturning is separated into a deep and a shallow overturning cell. In the deep cell, 22–27 Sv of deep water flows southward and returns northward as bottom water. In the shallow cell, 9 Sv flows southward at low density and returns northward just above the intermediate water density. In all three oceans, the Tropics appear to dominate upwelling across isopycnals, including the migration of the deepest waters upward to the thermocline in the Indian and Pacific. Estimated diffusivities associated with this tropical upwelling are the same order of magnitude in all three oceans. It is shown that vertically varying diffusivity associated with topography can produce deep downwelling in the absence of external buoyancy loss. The rate of such downwelling for the northern North Pacific is estimated as 2 Sv at most, which is smaller than the questionable downwelling derived from the velocity analysis.

Control on dissolved iron concentrations in deep waters in the western North Pacific: Iron(III) hydroxide solubility

The vertical profiles of nutrient, AOU (apparent oxygen utilization), and Fe(III) hydroxide solubility (Fe(III) solubility) in the western North Pacific commonly show that their concentrations increased with depth below the surface mixed layer and have strong gradients around the North Pacific Intermediate Water (NPIW) in a potential density (σθ) range of 26.7–27.0. They had the maximum values at a σθ range of 27.0–27.5, and then rapidly decreased with depth at higher σθ than 27.5. The similarity of their profiles versus σθ suggests that they are controlled by similar processes with an intrinsic timescale such as deep‐ocean circulation. The vertical profiles of dissolved Fe were also characterized by mid‐depth maxima and, subsequently, a slight decrease with depth, which is markedly similar to nutrient and Fe(III) solubility depth profiles. This implies that the major source of dissolved Fe in the deep ocean is release during the remineralization of biogenic organic matter. In the present study, we attempted to confirm that the dissolved Fe depth profiles are controlled by the sinking particulate organic matter (POM), the production of dissolved Fe from POM during carbon remineralization, the scavenging of dissolved Fe and the temporal fixed Fe(III) solubility depth profile. Using the production rate (α) of dissolved Fe (1/α = 5–10 years), the calculated depth profile of dissolved Fe is in remarkable agreement with the observed profile of dissolved Fe with mid‐depth maxima. Therefore we concluded that dissolved Fe concentrations in deep ocean waters are controlled primary by the Fe(III) solubility.

Improved estimates of the dynamical state of the North Pacific Ocean from a 4 dimensional variational data assimilation

A 4‐dimensional variational data assimilation system has been used to better define the mean seasonal state of the North Pacific. The synthesis of available observational records and a sophisticated general circulation model produces a dynamically consistent time‐varying dataset which exhibits realistic features of the global ocean circulation and requires no artificial sources or sinks for the temperature and salinity fields. The dataset enables us to clarify the water mass formation and movement processes. A sensitivity experiment using our system reveals that the origin of the North Pacific Intermediate Water can be traced back to the Okhotsk and Bering Seas in the subarctic region and to the subtropical Kuroshio region further south, consistent with recent observational findings. This result illustrates that the ocean state derived from our data assimilation has greater information and forecast potential than that obtained from earlier methods.

Transport of North Pacific Intermediate Water across Japanese WOCE sections

Under the auspices of the World Ocean Circulation Experiment (WOCE) Hydrographic Program, Japan carried out a total of four sections in the North Pacific, P2 at 30°N, P13 at 165°E, P9 at 137°E, and P8 at 130°E. These Japanese and several other WOCE sections are used to estimate the transport of North Pacific Intermediate Water (NPIW) in the subtropical gyre. Previous studies speculate about 3 Sv (1 Sv = 106 m3 s−1) of NPIW transport, originating mainly in Okhotsk Sea, entering the subtropical gyre southward via the mixed water region in the east of Japan. The total subpolar input is 3.2 Sv when another source of NPIW in the Gulf of Alaska, estimated at 0.2 Sv in this study, is added. This transport from the Japanese WOCE sections (a mean of 2.8 ± 0.2 Sv) compares well with several other sections (a mean of 2.5 ± 0.2 Sv). From a total of five meridional and two zonal sections, a mean transport of 2.7 ± 0.2 Sv is found in the defining NPIW layer between σN = 26.5 and 27.4 neutral density surfaces. The transport shows a steady southward and westward increase. About 3 Sv transport is found mainly in the southern (south of 25°N) and western (west of the dateline) sections. Closer to the subarctic‐tropical frontal zone and in the eastern subtropical gyre, NPIW transport is less than 2 Sv. This implies that a lower subpolar input of about 2 Sv is sufficient for subtropical ventilation. Recirculation, entrainment with the upper and lower waters, and epineutral and dianeutral mixing likely add about 1 Sv to the NPIW transport. To interpret the transport distribution, a simple water mass mixing scheme is applied, which includes two formation sources, Okhotsk Intermediate Water (OIW) and Gulf of Alaska Intermediate Water (GAIW) in the subpolar regions, and a transformed aged NPIW (aNPIW) in the northwestern subtropical gyre southeast of Japan. The aNPIW mixing fraction increases southward and westward, while OIW and GAIW fractions decrease. This suggests that after NPIW source water parcels enter the subtropical gyre, they largely recirculate within the wind‐driven gyre and thus become aged. Therefore one usually sees very low oxygen and CFC values in the NPIW layer in the subtropical gyre.

Implications of cabbeling on the formation and transformation mechanism of North Pacific Intermediate Water

The World Ocean Atlas 1994 is used to study the formation and transformation of North Pacific Intermediate Water (NPIW), paying attention to the broad subarctic‐tropical frontal zone (SATFZ). Four neutral density surfaces, σN = 26.5, 26.9, 27.2, and 27.4, are mapped to span the NPIW layer with the σN = 26.9 surface identified with the salinity minimum of NPIW. Various dianeutral motion processes, cabbeling, thermobaricity, vertical turbulent mixing and double‐diffusion, and water‐mass property conversion are quantified; cabbeling appears to be the most important. Spatially, dianeutral transport is strongest in the SATFZ. Within the SATFZ subdomain 36°N–44°N, 142°E–140°W, with assumed epineutral and dianeutral diffusivities of K = 103 m2 s−1 and D = 10−5 m2 s−1 cabbeling contributes to a maximum dianeutral downwelling transport of 0.9 Sv on the σN = 26.5 neutral density surface (the upper boundary of NPIW) and 0.7 Sv on the σN = 26.9 surface (at the NPIW core). In meridional sections, cabbeling shows epineutral extension from the SATFZ towards the subtropical gyre, most apparent along the σN = 26.9 surface. Dianeutral transport is found to be sensitive to the epineutral diffusivity K on the upper two surfaces σN = 26.5 and 26.9 where the transport is dominated by cabbeling. With only half an order of magnitude increase in diffusivity K = 103.5m2 s−1, the total NPIW (about 3 Sv) can be accomplished by cabbeling alone. These results suggest that cabbeling plays a crucial role in the NPIW transformation. Since cabbeling describes physically the dianeutral downwelling resulted from epineutral mixing, strong cabbeling in the SATFZ implies that NPIW sources are first transformed by cabbeling from the upper to the lower neutral density surfaces, and subsequently, NPIW is carried by epineutral advection and gyre circulation into the subtropical interior. Thus cabbeling is revealed as the principle mechanism responsible for transforming subpolar source waters into subtropical NPIW water via the SATFZ.

Interleaving Phenomena of the North Pacific Intermediate Water in the Offshore Area of the Kuroshio

To study the intruded phenomena of North Pacific Ocean around Boso peninsular, water property distribution in the adjacent seas to Japan is studied using the hydrographic data obtained by Japan Maritime Agency and Japan Fisheries Agency from 1973 to 1996, The scattering of water type in T-5 diagram is relatively small in the Kuroshio Region. Both the envelopes of saline side and of fresh side of the scattered data points shifts gradually from saline side to fresh side as the observation Line moves from southwest to northeast. In mixed water region, the scattering of water type increases rapidly as the observation line moves north; the envelope of fresh cold side moves towards fresh cold side much faster than that of saline side. This suggests that the water does not advect along the salinity minimum layer, but the salinity minimum layer can be understood as a boundary of two different waters aligned vertically, We defined the typical water masses as the Oyashio Water and the Kuroshio Water. The water mass below the salinity minimum layer may be created by isopycnal mixing of these two water masses with a fixed mixing rate. While the water mass above the salinity minimum cannot be created simply by isopycnal mixing. The salinity minimum layer may be eroded from upper side due to active minxing processes in the surface layer, while the water of the salinity minimum layer moves gradually southward. This appears to give an explanation why the thermosteric anomaly value at salinity minimun decereases towards south.

Last Glacial–Holocene change in intermediate-water ventilation in the Northwestern Pacific

The modern North Pacific is the terminus for deep-water circulation. While no deep-water mass is formed today, the region is the source of North Pacific Intermediate Water (300–800 m) formed primarily in the Sea of Okhotsk. However, the source and strength of intermediate-water in the glacial North Pacific was likely different from the present intermediate-water. Here we present microfossil evidence of Late Quaternary ventilation changes associated with changes in intermediate-water. Intermediate-water ventilation decreased drastically at the glacial termination when it switched to the modern circulation system. The ventilation change appears to have been widespread and synchronous in the North Pacific. Glacial microfossil assemblages indicate that there was relatively strong ventilation of intermediate-water, which seems to have been cold and well-oxygenated as if produced by brine rejection during the formation of sea ice. On the basis of the glacial distribution of microfossils in the North Pacific, we suggest that intermediate-water was derived from the Bering Sea during the glacials, rather than the Sea of Okhotsk.

Cabbeling and the density of the North Pacific Intermediate Water quantified by an inverse method

North Pacific Intermediate Water (NPIW), defined as the main salinity minimum in the subtropical North Pacific, at a density of 26.7–26.8σθ, is denser than the winter surface water in the Oyashio which is the source of the salinity minimum. We showed previously that cabbeling and double diffusion during mixing between the Oyashio water and more saline Kuroshio water can account for the density increase from the surface source water to the salinity minimum. An inverse method is employed herein to quantify the effect of cabbeling, using CTD data from the western North Pacific. The difference between proportional mixing between parcels of Oyashio and Kuroshio waters and mixing along isopycnals is exploited to compute the convergence of water into density layers. The diapycnal transport convergence associated with cabbeling into the NPIW density layer is estimated to be 0.56 Sv for an assumed turnover time of 1 year in the region between 142°E and 152°E. Diapycnal transport convergences in the regions 152°E–165°E, 165°E–175°W, and 175°W–136°W are similarly estimated by assuming longer turnover times. We estimate that the total diapycnal transport convergence into the NPIW density layer may be up to 2.3 Sv in the entire NPIW region.

Evidence of change in the sea of okhotsk: Implications for the north pacific

Russian data from five cruises during the period 1949 to 1952 are compared with observations taken during the World Ocean Circulation Experiment (WOCE) P1W in 1993 to examine changes which may have occurred in the Sea of Okhotsk during the latter half of the last century. A basin‐wide warming (0.1°‐0.3°C) and freshening (0.05–0.1 psu) of the Sea of Okhotsk was found to have occurred over the latter part of the 20th century. Since the Sea of Okhotsk is thought to be the major source for North Pacific Intermediate Water (NPIW), calculations were made to determine whether or not these changes in the Sea of Okhotsk water properties were consistent with evidence of large‐scale freshening of intermediate waters in the North Pacific. From several Okhotsk‐to‐Pacific salt flux calculations, we conclude that the Sea of Okhotsk was capable of causing the freshening noted in the NPIW over the past half century under certain assumed outflow conditions.

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 26.6–27.4σθ 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.

Re-Estimation of Annual Anthropogenic Carbon Input from Oyashio into North Pacific Intermediate Water

The annual transport of anthropogenic carbon (C anth ) to the North Pacific Intermediate Water (NPIW) from the Western Subarctic Gyre (WSG) has been re-estimated by using newly estimated Oyashio transport and C anth concentration, the latter calculated by the recently-established “∆ C*” method with some modifications. Estimated annual C anth transport through the nearshore Oyashio west of 146°E was 0.020 ± 0.010 GtC y ‐1 , closely approximating the previous estimation based on a 1-D model calibrated with the CFC vertical distribution. The present study, however, found that an additional 0.025 ± 0.010 GtC y ‐1 of C anth was transported into NPIW in the region east of 146°E. Total C anth transport, 0.045 GtC y ‐1 , contributes about 35% of annual C anth accumulation of the whole temperate North Pacific.

ADCP-Referenced Kuroshio and Oyashio Water Transports for North Pacific Intermediate Water Formation*

Abstract The ADCP-referenced geostrophic circulation of the Oyashio and Kuroshio waters in the density range 26.6–27.2 σθ in the Kuroshio–Oyashio interfrontal zone is examined by synoptic observations with a CTD and shipboard ADCP in May 1992 and May 1994 in order to investigate the absolute transports of the source waters for North Pacific Intermediate Water (NPIW) formation. The Kuroshio and Oyashio components are calculated by the ADCP-referenced transport combined with the mixing ratio assuming isopycnal mixing between pure Kuroshio and Oyashio waters. As a result, the Kuroshio transport involved in the NPIW formation is estimated to be 7–8 Sv (Sv ≡ 106 m3 s−1), which is the net component along the Kuroshio Extension (KE) from the separation point near the Cape Nojima to the east side of the first crest of the KE. The Oyashio transport is estimated to be 5–6 Sv, which consists of coastal 1–2 Sv and offshore 3–4 Sv separated by warm core rings, and which subsequently merges with the KE at the east side...

Modification of North Pacific Intermediate Water around Mixed Water Region

The mixing processes in the Mixed Water Region (MWR) that lead to changes in the properties of North Pacific Intermediate Water (NPIW) have been studied using observational data sets obtained in May–June 1998. Neutral surfaces, the equation of water mass conversion rate on neutral surfaces and the equation of vertical velocity across neutral surfaces have been used to distinguish dominant processes by assuming the horizontal scale to be the streamer scale (under 100 km). The possibility of double diffusive convection is also discussed in relation to the density ratio. These results may be summarized as follows: (1) the difference between the potential density surface and the neutral surface may rise to −0.04 kg/m3 around the source water of NPIW; (2) horizontal diffusion causes strong modifications of the source water of NPIW; (3) the density range within which strong modification of the source water of NPIW occurs becomes dense from the northern part of MWR near the Oyashio Front to the southern part near the Kuroshio Front, and to the eastern part. Our modeling of these processes shows that cabbeling has effects on the density increment of the source water of NPIW in the northern and southern part of MWR. Double diffusive convection has effects on the density increment of the source water of NPIW, mainly in the northern part of MWR. The possible density increment due to cabbeling in these areas is estimated to be 0.01≈0.03 kg/m3. The possible density increment due to double diffusive convection is 0.01≈0.03 kg/m3. The total density increment due to cabbeling and double diffusive convection amounts to 0.06 kg/m3.

Long‐term hydrographic variability in the Northwest Pacific Ocean

Hydrographic changes in the Northwest Pacific Ocean are examined using data for two time periods: 1945: 1975 and 1976:1998. The largest changes in T/S (2°C,0.2 psu) are within the interfrontal zone between the Kuroshio Extension (KE) and the subarctic (or Oyashio) front near the Shatsky Rise, and are consistent with a southward shift of the Kuroshio Bifurcation Front (KBF). Other major changes seen are a freshening of ∼0.04 psu of the newly formed North Pacific Intermediate Water (NPIW); a shoaling over time of the halocline in the center of the Western Subarctic Gyre (WSAG); and a southward shift of the subarctic front between the dateline and 150°W. Because long‐term T/S changes near the Shatsky Rise are well‐correlated, we have examined subsurface thermal data at 100 and 200‐m depth in the region and found that shifts in front locations show a high degree of correlation with the PDO index on an interannual basis, and lagging it by ∼1 year at 200‐m depth.

Formation, Distribution and Volume Transport of the North Pacific Intermediate Water Studied by Repeat Hydrographic Observations

In order to examine the formation, distribution and transport of North Pacific Intermediate Water (NPIW), repeated hydrographic observations along several lines in the western North Pacific were carried out in the period from 1996 to 2001. NPIW formation can be described as follows: (1) Oyashio water extends south of the Subarctic Boundary and meets Kuroshio water in intermediate layers; (2) active mixing between Oyashio and Kuroshio waters occurs in intermediate layers; (3) the mixing of Oyashio and Kuroshio waters and salinity minimum formation around the potential density of 26.8σθ proceed to the east. It is found that Kuroshio water flows eastward even in the region north of 40°N across the 165°E line, showing that Kuroshio water extends north of the Subarctic Boundary. Volume transports of Oyashio and Kuroshio components (relative to 2000 dbar) integrated in the potential density range of 26.6–27.4σθ along the Kuroshio Extension across 152°E–165°E are estimated to be 7–8 Sv (106 m3s−1) and 9–10 Sv, respectively, which is consistent with recent work.

Hydrographic Structure and Variability in the Kuroshio-Oyashio Transition Area

The hydrographic structure and variability in the Kuroshio-Oyashio Transition Area of the northwestern Pacific are briefly reviewed, focusing on the circulation, frontal structure and water-mass formation from surface to intermediate depths. This area is a key to understanding climate and ecosystem variations because signals can be detected earlier than major climate regime shifts and also because species replacement among small pelagic fishes could be related to environmental changes in this area. We need further studies of the effect of North Pacific Intermediate Water on surface currents and frontal structures and also studies on the formation and variability of water-masses in surface mixed layer.