Salinity Minimum Research Articles

Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 2. Formation mechanism of salinity minimum from the view of the “critical gradient” of the Oyashio mixing ratio

In order to avoid the “false North Pacific Intermediate Water (NPIW)” problem a general circulation model is driven by the winter‐like North Pacific wind that can separate the modeled Kuroshio from the coast around 36°N. In the model a salinity minimum appears around 26.8σθ in the North Pacific subtropical region, which does not outcrop in the North Pacific. Formation processes of the salinity minimum are examined. Comparing them with observations and discussing them sufficiently, this study clarified that NPIW salinity minimum is formed just above the less saline water influenced largely by the Oyashio water. The formation of NPIW is basically controlled by the rapid increase of the Oyashio layer thickness around 26.5–26.8σθ, which layer structure is determined by Okhotsk Sea Mode Water pycnostad at 26.8–26.9σθ and the winter sea surface density of ∼26.5σθ. For NPIW salinity minimum formation it is most important that the gradient of the Oyashio mixing ratio exceeds the “critical gradient” around 26.4–26.7σθ. NPIW salinity minimum density is almost determined by the heaviest density of this range (26.7σθ), which is ∼0.1σθ less dense than the bottom of the rapid increase of the Oyashio layer thickness. Then cabbeling leads to an increase in its density to 26.8σθ. The Sea of Okhotsk is very important for the formation of NPIW through the formation of the Oyashio thick layers, and it practically determines NPIW salinity minimum density.

Salinity tolerance of Haliotis diversicolor supertexta at different salinity and temperature levels

Juvenile abalones, Haliotis diversicolor supertexta Lischke maintained in different environmental regimes (25, 30 and 35‰ combined with 20, 25 and 30°C) were examined for salinity tolerance by increasing and decreasing salinity at a rate of 2‰/h. They were also examined for the LS50 (median lethal salinity) when transferred directly into a series of higher salinity (32–48‰) and lower salinity (9–25‰) water baths. The 50% CSMax (critical salinity maximum), USTL (upper salinity tolerance limit), 50% CSMin (critical salinity minimum) and LSTL (lower salinity tolerance limit) increased directly with salinity, but were inversely related to temperature. The juveniles maintained in 25°C and 25, 30 and 35‰ had 50% CSMin–50% CSMax of 11.2–41.8‰, 12.5–43.9‰ and 13.5–49.8‰, respectively. The juveniles maintained in 25‰ and 30°C survived salinity of 14–33‰, and those maintained in 35‰ and 20°C survived salinity of 20–45‰, when salinity was decreased or increased gradually. It is suggested that H. diversicolor supertexta juveniles maintained in 35‰ or higher at 20°C or lower survive salinity higher than 45‰ when salinity is increased. It is also suggested that the juveniles maintained in 25‰ or a lower salinity at 30°C or a higher temperature survive salinities lower than 14‰ when salinity is decreased.

Field investigations of amoebic gill disease in Atlantic salmon, Salmo salar L., in Tasmania

Amoebic gill disease (AGD) is the most serious health problem in Atlantic salmon cultured in Tasmania. Our field investigation examined prevalence of AGD during 2 years, every year for up to 7 months after transfer to sea water. The relationship between environmental factors and AGD prevalence was determined. Additionally, effects of adding levamisole to freshwater baths were investigated in a field trial. AGD was recorded on all farms, except for farm A, which did not move salmon from a brackish site to a full‐salinity site during the study. The prevalence showed a bimodal distribution with the first larger peak in summer (usually in January) and the second smaller peak in autumn (between March and May). During both years the prevalence of AGD was significantly greater in January than any other month. Sampling month and the interaction between farm and month had a statistically significant effect on AGD prevalence. AGD was recorded at a minimum temperature of 10.6 °C and minimum salinity of 7.2 ppt. There was a positive relationship between the time since the freshwater bath and the prevalence of AGD for the first 30 days after the bath, with a dramatic increase in the AGD prevalence about 3 weeks after the bath. After 30 days, there was no statistically significant relationship between AGD prevalence and days since the last bath, except for the second bath. The addition of levamisole to the freshwater bath did not significantly increase the time between treatments. The relationship between diagnosis on the basis of gross signs and histological diagnosis was significant, however, the gross diagnosis was unreliable within the lower range, with 31.8% false negatives and 15.9% false positives and kappa value of 0.2742.

Origin and ages of mode waters in the Brazil‐Malvinas Confluence region during austral winter 1994

The ventilation of the main thermocline associated with the formation of Subtropical Mode Water (STMW) and Subantarctic Mode Water (SAMW) within the Brazil/Malvinas Confluence region is investigated using satellite data, hydrographic data, and dissolved CFCs as transient tracers. During the winter of 1994, two types of STMW were observed. Both types, 26.24 σθ and 26.46 σθ were formed within the warm pool of the South Atlantic Central Water (SACW). The SAMW is found on the 27.10 σθ density surface. This surface lies in the minimum salinity layer of the Antarctic Intermediate Water (AAIW). The mode waters are dated using CFC ratios. To our knowledge, this is the first estimate of the age of the mode waters in the region using the CFC‐113:CFC‐11 ratio method. The lighter STMW was being formed during the cruise. The heavier STMW was found to be 6±3 years old and is thought to have circulated in the subtropical gyre without losing its primary properties. The SAMW, or the lightest AAIW has an age of 3±2 years. This water is very young compared with the oldest (10±5 years) and heaviest component of the AAIW, which circulates within the subtropical gyre. The formation of the STMW in the confluence region is dependent on the southward extension of the Brazil Current.

Strait of Sicily water masses

We have derived a water mass model for the Strait of Sicily, based on 1994 and 1995 cruise data. The model consists of seven water masses, suggested by the measured shapes of the vertical temperature and salinity distributions. The core of the Atlantic water is distributed below the surface as a shallow layer, in a depth range of 40 to 100 m, with a salinity minimum. It is capped by upper and surface layers above and a mixed region below. At the bottom, Levantine water is present with a transition region above. Between the mixed and transition region there is, on occasion, a fresher water layer. The structure and statistics of water masses is analyzed over the Strait of Sicily region in terms of their temperature, salinity, and depth values. Objective analysis of the temperature, salinity, and depth parameters is performed in latitude and longitude. The water masses are tracked in terms of their parameter signatures. Changes in temperature and salinity distributions are interpreted. 2-D ellipses that represent the water masses, in terms of means and standard deviation, are derived in a space of temperature, salinity, and depth. Their axes are the standard deviations of parameter space ranges. The areas of the ellipses are compared against the temperature and salinity data distribution. The water mass composition ratios are computed and analyzed. Hypotheses and mechanisms for the origin and mixing of water masses are suggested.

On the Hydrographic Distribution in the Source Region of the Delaware Coastal Current

The buoyant discharge from Delaware Bay forms two separate branches of residual outflow near the bay mouth, one along each shore. Upon exiting the bay, the branch along the Delaware shore turns right to form the southward flowing Delaware coastal current along the inner continental shelf off the Delaware, Maryland, and Virginia coasts. CTD and thermosalinograph, data collected at the mouth of Delaware Bay over two semidiurnal tidal cycles are used to examine the hydrographic distribution at the source region of the Delaware coastal current. In this region the buoyant source water of the coastal current, is largely detached from the shoreline and confined to the top 15 m of the water column over much of the tidal cycles. The core of the coastal current's source water, as defined by the point of salinity minimum, is located over the deep channel well offshore of the Delaware coast. The separation between this buoyant water and the more saline waters right along the Delaware coast and that in the central part of the bay mouth are marked by regions of high horizontal salinity gradients. The horizontal salinity gradients around the inshore and offshore boundaries of the source water of the coastal current are intensified during the flood tide, and clearly defined fronts (with a change of 3‰ over a distance of 150 m) are present at the offshore boundary near the end of the flood tide. The structure of the mean flow and the distribution of the brackish coastal current on the inner continental shelf contribute to the persistence of stratification in the source region off the Delaware shore throughout the ebb and flood tides. In contrast, the ebb-induced stratification in the region off the New Jersey shore is quickly destroyed with the onset of the flood current.

Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 1. A pitfall of general circulation model studies

Formation processes of North Pacific Intermediate Water (NPIW) are examined by a numerical model with fine vertical grids. The model produces a salinity minimum in the North Pacific (calculated NPIW (C‐NPIW)), which is very similar to observed NPIW except for its lighter density. Upon examination with the particle‐tracking method and T‐S diagram, it is determined that C‐NPIW is formed around the Kuroshio Separation, most C‐NPIW is transported by the Kuroshio, and some originates from the subarctic by turbulent mixing. However, C‐NPIW cannot be formed by only a mixture of the Kuroshio and the subarctic waters. Cooling and freshening surface fluxes with a thick mixed layer are generated near the Kuroshio Separation, and less saline C‐NPIW is formed by 4.5 m yr−1 freshening surface flux. While the less saline mixed layer is carried eastward, its upper part becomes saline by the surface flux and a salinity minimum is formed. Thus its density is determined by the surface density at the Kuroshio Separation. These processes of C‐NPIW formation are inconsistent with observations, thus C‐NPIW is not the counterpart of NPIW and should be called “false NPIW.” The false NPIW results from the northward extension of the northwestern North Pacific subtropical gyre (NPSTG) beyond the Subarctic Front. In this situation the surface water near the Kuroshio Separation is the densest in NPSTG, and also, it has lowest salinity in the lower part of the ventilated layer in NPSTG. Hence the less saline water formed at the Kuroshio Separation spreads over NPSTG as false NPIW. The boundary of the subtropical/subarctic gyres expected from the annual mean wind is at the north of the Subarctic Front in the western Pacific, thus the problem will be common to most numerical models. Therefore models must be used that can simulate a realistic Kuroshio Separation in order to reproduce NPIW.

The Japan Sea Intermediate Water; Its Characteristics and Circulation

In the southern Japan Sea there is a salinity minimum layer between the Tsushima Current Water and the Japan Sea Proper Water. Since the salinity minimum corresponds to the North Pacific Intermediate Water, it is named the Japan Sea Intermediate Water (JIW). To examine the source and circulation of JIW, the basin-wide salinity minimum distribution was investigated on the basis of hydrographic data obtained in 1969. The young JIW, showing the highest oxygen concentration and the lowest salinity, is seen in the southwestern Japan Sea west of 133°E, while another JIW with lower oxygen and higher salinity occupies the southeastern Japan Sea south of the subpolar front. Since the young JIW shows high oxygen concentrations, high temperatures and low densities, the source of the water is probably in the surface layer. It is inferred that the most probable region of subduction is the subarctic front west of 132°E with the highest oxygen and the lowest salinity at shallow salinity minimum. In addition, property distributions suggest that JIW takes two flow paths: a eastward flow along the subarctic front and an southward flow toward the Ulleung Basin. On the other hand, a different salinity minimum from JIW occupies the northern Japan Sea north of the subarctic front, which shows an apparently higher salinity and high oxygen concentration than JIW. However, this salinity minimum is considered not to be a water mass but to be a boundary between overlying and underlying water masses.

Decadal variability in the composition of Faroe Shetland Channel bottom water

Two standard sections across the deep water channel separating the Faroese Plateau from the Scottish continental shelf have been surveyed regularly since the start of the 20th century. There have been significant changes in the characteristics of surface, intermediate and deep water masses during this period. At intermediate depths, the presence of Norwegian Sea Arctic Intermediate Water (NSAIW) was evident as a salinity minimum during the first decade of the century. During the decades 1960–1980 this salinity minimum disappeared, and only four water types were identified in the Channel. Since 1980 the salinity of the intermediate water has again decreased, due to changes in the atmospheric forcing over the Nordic Seas, and it is again evident on a θS curve as a distinct minimum. The salinity of the bottom water in the Channel has also decreased (0.01/decade) linearly since the mid-1970s, although at a slower rate than the intermediate water (0.02/decade). The decline in salinity of the bottom water cannot be accounted for by changes in the salinity of upper Norwegian Sea Deep Water (NSDW), which Faroe Shetland Channel Bottom Water (FSCBW) has traditionally been assumed to be composed of. There is evidence that the upper level of NSDW has become deeper outside the Channel owing to a reduced supply from the Greenland Sea. This has resulted in a change in the composition of FSCBW, from being approximately 60% NSDW during the period 1970–1985 to 40% NSDW since 1990. Thus, the thermohaline circulation of the Nordic Seas has lost its deep water connection. The associated freshening of FSCBW has propagated out through the Channel into the North Atlantic and has resulted in a reduction of the salinity (0.02/decade) and transport (1–7%/decade) of Iceland Scotland Overflow Water (ISOW) into the North Atlantic.

Formation and Movement of the ESIW as Modeled by MICOM

The presence of the minimum salinity and oxygen-rich East Sea Intermediate Water (ESIW) off Korean coast has raised many questions about its origin and movement. This study attempts to answer these questions in the context of numerical model. For this, the Miami Isopycnic Coordinate Ocean Model is used with surface and inflow boundary conditions based on available data. Model results show that the ESIW forms most distinctly off Vladivostok, where the center of doming structure is observed, and moves southward along Korean coast forming a large cyclonic gyre around the doming. The renewed ESIW has the character of minimum potential vorticity common to usual mode waters. These results are compatible with various indications made in previous observational and theoretical studies. However, it is not known whether the doming structure is a result of convection or the latter is favored by the former.

Formation of North Pacific Intermediate Water in Meteorological Research Institute ocean general circulation model: 1. Subgrid‐scale mixing and marginal sea fresh water

Formation of North Pacific Intermediate Water (NPIW) is investigated using an ocean general circulation model for the North Pacific with realistic bottom topography. The model has a horizontal resolution of 2° longitude by 1° latitude and 28 vertical levels. Experiments explored the sensitivity of the NPIW property to (1) subgrid‐scale mixing parameterization, (2) surface flux conditions for temperature and salinity, and (3) fresh water sources in marginal seas. A combination of horizontal mixing and sea surface temperature and salinity relaxation toward annual mean climatological values simulates the salinity minimum of the modeled NPIW very poorly. Inclusion of isopycnal subgridscale mixing substantially improves the salinity structure. A low‐salinity tongue associated with the salinity minimum, for example, extends from the subarctic gyre to the subtropical gyre at intermediate depths, while the salinity minimum in the modeled NPIW appears at 26.0–26.2 σθ, compared to 26.8 σθ for the observed NPIW. When surface condition for temperature and salinity restored to perpetual winter is imposed, the isopycnal surface of 26.0–26.2 σθ outcrops in the northwestern subtropical gyre, and salinity increases effectively due to excessive evaporation over precipitation. This allows the NPIW density to increase to 26.4–26.6 σθ. When temperature and salinity are restored to climatological values from the surface to the bottom in the Okhotsk and Bering Seas, less saline water flowing from marginal seas sets the NPIW density to the observed 26.8 σθ. Spatial distribution of modeled NPIW is found sensitive to the isopycnal diffusion. If the isopycnal diffusion is reduced, a low‐salinity tongue of the modeled NPIW disappears in the western North Pacific; relatively large isopycnal diffusion is necessary to simulate a fresh NPIW tongue in this area. Experiments with and without water flux from the northern marginal seas to the North Pacific confirm that the modeled NPIW has two fresh water sources. It is found that the northwestern subarctic sea surface supplies fresh water to the upper NPIW (26.4–26.7 σθ), whereas the Okhotsk Sea supplies fresh water to the lower NPIW (26.8–27.4 σθ).

Formation of the salinity minimum in the Mixed Water Region between the Oyashio and Kuroshio Fronts

The salinity minimum frequently occurring in the Mixed Water Region between the Oyashio and Kuroshio Fronts seems to originate from the salinity minimum at the density of 26.8 σ θ called the North Pacific Intermediate Water. We examined water exchange of this region with the Oyashio and the Kuroshio Extension using mixing ratio R K defined as ( θ− θ OY)/( θ K− θ OY)×100, where θ OY , θ K , and θ represent potential temperature of the Oyashio and Kuroshio Waters and their mixture on the isopycnal surfaces, respectively. CTD data were obtained by repeated observation from January 1990 to May 1991. R K increases southward from the Oyashio Front to the Kuroshio Front with the range of −20 to 120%. The gradient of R K on the isopycnal surfaces is large around the Oyashio Front above the 26.8 σ θ surface, while it is large around the Kuroshio Front below it. This agrees with the average R K in the Mixed Water Region decreasing greatly with the increase of density at densities less dense than 26.8 σ θ . We calculated thickness and volume transport of the Oyashio between the isopycnal surfaces near the coast of Hokkaido. They increase largely with density at densities less dense than 26.8 σ θ . It is supposed that the salinity minimum in the Mixed Water Region is the upper limit of the water largely influenced by the Oyashio Water. Its density could depend only on the density structure of the Oyashio.

Water masses and their circulation off western Iberia during May 1993

The distribution of the water masses and their circulation in the eastern North Atlantic region off the northern half of the west coast of the Iberian Peninsula are analysed here based on CTD measurements made in May 1993 during the “MORENA 1” cruise. The upper layers of the ocean, down to about 100m, showed a typical spring, non-upwelling situation with a very shallow seasonal thermocline overlying thicker remnants of the mixed layer from the previous winter. Below, Central Water extended between a subsurface salinity maximum near depths of 100–140m and a salinity minimum at about 500m. Using quantitative water mass analysis and geostrophic calculations, a poleward surface current reaching down to depths of 250m and transporting relatively warm and saline Eastern North Atlantic Central Water (ENACW) of subtropical origin was characterised, centred at a few tens of km offshore the shelf break, therefore indicating that the northward current identified previously in this region during winter (the Portugal Coastal Countercurrent) continues to be present through the spring season. A large-scale transition zone, the Galicia Front, was found in the northwest area of the study region, reaching from the surface layers down to the depths of the Mediterranean Water. To the north of this front, relatively fresh Central Water of Western Atlantic origin replaced the subtropical branch of ENACW. At the deeper Central Water levels, ENACW from subpolar North Atlantic origin was present through the whole study region. The influence of the upper and lower cores of Mediterranean Water was present, centred at levels of 800 and 1100m, as indicated by their characteristic thermohaline maxima. The deduced flow of Mediterranean Water was towards the north in the whole study region, in clear relation with the Portugal Slope Undercurrent, and showed some tendency to branch towards the west through the gap between the Vigo Seamount and the Galicia Bank. The Mediterranean upper core occupied a relatively thin layer, O (100m), and underwent a strong dilution of about 45% from the south to the north of the study region; the lower core spread over a larger depth range, of about 400–500m, and decayed only by about 20% through the region. Below the Mediterranean Water, the influence of North Atlantic Deep Water spread through the study region, and an intermediate layer of Labrador Sea Water was also present, particularly offshore in the northwest zone.

Regional Characteristics of the North Pacific Intermediate Salinity Minimum

Regional Characteristics of the North Pacific Intermediate Salinity Minimum

Northern and southern water masses in the equatorial Atlantic: distribution of nutrients on the WOCE A6 and A7 lines

In the framework of the WOCE Hydrographic Program, two trans-Atlantic CTDO/tracer sections with closely-spaced stations, along 7°30′N and 4°30′S (WHP Lines A6 and A7), and two meridional sections, along 3°50′W and 35°W joining the two zonal sections, were occupied in January–March 1993 (CITHER 1 cruise on board the N/OL′ATALANTE). CTD profiles and nutrient (silicate, phosphate and nitrate) data at 32 depths between surface and bottom were obtained at each station. The distributions on vertical sections, and on isopycnal surfaces, of these three chemical tracers are presented and discussed in the context of large-scale circulation in the equatorial Atlantic Ocean.The nutrient fields are used to show the main components of the circulation on four main levels: near-surface, intermediate, deep and bottom layers. Near the surface the nutrient distribution pattern is dominated by westward advective flows on either side of the equator from the eastern regions enriched by coastal upwellings. Beneath the lower thermocline water, high silicate concentrations (at about 1000m depth), at a larger depth than that of the salinity minimum of the Antarctic Intermediate Water, enable a differentiation of the Upper Circumpolar Water (UCPW) from the former. In the deeper layers, the nutrient distribution confirms the bifurcation of the Deep Western Boundary Current (DWBC), carrying the North Atlantic Deep Water, at the equator into an eastward flow and another one continuing southward along the western boundary. The eastward flow of the UNADW along the equator can be traced as far as 3°50′W. The analysis of nutrient distribution on isopycnal surfaces also shows the existence of recirculation components of the DWBC in the western equatorial region. The northward succession of bottom concentrations of silicate and phosphate indicates that the flow of the Antarctic Bottom Water (AABW) exiting the Brazil Basin is topographically constrained along the equator. The higher silicate and phosphate concentrations in the near-bottom waters west of the Mid-Atlantic Ridge (MAR) and north of the equator indicate that the main flow of bottom waters is northwestward rather than through the equatorial fracture zones into the eastern Atlantic. Finally, the distributions of silicate and phosphate on near-bottom and isopycnal surfaces suggests a recirculation of AABW in the northern part of the Brazil Basin.

Preliminary results from WOCE hydrographic sections at 80°E and 32°S in the central Indian Ocean

The hydrographic properties and circulation along sections at 80°E and 32°S in March, 1995, in the Indian Ocean are described very briefly. A halocline was well‐developed in the tropics. A westward coastal jet of fresh Bay of Bengal water was present at the sea surface at Sri Lanka with eastward flow of saline Arabian Sea water below. The Equatorial Undercurrent was well developed as were the deep equatorial jets. The Indonesian throughflow jet presented a large dynamic signature at 10 to 14°S coinciding with a strong front in all properties to great depth. Its mid‐depth salinity minimum is separated from that of the Antarctic Intermediate Water. The Subantarctic Mode Water of the southeastern Indian Ocean imparts its high oxygen ventilation signature to the whole of the transects, including the tropical portion. The deepest water in the Central Indian Basin is pooled in the center of the basin, and its principal source appears to be the sill at 11°S through the Ninetyeast Ridge. Northward deep water transports across the 32°S section were similar to those observed in 1987 but the deep water was lower in oxygen and fresher than in 1987. Upper ocean waters at 32°S were more saline and warmer in 1995.

Ocean currents across the entrance to the Gulf of California

Observations of transport and currents in April, May, and December 1992 and January 1993 were made across the entrance to the Gulf of California with an acoustically tracked dropsonde. Flow was into the gulf along Sinaloa and out of the gulf along Baja California. The transports were 5–10 Sv and the currents were deep, with 10 cm s−1 flow extending to depths greater than 1000 m. The currents were intensified in the upper 300 m. Geostrophic flows compared well with observed currents when smoothed to 2–3 times the Rossby radius. Salinity in the upper 300 m was higher on the Baja California side of the gulf, indicating modification of Subtropical Subsurface waters within the Gulf as well as the presence of surface and near‐surface gulf waters. The salinity minimum associated with California Current waters at 50 m had narrow cores that can be resolved only with closely spaced conductivity‐temperature‐depth casts. Mass and heat fluxes for the upper 300 m were estimated as 280 t s−1 and −0.1×1012 W in May and 170 t s−1 and 2.0×1012 W in December.

Meltwater pulses in the northern North Atlantic: retrodiction and forecast by numerical modelling

Changes in sea surface salinity, especially by sudden meltwater pulses, are the most effective process to modify the circulation in the Greenland–Iceland–Norwegian (GIN) seas. With “Sensitivity and Circulation of the Northern North Atlantic” (SCINNA), a three-dimensional ocean general circulation model, several experiments addressing the possible effects of meltwater inputs of different intensities were carried out. The experiments used (a) the last glacial maximum (LGM) reconstruction based on oxygen isotopes data from sediment cores and (b) the modern conditions of the GIN seas for their initial states. Meltwater inputs from Europe as recorded during the last deglaciation succeeding the LGM change the circulation pattern drastically. These pulses can push the high-salinity inflow from the northeast Atlantic away from Europe over to the southern coast of Iceland, thus allowing the low-salinity meltwater to spread all over the GIN seas. As a result, the deepwater formation in this region can be turned off and the circulation system shifts from the normal cyclonal-antiestuarine into an anticyclonal-estuarine mode. On the contrary, meltwater pulses originating from Greenland due to global warming mainly intensify the East Greenland Current without altering the overall circulation and temperature/salinity patterns significantly because they chiefly enhance the salinity minimum off the Greenland coast.

Sensitivity of simulated salinity in a three‐dimensional ocean model to upper ocean transport of salt from sea‐ice formation

We show that explicit representation of sinking of salt rejected during sea‐ice formation dramatically improves simulated salinity in an ocean general circulation model (OGCM). In our “control” simulation, rejected salt goes into the top model layer, and simulated salinities are typical of OGCMs: the deep ocean is too fresh, and the intermediate‐depth salinity minimum associated with Antarctic Intermediate Water is absent. These problems are eliminated in our “test” simulation, in which we distribute rejected salt uniformly over the upper 160 m. Also, the strength of the Antarctic Circumpolar Current is more realistic in this simulation. These results show the need for, but do not provide, a better representation of sinking of rejected salt. The sensitivity of our model to sinking of rejected salt suggests that a similar sensitivity may exist in the real ocean, and that loss of Antarctic sea ice might have major effects.

Modification of the Oyashio Water in the Hokkaido and Tohoku areas

Changes of the Oyashio Water properties are examined along the Oyashio northeast of Honshu, Japan. CTD data from simultaneous observations made by multiple ships in September 1989 are used. The Oyashio Water is modified by layering and vertical mixing with warm, saline waters flowing out from the Okhotsk Sea and the Japan Sea at densities lower than 130 cl/ton (26.7 σ δ). At densities greater than 130 cl/ton, the properties of Oyashio Water do not change significantly from the southeast of Iturup Island to the coast of Tohoku district at 39°20′N. The large and abrupt modification of Oyashio Water occurs within 400 km of the coast of Tohoku district, i.e. at the westernmost part of the Oyashio Front at about 39°N, by mixing with Kuroshio Water. By this modification, a salinity minimum is formed at densities around 26.8 6e corresponding to North Pacific Intermediate Water, which is the salinity minimum water broadly located in the subtropical region. The modified Oyashio Water flows northeastward along the Oyashio Front and joins the offshore flow of the Oyashio, which is separated from the main flow of the Oyashio southeast of the southern Kuril Islands carrying the typical western subarctic water.