Dense Shelf Water Research Articles

Introduction to special section: Oceanography of the Okhotsk Sea

[1] The Okhotsk Sea is a 1.5 10 km marginal sea adjacent to the Pacific Ocean in the Northern Hemisphere. Its defining boundaries are Hokkaido, Japan, Sakhalin Island, the Russian mainland, the Kamchatka Peninsula, and the Kuril Islands. In the Northern Hemisphere the Okhotsk is the southernmost sea with a sizable seasonal ice cover, which experiences large interannual variations in extent. Because of the coupling of this ice cover with the Okhotsk oceanography, the ice can be considered as a sensitive indicator of climate change. Despite the recent realization of the importance of the Okhotsk Sea, because of severe weather conditions and the presence of sea ice, in situ observations have been limited, especially in winter. [2] Part of the importance of the Okhotsk is that the North Pacific Intermediate Water (NPIW) is believed to be ventilated in and around it, both from hydrographic data [Talley, 1991] and chlorofluorocarbons (CFCs) observations [Warner et al., 1996]. Specifically, the Okhotsk is regarded as the only site where the atmosphere can directly exchange the heat and material (including CO2) with the NPIW. Ventilation of this water is an important component of the global overturning circulation. There are two ventilation sites for the intermediate water in the Okhotsk Sea. The first is the northwest shelf, which has large active coastal polynyas that are regarded as the primary area of dense shelf water (DSW) formation [Kitani, 1973; Wong et al., 1998; Martin et al., 1998; Gladyshev et al., 2000]. The second is the area in and around the straits in the Kuril Islands, where strong tidal currents generate diapycnal mixing [Talley, 1991]. [3] As part of a joint Japanese-Russian-U.S. study of the Okhotsk, four international joint cruises were carried out in the Okhotsk during 1998–2001 on the R/V Professor Khromov. These were supported by U.S. National Science Foundation and the Core Research for Evolutional Science and Technology of Japan Science and Technology Corporation. The participants consisted of representatives from the Russian Far Eastern Regional Hydrometeorological Research Institute, the U.S. Scripps Institution of Oceanography and the University of Washington, and the Japanese Hokkaido University and Japan Agency for MarineEarth Science and Technology (JAMSTEC, previously Japan Marine Science and Technology Center). The main focus of the study was the ventilation of the NPIW by the Okhotsk Sea. This divides into three parts: the northwest Okhotsk shelf, where brine rejection generates the DSW; the East Sakhalin Current, which is the western Okhotsk boundary current and DSW pathway to the southern Okhotsk [Mizuta et al., 2003]; and the Bussol Strait, which is the main exchange between the Okhotsk and the Pacific and a site of diapycnal mixing. [4] As the accompanying papers show, the project was an interdisciplinary one, consisting of studies of physical oceanography, sea ice, biogeochemistry, paleo-oceanography, and atmospheric sciences. The field methods consisted of hydrographical and chemical observations, moorings, bottom lander observations, Lagrangian float measurements, lowered acoustic Doppler current profiler (ADCP) measurements, and sediment corings. At the Ocean Sciences Meeting at Honolulu in 2002 a meeting of the Okhotsk participants showed that there were sufficient potential papers and new results to justify a Journal of Geophysical special section. In this special section, eight papers of the thirteen papers are based on the field results; others are based on numerical and satellite studies. [5] As the papers derived from these field experiments describe, direct measurements on the northwest shelf of brine rejection and formation of DSW were carried out for the first time using bottom landers [Shcherbina et al., 2003, 2004a]. These landers directly observed the ventilation of the intermediate water and also suggest that baroclinic instabilities occur on the density front at the JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, C09S01, doi:10.1029/2004JC002604, 2004

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.

Dissolved and particulate organic carbon in the Sea of Okhotsk: Transport from continental shelf to ocean interior

Dissolved and particulate organic carbon (DOC and POC) were measured in water columns of the Sea of Okhotsk together with other hydrological and chemical properties, including phytoplankton pigments (Chl‐a) and stable carbon isotopic ratios (δ13C) of POC. Transects from shelf to slope off the northeast coast of Sakhalin showed the existence of a cold‐water mass in the intermediate layer on the slope area having a density of 26.7–27.0 σθ, which is equivalent to the cold and dense water on the shelf (dense shelf water: DSW). The cold intermediate water mass was rich in DOC, POC, and Chl‐a, and the δ13C of POC was high, similar to that in DSW, indicating that the organic matter in the cold intermediate water is exported from the highly productive shelf area by the outflow of DSW. On the other hand, surface waters near the coast of northern Sakhalin had a very low salinity, which must be affected by the fresh water discharge from the Amur River. The low‐salinity water contained a large amount of DOC that was more than 3 times greater than the pelagic surface waters. The linear regression line between the salinity and the DOC in the surface layer of the studied area indicates that the Amur River water contains approximately 690 μM of DOC, and the input of DOC from the Amur to the Sea of Okhotsk was estimated to be approximately 2.5 TgC/yr. The relationships between the temperature and the DOC and POC along a density surface from 26.7 to 27.0 σθ suggest that DOC and POC in the DSW are transported into the intermediate layer of the pelagic region without being removed at the shelf edge. Simple estimates for additions of organic carbon into the intermediate layer of the area off the east coast of Sakhalin suggests that DSW outflow from the shelf brings into this layer approximately 13.6 and 0.9 TgC/yr of DOC and POC, respectively. These fluxes are much larger than the sinking POC flux from the in situ surface water, and so are fundamental in characterizing the biogeochemical cycles in the Sea of Okhotsk.

Methane in the western part of the Sea of Okhotsk in 1998–2000

We measured oceanic methane in the western part of the Sea of Okhotsk in 1998, 1999, and 2000. An anomalously high methane concentration was found in near‐bottom water above the shelfbreak (∼200 m) off northeast Sakhalin every year: up to 488 nmol kg−1 in 1998, 981 nmol kg−1 in 1999, and 556 nmol kg−1 in 2000. This anomalously high concentration can be used to trace the water with density range 26.6 to 26.8 σθ in the upper Dense Shelf Water. In the shelf off east Sakhalin, a strong stratification caused by freshwater from Amur River controlled the upward transport of methane through the suppression of vertical convection. The calculated methane flux was largest in the northeastern shelf region of Sakhalin (88 mol CH4 km−2 d−1). In the western part of the Sea of Okhotsk (0.73 × 106 km2, 51% of total Sea of Okhotsk area), the emission rate of methane was 11 Gg CH4 yr−1.

Biogenic and lithogenic particle fluxes in the western region of the Sea of Okhotsk: Implications for lateral material transport and biological productivity

Sinking particles collected at two depths at two sites off Sakhalin Island in the western region of the Sea of Okhotsk were analyzed to determine their major chemical components (lithogenic material, biogenic opal, calcium carbonate (CaCO3), and total organic carbon (TOC)). The lithogenic and opal fluxes increased downward, indicating the lateral input of resuspended sedimentary particles into the lower traps, while the CaCO3 or TOC fluxes did not change greatly between the upper and lower traps. The lithogenic particle flux in the lower traps showed intermittent increase events frequently during periods when cold water masses penetrated into the intermediate layer at the mooring sites. This coincidence between the flux increase and the cooling of the intermediate layer clearly indicates that the cold and turbid Dense Shelf Water (DSW) is discharged from the bottom of the northwestern continental shelf to the pelagic intermediate layer, supplying large amounts of particulate matter onto the bottom of the slope and deep basin. After removing the effect of the lateral input, the time series of the opal export from the surface water at the mooring sites were reconstructed using the upper trap data. There were two predominant characteristics in the seasonal variations of the opal export, which is nearly equal to the diatom production, in the area off Sakhalin. One is the relatively large opal flux in autumn, and the other is the southward propagation of the spring diatom bloom, although the sea ice retreats northward every spring. Both of these characteristics suggest that the river water discharged from the Amur, together with the seasonal sea ice cover, regulates primary production in this area.

Transport and modification processes of dense shelf water revealed by long‐term moorings off Sakhalin in the Sea of Okhotsk

The region off the east coast of Sakhalin is thought of as an important pathway of dense shelf water (DSW) from its production region in the northwestern Okhotsk Sea to the southern Okhotsk Sea. From July 1998 to June 2000, the first long‐term mooring experiment was carried out in this region to observe the southward flowing East Sakhalin Current (ESC) and DSW. Moored and associated hydrographic data show considerable modification of cold dense water via mixing with warm offshore water in the slope region off northern Sakhalin. Significant onshore eddy heat flux was observed at the northernmost mooring (54.9°N), which suggests the occurrence of baroclinic instability. The eddy heat flux was not significant farther south. At moorings along 53°N, cold anticyclonic eddies were identified that were consistent with isolated eddies seen in the hydrographic data. The three years of hydrographic data also showed large differences in extent and properties of DSW. Furthermore, the mooring data show that seasonal variability of DSW was quite different in the two years. The average DSW transport for σθ > 26.7 evaluated using the moored data at 53°N for 1 year (1998–1999) was ∼0.21 Sv (= 106 m3 s−1). This value is at the lower end of the previous indirect estimates. Along with the DSW modification, this transport estimate indicates that DSW was not only carried southward by the ESC but was spread offshore by eddies off northern Sakhalin.

Dense water formation on the northwestern shelf of the Okhotsk Sea: 2. Quantifying the transports

A combination of direct bottom mooring measurements, hydrographic and satellite observations, and meteorological reanalysis was used to estimate the rate of formation of Dense Shelf Water (DSW) due to brine rejection on the Okhotsk Sea northwestern shelf and the rate of export of DSW from this region. On the basis of remote sensing data, an estimated 8.6 × 1012 m3 of DSW was formed during the winter of 1999–2000, resulting in a mean annual production rate of 0.3 Sv. According to direct observations, the export rate of DSW during this period varied from negligibly small in autumn to 0.75 ± 0.27 Sv in winter (January–February), to 0.34 ± 0.12 Sv in spring (March–April). From these observations the mean annual export rate can be estimated to be 0.27 Sv. The same relationships used to obtain the integral estimates were also applied differentially using an advective approach incorporating realistic flow and heat flux fields, which allowed direct comparison with the moored observations. The comparison highlights the importance of along‐shelf advection and cross‐shelf eddy transport to the accurate parameterization of DSW formation.

Dense water formation on the northwestern shelf of the Okhotsk Sea: 1. Direct observations of brine rejection

Dense Shelf Water (DSW) formation due to brine rejection in the coastal polynya on the northwestern shelf of the Okhotsk Sea was studied using two bottom moorings during the winter of 1999–2000. A steady salinity and density increase that continued for over a month was observed at the shallower mooring. The maximum density of σθ = 26.92 kg m−3 was reached during this period. The density increase terminated abruptly in late February, while the active brine rejection continued for several more weeks based on indirect evidence from water properties and ice cover. This termination was possibly due to the onset of baroclinic instability of the density front at the polynya edge facilitating offshore eddy transport of the density anomaly. Observed periodic baroclinic tide intensification events are hypothesized to be an indicator of the presence of such baroclinic eddies. No significant density increase was observed at the deeper, offshore mooring, indicating a robust demarcation of the offshore extent of newly formed DSW. The relatively fresh water of the tidally mixed zone inshore of the shelf front was the precursor of the DSW, aided by the late‐autumn offshore transition of the front.

North Pacific Intermediate Water: Progress in SAGE (SubArctic Gyre Experiment) and Related Projects

We survey the recent progress in studies of North Pacific Intermediate Water (NPIW) in SAGE (SubArctic Gyre Experiment), including important results obtained from related projects. Intensive observations have provided the transport distributions relating to NPIW and revealed the existence of the cross-wind-driven gyre Oyashio water transport that flows directly from the subarctic to subtropical gyres through the western boundary current as well as the diffusive contribution across the subarctic front. The anthropogenic CO2 transport into NPIW has been estimated. The northern part of NPIW in the Transition Domain east of Japan is transported to the Gulf of Alaska, feeding the mesothermal (intermediate temperature maximum) structure in the North Pacific subarctic region where deep convection is restricted by the strong halocline maintained by the warm and salty water transport originating from NPIW. This heat and salt transport is mostly balanced by the cooling and freshening in the formation of dense shelf water accompanied by sea-ice formation and convection in the Okhotsk Sea. Intensive observational and modeling studies have substantially altered our view of the intermediate-depth circulation in the North Pacific. NPIW circulations are related to diapycnal-meridional overturning, generated around the Okhotsk Sea due to tide-induced diapycnal mixing and dense shelf water formation accompanied by sea-ice formation in the Okhotsk Sea. This overturning circulation may possibly explain the direct cross-gyre transport through the Oyashio along the western boundary from the subarctic to subtropical gyres.

Effects of the physical properties of water masses on microbial activity during an Ice Shelf Water overflow in the central Ross Sea

During the 1997–98 Italian Expedition to Antarctica a five-day mesoscale experiment was carried out on the continental shelf-break in the central Ross Sea. This area is oceanographically characterized by shelf/slope interactions, through intense mixing processes, between the Circumpolar Deep Water (CDW) and the Ice Shelf Water (ISW), coming from beneath the Ross Ice Shelf and spilling over the shelf edge. The export of dense shelf waters is of crucial importance not only for the mass balance of the basin, but also for carbon export from the upper layers into the abyssal ones. The study investigated how the ISW interactions with the CDW may influence bacterial metabolism during an ISW downslope event. In particular, what effect does this have on the bacterial activities related to the utilization and transformation of the organic carbon substrate (ectoenzymatic activities, carbon production, growth rate) within the ISW and the CDW cores? Our data show that in the CDW the metabolic response was to increase the biomass and enzymes were less active due to a higher nutritional value for the substrate. In the ISW the bacterial metabolic activity shifted towards degradative processes. These results suggest differences in the quality of the organic carbon pool with a greater concentration of labile organic matter in the CDW and of low-degradable compounds in the ISW. The use of microbial parameters seems to be very promising in the evaluation of the carbon export during mixing processes, when the refractory fraction of the organic carbon pool might play a key role.

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).

Interannual variability of dense shelf water salinities in the north-western Barents Sea

The maximum dense shelf water salinity formed during winter in the Svalbard Bank area of the north-western Barents Sea is reconstructed for the period 1952–2000 by analysing the transformation of summer remnants. The variability of 34.7 - 35.4, waters being at the freezing point, is mainly generated by interannual variations in the near surface salinity. On interannual time scales the latter is strongly linked to the sea ice import. In contrast, no correlation of the salinity of the Atlantic Water (AW) throughflow to the Arctic Ocean with the ice import is found. Salinities of both the dense shelf water site in the north-west Barents Sea and the north-eastward AW throughflow show a long term decrease, which can partly be explained by a less saline inflow of AW from the Norwegian Sea. The unusually low dense water salinities in the north-west Barents Sea during the 1990s appear to have a different origin, consistent with a response to oceanic heat advection and decreasing sea ice extent.

Distribution, formation, and seasonal variability of Okhotsk Sea Mode Water

Russian historical data and recently completed conductivity‐temperature‐depth surveys are used to examine the formation and spread in the deep Ohkotsk Sea of dense shelf water (DSW) produced in the Okhotsk Sea polynyas. Isopycnal analysis indicates that all of the main polynyas contribute to the ventilation at σθ < 26.80, including the Okhotsk Sea Mode Water (OSMW), which has densities σθ = 26.7–27.0. At densities greater than 26.9 σθ the northwest polynya is the only contributor to OSMW. (Although Shelikhov Bay polynyas produce the densest water with σθ > 27.1, vigorous tidal mixing leads to outflow of water with a density of only about 26.7 σθ). In the western Okhotsk Sea the East Sakhalin Current rapidly transports modified dense shelf water along the eastern Sakhalin slope to the Kuril Basin, where it is subject to further mixing because of the large anticyclonic eddies and tides. Most of the dense water flows off the shelves in spring. Their average flux does not exceed 0.2 Sv in summer and fall. The shelf water transport and water exchange with the North Pacific cause large seasonal variations of temperature at densities of 26.7–27.0 σθ (depths of 150–500 m) in the Kuril Basin, where the average temperature minimum occurs in April–May, and the average temperature maximum occurs in September, with a range of 0.2°–0.7°C. The average seasonal variations of salinity are quite small and do not exceed 0.05 psu. The Soya Water mixed by winter convection, penetrating to depths greater than 200 m, in the southern Kuril Basin also produces freezing water with density greater than 26.7 σθ. Using a simple isopycnal box model and seasonal observations, the OSMW production rate is seen to increase in summer up to 2.2 ± 1.7 Sv, mainly because of increased North Pacific inflow, and drops in winter to 0.2 ± 0.1 Sv. A compensating decrease in temperature in the Kuril Basin implies a DSW volume transport of 1.4 ± 1.1 Sv from February through May. The residence time of the OSMW in the Kuril Basin is 2 ± 1.7 years.

Interannual variability of dense shelf water salinities in the north-western Barents Sea

Interannual variability of dense shelf water salinities in the north-western Barents Sea

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.

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.

SAKHALIN-1 CONSORTIUM OUTLINES ORLAN MODIFICATION

From December to June 2002–2003, sea-ice and oceanic data were obtained from moorings near Sakhalin in the west central Okhotsk Sea. Ice draft measured by sonar reveals distinct periods of thin and thick ice. Thin-ice periods in January–March corresponded to offshore ice movement and increasing seawater salinity. The measured change in salinity corresponds well with that derived from heat-flux calculations using the observed ice thickness. Brine rejection from ice growing in a coastal polynya off northern Sakhalin is responsible for much of the observed salinity increase. The simultaneous observation of dense shelf water (>26.7σθ) suggests that this region is one possible source. The periods of thick-ice incursion are likely indicative of heavily deformed pack formed further north and drifting south with the current. The mean draft (1.95 m), thick-ice ratio, and keel frequency during these periods are close to values observed in the Beaufort Sea. Freshwater transport estimated from the observed ice thickness and velocity is larger than that of the Amur River discharge.

Contribution of entrainment and vertical plumes to the winter cascading of cold shelf waters in a deep lake

Observations made from vertical moorings of thermistors and current meters are used to analyze the relative contribution of the entrainment of overlying water and the cold plumes convecting from the surface to the cascade of dense shelf waters down the sides of Lake Geneva. Assuming a steady state and ignoring the effects of the Earth’s rotation, a simple model was introduced and 25 individual slugs of gravity currents were analyzed. The average drag coefficient, which includes the effects of friction on the slope and the interface between the gravity current and the ambient water, is found to be equal to CD = (4.6 ± 1.5) × 10−3. The volume flux in the gravity current increases with downslope distance—a consequence of both the entrainment of water into the gravity current and the supply of cold water from the plumes. Entrainment was found to be, on average, 1.9 ± 0.9 times the contribution of vertical plumes, suggesting that the latter may be important in the dynamics of the cascade.

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<σθ<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.

The sources of Antarctic bottom water in a global ice–ocean model

Two mechanisms contribute to the formation of Antarctic bottom water (AABW). The first, and probably the most important, is initiated by the brine released on the Antarctic continental shelf during ice formation which is responsible for an increase in salinity. After mixing with ambient water at the shelf break, this salty and dense water sinks along the shelf slope and invades the deepest part of the global ocean. For the second one, the increase of surface water density is due to strong cooling at the ocean–atmosphere interface, together with a contribution from brine release. This induces deep convection and the renewal of deep waters. The relative importance of these two mechanisms is investigated in a global coupled ice–ocean model. Chlorofluorocarbon (CFC) concentrations simulated by the model compare favourably with observations, suggesting a reasonable deep water ventilation in the Southern Ocean, except close to Antarctica where concentrations are too high. Two artificial passive tracers released at surface on the Antarctic continental shelf and in the open-ocean allow to show clearly that the two mechanisms contribute significantly to the renewal of AABW in the model. This indicates that open-ocean convection is overestimated in our simulation. Additional experiments show that the amount of AABW production due to the export of dense shelf waters is quite sensitive to the parameterisation of the effect of downsloping and meso-scale eddies. Nevertheless, shelf waters always contribute significantly to deep water renewal. Besides, increasing the P.R. Gent, J.C. McWilliams [Journal of Physical Oceanography 20 (1990) 150–155] thickness diffusion can nearly suppress the AABW formation by open-ocean convection.