Origin of freshwater and polynya water in the Arctic Ocean halocline in summer 2007

Extremely low summer sea-ice coverage in the Arctic Ocean in 2007 allowed extensive sampling and a wide quasi-synoptic hydrographic and δ¹⁸O dataset could be collected in the Eurasian Basin and the Makarov Basin up to the Alpha Ridge and the East Siberian continental margin. With the aim of determining the origin of freshwater in the halocline, fractions of river water and sea-ice meltwater in the upper 150 m were quantified by a combination of salinity and δ¹⁸O in the Eurasian Basin. Two methods, applying the preformed phosphate concentration (PO^*) and the nitrate-to-phosphate ratio (N/P), were compared to further differentiate the marine fraction into Atlantic and Pacific-derived contributions. While PO^*-based assessments systematically underestimate the contribution of Pacific-derived waters, N/P-based calculations overestimate Pacific-derived waters within the Transpolar Drift due to denitrification in bottom sediments at the Laptev Sea continental margin.Within the Eurasian Basin a west to east oriented front between net melting and production of sea-ice is observed. Outside the Atlantic regime dominated by net sea-ice melting, a pronounced layer influenced by brines released during sea-ice formation is present at about 30–50 m water depth with a maximum over the Lomonosov Ridge. The geographically distinct definition of this maximum demonstrates the rapid release and transport of signals from the shelf regions in discrete pulses within the Transpolar Drift.The ratio of sea-ice derived brine influence and river water is roughly constant within each layer of the Arctic Ocean halocline. The correlation between brine influence and river water reveals two clusters that can be assigned to the two main mechanisms of sea-ice formation within the Arctic Ocean. Over the open ocean or in polynyas at the continental slope where relatively small amounts of river water are found, sea-ice formation results in a linear correlation between brine influence and river water at salinities of about 32–34. In coastal polynyas in the shallow regions of the Laptev Sea and southern Kara Sea, sea-ice formation transports river water into the shelf’s bottom layer due to the close proximity to the river mouths. This process therefore results in waters that form a second linear correlation between brine influence and river water at salinities of about 30–32. Our study indicates which layers of the Arctic Ocean halocline are primarily influenced by sea-ice formation in coastal polynyas and which layers are primarily influenced by sea-ice formation over the open ocean. Accordingly we use the ratio of sea-ice derived brine influence and river water to link the maximum in brine influence within the Transpolar Drift with a pulse of shelf waters from the Laptev Sea that was likely released in summer 2005.     

Depth distribution of radiolarians from the Chukchi and Beaufort Seas, western Arctic

The depth distributions of the radiolarian fauna in the Chukchi and Beaufort Seas, marginal seas of the western Arctic Ocean, were examined quantitatively in depth-stratified plankton tows from 4 or 5 intervals above 500 m and in surface sediments from various depths between 163 and 2907 m. The radiolarian assemblage from the water column in September 2000 was dominated by Amphimelissa setosa and followed by the Actinomma boreale/leptoderma group, Pseudodictyophimus gracilipes and Spongotrochus glacialis. These species are related to the Arctic Surface Water shallower than 150 m. This assemblage is similar to that in the Greenland Sea relating to the ice edge, but did not contain typical Pacific radiolarians in spite of the flow of water of Pacific origin in this region. The living depth of Ceratocyrtis historicosa was restricted to the relatively warm water between 300 and 500 m corresponding to the upper Arctic Intermediate Water (AIW) originating from the Atlantic Ocean. Radiolarian assemblages in the surface sediments are similar to those in the plankton tows, except for common Cycladophora davisiana in sediment samples below 500 m. C. davisiana is probably a deep-water species adapted to the lower AIW or the Canadian Basin Deep Water ventilated from the shelves.     

CMIP6模式对北冰洋海洋热含量的模拟能力评估

本文利用PHC、ECCO2、SODA、GECCO3和CMIP6资料,分析了北冰洋热含量的水平分布特征、季节变化和长期变化趋势等,评估了CMIP6模式对北冰洋海洋热含量的模拟能力。研究发现,北冰洋海洋热含量表现出明显的季节变化:热含量在4月份最低,9月份最高;在历史情形下(1850?2014年),相较观测和再分析资料,CMIP6多模式集合平均(MME)的上层500 m热含量在格陵兰海偏暖,在挪威海、巴伦支海和欧亚海盆偏冷,MME的全水深热含量在北冰洋几乎所有区域均偏暖,在格陵兰海偏差最大;CMIP6模式对北冰洋温度剖面模拟偏差较大,MME平均温度在1 000 m以深均高于观测和再分析资料。在未来情形下(2015?2100年),MME表现出明显的北冰洋增暖情形,但绝大多数中国模式没有表现出明显的增暖情形。中国模式中,BCC-CSM2-MR和BCC-ESM1对北冰洋年平均热含量的模拟较差,CIESM对热含量季节和年代际变化模拟较差,FIO-ESM-2-0对北冰洋上层500 m年平均热含量及热含量季节和年代际变化的模拟都比较好。     

An isopycnal view of the Nordic Seas hydrography with focus on properties of the Lofoten Basin

Few basins in the world exhibit such a wide range of water properties as those of the Nordic Seas with cold freshwaters from the Arctic in the western basins and warm saline waters from the Atlantic in the eastern basins. In this study we present a 50-year hydrographic climatology of the Nordic Seas in terms of depth and temperature patterns on four upper ocean specific volume anomaly surfaces. This approach allows us to better distinguish between change due to variations along such surfaces and change due to depth variations of the stratified water column. Depth variations indicate changes in the mass field while property variations along isopycnals give insight into isopycnal advection and mixing, as well as diapycnal processes. We find that the warmest waters on each surface are found in the north, close to where the isopycnal outcrops, a clear indication of downward mixing of the warmer, more saline waters on shallower isopycnals due to convective cooling at the surface. These saline waters come from the Norwegian Atlantic Slope Current by means of a very high level of eddy activity in the Lofoten Basin.The isopycnal analyses further show that the principal water mass boundary between the waters of Arctic origin in the west and Atlantic waters in the east aligns quite tightly with the Jan Mayen, Mohn, Knipovich Ridge system suggesting little cross-ridge exchange. Instead, the main routes of exchange between the eastern and western basins appear to be limited to the northern and southern ends of ridge system: Atlantic waters into the Greenland Sea in the Fram St and Artic waters into the southern Norwegian Sea just north of the Iceland-Faroe Ridge.Analysis of a representative isopycnal in the main pycnocline shows it to be stable over time with only small variations with season (except where it outcrops in winter in the Greenland and Iceland Seas). However, two very cold winters, 1968–1969, led to greater than average heat losses across the entire Lofoten Basin that eroded away much of the Lofoten eddy and induced the greatest temperature anomaly in the entire 50-year record. Interannual variations in isopycnal layer temperature correlate with the NAO index such that waters in the Iceland Sea become warmer than average with warming air temperatures and conversely in the Lofoten Basin.     

Transports of Nordic Seas water masses and excess SF6 through Fram Strait to the Arctic Ocean

To determine the exchanges between the Nordic Seas and the Arctic Ocean through Fram Strait is one of the most important aspects, and one of the major challenges, in describing the circulation in the Arctic Mediterranean Sea. Especially the northward transport of Arctic Intermediate Water (AIW) from the Nordic Seas into the Arctic Ocean is little known. In the two-ship study of the circulation in the Nordic Seas, Arctic Ocean - 2002, the Swedish icebreaker Oden operated in the ice-covered areas in and north of Fram Strait and in the western margins of Greenland and Iceland seas, while RV Knorr of Woods Hole worked in the ice free part of the Nordic Seas. Here two hydrographic sections obtained by Oden, augmented by tracer and velocity measurements with Lowered Acoustic Doppler Current Profiler (LADCP), are examined. The first section, reaching from the Svalbard shelf across the Yermak Plateau, covers the region north of Svalbard where inflow to the Arctic Ocean takes place. The second, western, section spans the outflow area extending from west of the Yermak Plateau onto the Greenland shelf. Geostrophic and LADCP derived velocities are both used to estimate the exchanges of water masses between the Nordic Seas and the Arctic Ocean. The geostrophic computations indicate a total flow of 3.6 Sv entering the Arctic on the eastern section. The southward flow on the western section is found to be 5.1 Sv. The total inflow to the Arctic Ocean obtained using the LADCP derived velocities is much larger, 13.6 Sv, and the southward transport on the western section is 13.7 Sv, equal to the northward transport north of Svalbard. Sulphur hexafluoride (SF₆) originating from a tracer release experiment in the Greenland Sea in 1996 has become a marker for the circulation of AIW. From the geostrophic velocities we obtain 0.5 Sv and from the LADCP derived velocities 2.8 Sv of AIW flowing into the Arctic. The annual transport of SF₆ into the Arctic Ocean derived from geostrophy is 5 kg/year, which is of the same magnitude as the observed total annual transport into the North Atlantic, while the LADCP measurements (19 kg/year) imply that it is substantially larger. Little SF₆ was found on the western section, confirming the dominance of the Arctic Ocean water masses and indicating that the major recirculation in Fram Strait takes place farther to the south.     

The Arctic Circumpolar Boundary Current

The main water transformations in the Arctic Mediterranean take place in the boundary current of Atlantic Water, which crosses the Greenland–Scotland ridge from the North Atlantic into the eastern Norwegian Sea. It enters and flows around the Arctic Ocean before it exits the Arctic Mediterranean as the East Greenland Current, primarily through Denmark Strait. On route, it experiences numerous branchings and mergings. By examining how the properties of this “circumpolar” boundary current evolve, it is possible to identify and describe the processes causing the water mass transformations in the Arctic Mediterranean. It is also possible to follow the Arctic Ocean deep waters as they spread into the Nordic Seas and eventually provide 40% of the overflow water supplying the North Atlantic Deep Water.     

北冰洋水体对格陵兰海混合增密对流的可能影响分析

格陵兰海内发生的等密度混合后产生的增密对流是重要的对流现象之一。北冰洋正在发生快速变化,其内水团变性以及环流系统的改变都将使格陵兰海等密度混合对流发生明显变化,继而对全球气候变化产生影响。以往关于等密度混合对流的研究很少,大都集中在对流发生海域。由于等密度混合的主体是大西洋回流水与北冰洋流出水体,本文目的是探讨北极内部不同海域的水体会对混合增密对流造成的可能影响。文中定义了有效对流速度,强调水平温度梯度和垂向层化强度是影响有效对流速度的决定性因素;水平温度差越大,垂向层化越弱,产生的对流越强。发生在东格陵兰极锋处的有效对流都是大西洋的水体,一部分是在格陵兰海回流的大西洋回流水;一部分是在北冰洋潜沉并回流的北极大西洋水,该水体在北冰洋循环的时间越长,温度差越大,产生的有效对流越强。而横越北冰洋的太平洋水因密度过低而不能参与等密度混合对流,加拿大海盆主盐跃层之上的水体也都不能参与对流。北冰洋几个海盆深层水的温度差异明显,有可能与格陵兰海深层水形成有效对流;但是,由于深层水流速低、湍流混合弱、水平温度梯度小,是否可以产生有效对流尚不清楚。     

The Greenland Sea tracer experiment 1996-2002: Horizontal mixing and transport of Greenland Sea Intermediate Water

In summer 1996, a tracer release experiment using sulphur hexafluoride (SF₆) was launched in the intermediate-depth waters of the central Greenland Sea (GS), to study the mixing and ventilation processes in the region and its role in the northern limb of the Atlantic overturning circulation. Here we describe the hydrographic context of the experiment, the methods adopted and the results from the monitoring of the horizontal tracer spread for the 1996-2002 period documented by ∼10 shipboard surveys. The tracer marked “Greenland Sea Arctic Intermediate Water” (GSAIW). This was redistributed in the gyre by variable winter convection penetrating only to mid-depths, reaching at most 1800 m depth during the strongest event observed in 2002.For the first 18 months, the tracer remained mainly in the Greenland Sea. Vigorous horizontal mixing within the Greenland Sea gyre and a tight circulation of the gyre interacting slowly with the other basins under strong topographic influences were identified. We use the tracer distributions to derive the horizontal shear at the scale of the Greenland Sea gyre, and rates of horizontal mixing at ∼10 and ∼300 km scales. Mixing rates at small scale are high, several times those observed at comparable depths at lower latitudes. Horizontal stirring at the sub-gyre scale is mediated by numerous and vigorous eddies. Evidence obtained during the tracer release suggests that these play an important role in mixing water masses to form the intermediate waters of the central Greenland Sea.By year two, the tracer had entered the surrounding current systems at intermediate depths and small concentrations were in proximity to the overflows into the North Atlantic. After 3 years, the tracer had spread over the Nordic Seas basins. Finally by year six, an intensive large survey provided an overall synoptic documentation of the spreading of the tagged GSAIW in the Nordic Seas. A circulation scheme of the tagged water originating from the centre of the GS is deduced from the horizontal spread of the tracer. We present this circulation and evaluate the transport budgets of the tracer between the GS and the surroundings basins. The overall residence time for the tagged GSAIW in the Greenland Sea was about 2.5 years. We infer an export of intermediate water of GSAIW from the GS of 1 to 1.85 Sv (1 Sv = 10⁶ m³ s⁻¹) for the period from September 1998 to June 2002 based on the evolution of the amount of tracer leaving the GS gyre. There is strong exchange between the Greenland Sea and Arctic Ocean via Fram Strait, but the contribution of the Greenland Sea to the Denmark Strait and Iceland Scotland overflows is modest, probably not exceeding 6% during the period under study.     

Distribution of Th and Pa in the water column in relation to the ventilation of the deep Arctic basins

Distribution of ²³⁰Th and ²³¹Pa in the water column was measured in the Eurasian basins of the Arctic Ocean, i.e. the Makarov, Amundsen and Nansen basins. The profiles obtained were compared to one previously published from the Alpha Ridge (Bacon et al., Earth and Planetary Science Letters, 95 (1989) 15–22. The Amundsen and Nansen basins are similar in their ^23OTh and ²³¹Pa distribution, whereas in the Makarov Basin and at the Alpha Ridge these radionuclides are comparatively enriched in the deep basin. This may be due to low particle flux in conjunction with low water mass renewal rates. The ²³⁰Th concentration in water depth < 1000 m in the Makarov Basin is relatively low compared with the Alpha Ridge, which may be caused by an exchange of water masses between the Amundsen and Makarov basins.In the Eurasian Basin Deep Water, ²³⁰Th profiles can be explained both by a scavenging model and by a mixing-scavenging model. Enhanced particle load caused by resuspension may explain the increased scavenging in the Eurasian Basin Bottom Water. The differences in scavenging between the Norwegian-Greenland Sea and the Eurasian basins at present are not large enough to cause significant fractionation between ²³⁰Th and ²³¹Pa.     

Trace metals (Co,Ni, Cu,Cd, and Pb) in the southern East/Japan Sea

Total dissolvable metals (Co, Ni, Cu, Cd, and Pb) in both surface waters and the water columns were acquired in the southern East/Japan Sea during a cruise around the Ulleung Basin in June 2001 to understand the spatial distributions of the metals. Concentrations in offshore surface waters were found to be Co 60 ± 12 pM, Ni 2.16 ± 0.25 nM, Cu 1.85 ± 0.55 nM, Cd 0.134 ± 0.018 nM, and Pb 155 ± 40 pM. Spatial distributions in surface waters showed that metal levels were generally enhanced at coastal sites in both Korea and Japan, where the metal distributions indicated complex patterns due to inputs, biogeochemical processes, and physical factors including upwelling. The Co distributions in the water columns seemed to be influenced predominantly by surface and bottom inputs, scavenged rather than regenerated at depth. For Cd, there was generally good agreement between the Cd and PO₄ depth distributions, in agreement with the literature. The Cd/PO₄ ratio from the water columns was found to be 0.133–0.203, lower than that in other marginal seas (e.g. the East/South China Seas and the Philippine Sea) of the western Pacific Ocean; this might be a result of the fast ventilation rate in this sea. The vertical Pb profile showed typical scavenged-type behavior with a surface maximum and deep minimum. From a comparison of inputs from the atmosphere and the Tsushima Warm Current, atmospheric deposition is substantial enough that it cannot be ignored, and its role in metal cycling is more significant in the offshore zone.     

Ventilation time and anthropogenic CO<Subscript>2</Subscript> in the Bering Sea and the Arctic Ocean based on carbon tetrachloride measurements

The Arctic Ocean is connected to the Pacific by the Bering Sea and the Bering Strait. During the 4th Chinese National Arctic Research Expedition, measurements of carbon tetrachloride (CCl₄) were used to estimate ventilation time-scales and anthropogenic CO₂ (C_ant) concentrations in the Arctic Ocean and Bering Sea based on the transit time distribution method. The profile distribution showed that there was a high-CCl₄ tongue entering through the Canada Basin in the intermediate layer (27.6?<?σ_θ?<?28), at latitudes between 78 and 85°N, which may be related to the inflow of Atlantic water. Between stations B09 and B10, upwelling appeared to occur near the continental slope in the Bering Sea. The ventilation time scales (mean ages) for deep and bottom water in the Arctic Ocean (~?230–380 years) were shorter than in the Bering Sea (~?430–970 years). Higher mean ages show that ventilation processes are weaker in the intermediate water of the Bering Sea than in the Arctic Ocean. The mean C_ant column inventory in the upper 4000 m was higher (60–82 mol m^?2) in the Arctic Ocean compared to the Bering Sea (35–48 mol m^?2).     

An inverse modeling study in Fram Strait. Part II: Water mass distribution and transports

A water-mass analysis is carried out in Fram Strait, between 77.15 and 81.15°N, based on three-dimensional large-scale potential temperature and salinity distributions reconstructed from the MIZEX 84 hydrographic data collected in summer 1984. Combining these distributions with the geostrophic flow field derived from the same data in a companion paper (Schlichtholz and Houssais, 1999), the heat, fresh water and volume transports are estimated for each of the water masses identified in the strait. Twelve water masses are selected based on their different origins. Among them, the Polar Water (PW) enters Fram Strait from the Arctic Ocean both over the Greenland Slope and over the western slope of the Yermak Plateau. In the Atlantic Water (AW) range, four modes with distinct geographical distributions are indentified. In the Deep Water range, the Eurasian Basin Deep Water (EBDW) is confined to the Lena Trough and to the Molloy Deep area where it is involved in a cyclonic circulation. The warm and shallower mode of the Norwegian Sea Deep Water (NSDW), concentrated to the west, is mainly seen as an outflow from the Arctic Ocean while the cold and deeper mode, essentially observed to the east, enters the strait from the Greenland Sea. Apart from the EBDW, there is a tendency for all water masses of polar origin to flow along the Greenland Slope. The two most abundant water masses, the AW and the NSDW, occupy as much as 67% of the total water volume. The southward net transport of PW through Fram Strait is about 1 Sv at 78.9°N. At the same latitude, the net transport of AW is southward and equal to about 1.7 Sv. Only the transport of the warm mode (AWw) is northward, amounting to 0.2 Sv. The overall net outflow of the Deep Waters to the Greenland Sea is about 2.6 Sv. Two upper water masses, the fresh (AWf) and the cold (AWc) mode of the AW, and one deep-water mass, the NSDW, appear to be produced in the strait, with production rates, between 77.6 and 79.9°N, of about 0.2, 1.0 and 1.7 Sv, respectively. A southward net fresh-water transport through the strait of about 2000 km³ yr⁻¹ (relative to a salinity of 34.93) is mainly due to the PW. The net heat transport relative to −0.1°C is northward, but undergoes a rapid northward decrease, suggesting an area-averaged surface heat loss of 50–100 W m⁻² in the strait.     

Distribution of Pacific-origin water in the region of the Chukchi Plateau in the Arctic Ocean in the summer of 2003

1Introduction Besidestheprecipitationandriverdischarges,the watersinthePacificOceanandtheAtlanticOceanare thesourcesoftheArcticOceanwater.TheAtlantic waterenterstheArcticOceanviatheFramStraitand theBarentsSea.Foritsdenserfeatureduetohigh salinity,mostofitsinkstothenorthofSvaldbardand circulatesinallthedeepbasinsintheArcticOcean, formingthedeepandbottomwatersoftheArcticO- cean(Aagaardetal.,1985;Rudelsetal.,1999).The BeringStraitistheonlychannelforthePacificwater toflowintotheArcticOce…     

The return of Pacific waters to the upper layers of the central Arctic Ocean

Temperature, salinity, and chemical measurements, including the nutrients silicic acid (Si), nitrate (NO₃), nitrite (NO₂), ammonium (NH₄), and phosphate (PO₄ or P), the oxygen isotopic composition of seawater (δ¹⁸O), and barium (Ba) concentrations were obtained from the central Arctic Ocean along transects radiating from the North Pole in early spring, 2000–2006. Stations that were reoccupied over this time period were grouped into five regions: from Ellesmere Island, (1) north along 70°W and (2) northwest along 90°W; near the North Pole, (3) on the Amundsen Basin flank and (4) directly over the Lomonosov Ridge; (5) through the Makarov Basin along 170–180°W. These regions had been shown by others to have undergone marked changes in water-mass assemblies in the early 1990s, but our time series tracer hydrographic data indicate a partial return of Pacific origin water within the mixed layer and the upper halocline layers beginning in 2003–2004. Back-trajectories derived from satellite-tracked ice buoys for these stations indicate that the upper levels of Pacific water in the central Arctic in 2004–2006 transited westward from the Bering Strait along the Siberian continental slope into the East Siberian Sea before entering the Transpolar Drift Stream (TPD). By 2004, the TPD shifted back from an alignment over the Alpha-Mendeleev Ridge toward the Lomonosov Ridge, as was characteristic prior to the early 1990s. At most stations occupied in 2006, a decrease in the Pacific influence was observed, both in the mixed layer and in the upper halocline, which suggests the Canadian branch of the TPD was shifting back toward North America. Clearly the system is more variable than has been previously appreciated.     

Sources to the East Greenland Current and its contribution to the Denmark Strait Overflow

Data from the East Greenland Current in 2002 are evaluated using optimum multiparameter analysis. The current is followed from north of Fram Strait to the Denmark Strait Sill and the contributions of different source waters, in mass fractions, are deduced. From the results it can be concluded that, at least in spring 2002, the East Greenland Current was the main source for the waters found at the Denmark Strait Sill, contributing to the overflow into the North Atlantic. The East Greenland Current carried water masses from different source regions in the Arctic Ocean, the West Spitsbergen Current and the Greenland Sea. The results agree well with the known circulation of the western Nordic Seas but also add knowledge both to the quantification and to the mixing processes, showing the importance of the locally formed Greenland Sea Arctic Intermediate Water for the East Greenland Current and the Denmark Strait.     

Sources and distribution of fresh water in the East Greenland Current

Fresh water flowing from the Arctic Ocean via the East Greenland Current influences deep water formation in the Nordic Seas as well as the salinity of the surface and deep waters flowing from there. This fresh water has three sources: Pacific water (relatively fresh cf. Atlantic water), river runoff, and sea ice meltwater. To determine the relative amounts of the three sources of fresh water, in May 2002 we collected water samples across the East Greenland Current in sections from 81.5°N to the Irminger Sea south of Denmark Strait. We used nitrate-phosphate relationships to distinguish Pacific waters from Atlantic waters, salinity to obtain the sum of sea ice melt water and river runoff water, and total alkalinity to distinguish the latter. River runoff contributed the largest part of the total fresh water component, in some regions with some inventories exceeding 12 m. Pacific fresh water (Pacific source water S ∼ 32 cf. Atlantic source water S ∼ 34.9) typically provided about 1/3 of the river runoff contribution. Sea ice meltwater was very nearly non-existent in the surface waters of all sections, likely at least in part as a result of the samples being collected before the onset of the melt season. The fresh water from the Arctic Ocean was strongly confined to near the Greenland coast. We thus conjecture that the main source of fresh water from the Arctic Ocean most strongly impacting deep convection in the Nordic Seas would be sea ice as opposed to fresh water in the liquid phase, i.e., river runoff, Pacific fresh water, and sea ice meltwater.     

The transfer of reprocessing wastes from north-west Europe to the Arctic

The discharge of radioactive waste, from nuclear fuel reprocessing facilities, into the coastal waters of north-west Europe has resulted in a significant increase in the inventories of a number of artificial radionuclides in the North Atlantic. Radiocaesium, ⁹⁰Sr and ⁹⁹Tc, which behave conservatively in seawater, have been used widely as tracers of water movement through the North Sea, Norwegian Coastal Current, Barents Sea, Greenland Sea, Fram Strait, Eurasian Basin, East Greenland Current and Denmark Strait overflow. These studies are summarised in the present paper. It has been estimated that 22% of the ¹³⁷Cs Sellafield discharge has passed into the Barents Sea, en route to the Nansen Basin, via the Bjomoya-Fugloya Section, with another 13% passing through the Fram Strait. This amounts to 14 PBq ¹³⁷Cs. Quantifying the influx of other radionuclides has been more problematic. The inflowing Atlantic water now appears to be diluting waters in the Arctic Basin, which were contaminated in the late 1970s and early 1980s as a result of the substantial decrease in the discharge of reprocessing wastes. Sellafield (U.K.) has dominated the supply of ¹³⁴Cs, ¹³⁷Cs, ⁹⁰Sr, ⁹⁹Tc and Pu, whereas La Hague (France) has contributed a larger proportion of ¹²⁹I and ¹²⁵Sb.     

Long-term bottom water warming in the north Ross Sea

We measured potential temperature, salinity, and dissolved oxygen profiles from the surface to the bottom at two locations in the north Ross Sea (65.2°S, 174.2°E and 67.2°S, 172.7°W) in December 2004. Comparison of our data with previous results from the same region reveals an increase in potential temperature and decreases in salinity and dissolved oxygen concentration in the bottom layer (deeper than 3000 m) over the past four decades. The changes were significantly different from the analytical precisions. Detailed investigation of the temperature, salinity, dissolved oxygen and σ ₃ value distributions and the bottom water flow in the north Ross Sea suggests a long-term change in water mass mixing balance. That is to say, it is speculated that the influence of cool, saline, high-oxygen bottom water (high-salinity Ross Sea Bottom Water) formed in the southwestern Ross Sea has possibly been decreased, while the influences of relatively warmer and fresher bottom water (low-salinity Ross Sea Bottom Water) and the Adélie Land Bottom Water coming from the Australia-Antarctic Basin have increased. The possible impact of global warming on ocean circulation needs much more investigation.     

Concentration and Regeneration of Cd in the Japan Sea Proper Water

The concentration level of cadmium (Cd) and the regeneration related to phosphate (PO₄) were examined at two stations (CM10, CM12) in the eastern Japan Basin in July 1998. The observed Cd concentrations were around 0.2–0.3 nM and 0.5–0.6 nM in the surface and deep layers (Japan Sea Proper Water; JSPW), respectively; the concentration of Cd in the JSPW was much lower than that in the Pacific deep water, which is attributed to its specific formation system (which driven by the winter convection of the surface layer within the Japan Sea, thereafter descending to the deep layer) connected with the relatively active vertical mixing in the Japan Sea. A plot of Cd against PO₄ showed good linearity with positive y-intercept values, suggesting that the excess Cd was apparently not available in the biogeochemical cycle. The molecular ratios of consumed O₂ to regenerated Cd and PO₄ in the JSPW were 688,000, 140 and 881,000, 146 for CM10 and CM12, respectively, and a lower preformed Cd concentration (around 0.37 nM) was also estimated in the JSPW, different from that of the North Pacific deep water (613,000 for Cd, 170 for PO₄, and 0.64 nM of preformed Cd).     

Deep convection seesaw controlled by freshwater transport through the Denmark Strait

Observations of deep ocean temperature and salinity in the Labrador and Greenland Seas indicate that there is negative correlation between the activities of deep convection in these two sites. A previous study suggests that this negative correlation is controlled by the North Atlantic Oscillation (NAO). In this study, we discuss this deep convection seesaw by using a coupled atmosphere and ocean general circulation model. In this simulation, the deep convection is realistically simulated in both the Labrador and Greenland Seas and their negative correlation is also recognized. Regression of sea level pressure to wintertime mixed layer depth in the Labrador Sea reveals strong correlation between the convection and the NAO as previous studies suggest, but a significant portion of their variability is not correlated. On the other hand, the convection in the Greenland Sea is not directly related to the NAO, and its variability is in phase with changes in the freshwater budget in the GIN Seas. The deep convection seesaw found in the model is controlled by freshwater transport through the Denmark Strait. When this transport is larger, more freshwater flows to the Labrador Sea and less to the Greenland Sea. This leads to lower upper-ocean surface salinity in the Labrador Sea and higher salinity in the Greenland Sea, which produces negative correlation between these two deep convective activities. The deep convection seesaw observed in the recent decades could be interpreted as induced by the changes in the freshwater transport through the Denmark Strait, whose role has not been discussed so far.