The hydrography of the mid-latitude Northeast Atlantic Ocean: II: The intermediate water masses

The intermediate water masses in the eastern Atlantic Ocean between 31°N and 53°N were studied by analysis of the distributions of potential temperature, salinity, dissolved nutrients and oxygen. Sub-surface salinity minima are encountered everywhere in the area. At the northern and southern boundary they are connected with the presence of Sub-Arctic Intermediate Water and Antarctic Intermediate Water, respectively, but towards the European ocean margin the sub-surface salinity minima shift to shallower density levels. The sub-surface salinity minima observed west of the Iberian Peninsula represent a water mass formed by winter convection in the Porcupine Sea Bight and the northern Bay of Biscay. These minima gain salt by diapycnal mixing with the underlying Mediterranean Sea Outflow water and with the overlying permanent thermocline. The core of Antarctic Intermediate Water appears to contribute to the formation of Mediterranean Sea Outflow Water since it becomes entrained into the overflow near Gibraltar. This entrainment gives rise to an enhanced concentration of the nutrients in the Mediterranean water in the North Atlantic. The deep salinity minimum, due to the presence of Labrador Sea Water, is restricted mainly to the Porcupine Abyssal Plain. In the Bay of Biscay this water type is strongly modified by enhanced diapycnal mixing near the continental slope. At all intermediate levels the continental slope in the Bay of Biscay seems to be a focal point for water mass modification by diapycnal mixing. Below the core of the Mediterranean Sea Outflow Water the Labrador Sea Water is also strongly modified. Its salinity is strongly enhanced by diapycnal mixing with the overlying core of Mediterranean Sea Outflow Water. An analysis of the oxygen and nutrient data indicates that the large spatial concentration differences at the level of the Labrador Sea Water are caused mainly by ageing of the water. The youngest water is observed at 52°N, and, especially in the Bay of Biscay and off south-west Portugal, the water at levels of about 1700 dbar are strongly enriched in nutrients and depleted in oxygen.     

Hydrography and through-flow in the north-eastern North Atlantic Ocean: the NANSEN project

The circulation and hydrography of the north-eastern North Atlantic has been studied with an emphasis on the upper layers and the deep water types which take part in the thermohaline overturning of the Oceanic Conveyor Belt. Over 900 hydrographic stations were used for this study, mainly from the 1987–1991 period. The hydrographic properties of Subpolar Mode Water in the upper layer, which is transported towards the Norwegian Sea, showed large regional variation. The deep water mass was dominated by the cold inflow of deep water from the Norwegian Sea and by a cyclonic recirculation of Lower Deep Water with a high Antarctic Bottom Water content. At intermediate levels the dominating water type was Labrador Sea Water with only minor influence of Mediterranean Sea Water. In the permanent pycnocline traces of Antarctic Intermediate Water were found.Geostrophic transports have been estimated, and these agreed in order of magnitude with the local heat budget, with current measurements, with data from surface drifters, and with the observed water mass modification. A total of 23 Sv of surface water entered the region, of which 20 Sv originated from the North Atlantic Current, while 3 Sv entered via an eastern boundary current. Of this total, 13 Sv of surface water left the area across the Reykjanes Ridge, and 7 Sv entered the Norwegian Sea, while 3 Sv was entrained by the cold overflow across the Iceland-Scotland Ridge. Approximately 1.4 Sv of Norwegian Sea Deep Water was involved in the overflow into the Iceland Basin, which, with about 1.1 Sv of entrained water and 1.1 Sv recirculating Lower Deep Water, formed a deep northern boundary current in the Iceland Basin. At intermediate depths, where Labrador Sea Water formed the dominant water type, about 2 Sv of entrained surface water contributed to a saline water mass which was transported westwards along the south Icelandic slope.     

Smectite composition as a tracer of deep circulation: the case of the Northern North Atlantic

The link between smectite composition in sediments from the northern North Atlantic and Labrador Sea, and deep circulation is being further investigated through detailed studies of the X-ray pattern of smectites and cation saturations. This allows clear distinction of dominant terrigenous sources associated to the main components of the modern Western Boundary Undercurrent. Time variations of smectite characteristics in two piston cores from the inlet and outlet of the Western Boundary Undercurrent gyre in the Labrador Sea indicate: (1) a more southern circulation of North East Atlantic Deep Water during the Late Glacial; (2) a step by step transition to the modern pattern of deep circulation during the Late Glacial/Holocene transition, with intensification of North East Atlantic Deep Water and Davis Strait Overflow; (3) an expansion of Davis Strait Overflow and Labrador Sea Water circulation in relation to ice surges and deposition of detrital layers; (4) an intensified circulation of North East Atlantic Deep Water during the Younger Dryas; and (5) a very recent increased influence of Denmark Strait Overflow Water beginning between 4.4 and <1 kyr.     

Decadal and multi-decadal variability of Labrador Sea Water in the north-western North Atlantic Ocean derived from tracer distributions: Heat budget, ventilation, and advection

Time series of profiles of potential temperature, salinity, dissolved oxygen, and planetary potential vorticity at intermediate depths in the Labrador Sea, the Irminger Sea, and the Iceland Basin have been constructed by combining the hydrographic sections crossing the sub-arctic gyre of the North Atlantic Ocean from the coast of Labrador to Europe, occupied nearly annually since 1990, and historic hydrographic data from the preceding years since 1950. The temperature data of the last 60 years mainly reflect a multi-decadal variability, with a characteristic time scale of about 50 years. With the use of a highly simplified heat budget model it was shown that this long-term temperature variability in the Labrador Sea mainly reflects the long-term variation of the net heat flux to the atmosphere. However, the analysis of the data on dissolved oxygen and planetary potential vorticity show that convective ventilation events, during which successive classes of Labrador Sea Water (LSW) are formed, occurring on decadal or shorter time scales. These convective ventilation events have performed the role of vertical mixing in the heat budget model, homogenising the properties of the intermediate layers (e.g. temperature) for significant periods of time. Both the long-term and the near-decadal temperature signals at a pressure of 1500 dbar are connected with successive deep LSW classes, emphasising the leading role of Labrador Sea convection in running the variability of the intermediate depth layers of the North Atlantic. These signals are advected to the neighbouring Irminger Sea and Iceland Basin. Advection time scales, estimated from the 60 year time series, are slightly shorter or of the same order as most earlier estimates, which were mainly based on the feature tracking of the spreading of the LSW₉₄ class formed in the period 1989-1994 in the Labrador Sea.     

Inorganic carbon in the Labrador Sea: Estimation of the anthropogenic component

The intermediate and deep waters of the Labrador Sea are dominated by recently ventilated water masses (ventilation ages <20 yr). Atmospheric gases such as CO₂ and chlorofluorocarbons are incorporated into these water masses at the time of formation and subsequently transported via boundary currents into the North Atlantic interior. Recent measurements of total carbonate were used in tandem with total alkalinity and oxygen to estimate the levels of anthropogenic carbon dioxide in the Labrador Sea region. Upper water column anthropogenic CO₂ estimated in this manner showed good agreement with levels calculated from CO₂ increase in the atmosphere. In spring 1997, anthropogenic contributions to total carbonate (CTant) were 40±3 μmol/kg in water penetrated by deep convection the previous winter and slightly lower (37±2 μmol/kg) in the deeper convective layer formed in the winters of 1992–1994. Consistent with the concurrent profiles of CFC-11, levels decrease into the older NEADW (North East Atlantic Deep Water) with levels of 30±3 μmol/kg and then increase near bottom within the layer of DSOW (Denmark Strait Overflow Water). The distribution of CTant shows the flow of new LSW southwards with the western boundary current and also eastwards into the Irminger Sea. We estimate that 0.15–0.35 Gt carbon of anthropogenic origin flow through the Labrador Sea within the Western Boundary Undercurrent per year.     

Rates of North Atlantic Deep Water formation calculated from chlorofluorocarbon inventories

Chlorofluorocarbon (CFC) inventories provide an independent method for calculating the rate of North Atlantic Deep Water (NADW) formation. From data collected between 1986 and 1992, the CFC-11 inventories for the major components of NADW are: 4.2 million moles for Upper Labrador Sea Water (ULSW), 14.7 million moles for Classical Labrador Sea Water (CLSW), 5.0 million moles for Iceland–Scotland Overflow Water (ISOW), and 5.9 million moles for Denmark Strait Overflow Water (DSOW). The inventories directly reflect the input of newly formed water into the deep Atlantic Ocean from the Greenland, Iceland and Norwegian Seas and from the surface of the subpolar North Atlantic during the time of the CFC-11 transient. Since about 90% of CFC-11 in the ocean as of 1990 entered the ocean between 1970 and 1990, the formation rates estimated by this method represent an average over this time period. Formation rates based on best estimates of source water CFC-11 saturations are: 2.2 Sv for ULSW, 7.4 Sv for CLSW, 5.2 Sv for ISOW (2.4 Sv pure ISOW, 1.8 Sv entrained CLSW, and 1.0 Sv entrained northeast Atlantic water) and 2.4 Sv for DSOW. To our knowledge, this is the first calculation for the rate of ULSW formation. The formation rate of CLSW was calculated for an assumed variable formation rate scaled to the thickness of CLSW in the central Labrador Sea with a 10 : 1 ratio of high to low rates. The best estimate of these rates are 12.5 and 1.3 Sv, which average to 7.4 Sv for the 1970–1990 time period. The average formation rate for the sum of CLSW, ISOW and DSOW is 15.0 Sv, which is similar to (within our error) previous estimates (which do not include ULSW) using other techniques. Including ULSW, the total NADW formation rate is about 17.2 Sv. Although ULSW has not been considered as part of the North Atlantic thermohaline circulation in the past, it is clearly an important component that is exported out of the North Atlantic with other NADW components.     

Hydrographic structure and circulation in the central area of the North Eastern Atlantic

On the basis of the hydrographic data observed within the Canary Basin in autumn 1985, temperature-salinity properties, distributions of water masses and barocltne flow field, as well as the volume transports in this area are described more detailly. The analyses indicate that the activity in the waters of the Canary Basin is mainly attributed to the interleaving and mixing between the originated water masses (e. g. Surface Water, North Atlantic Central Water, Mediterranean Water and Deep Water) and the modified water masses (Subpolar Mode Water, Labrador Sea Water and Antarctic Intermediate Water) from the outside of the study area and the variation of themselves. The east recirculation of the Subtropic Gyre in the North Atlantic consists of Azores Current and Canary Current.Azores Current is formed with several flow branches around the Azores Island, while the main flow lies at 35?N south of the Azores Island. It begins to diverge near the 15?W. The return flow found off the Portugal coast may be its     

Analysis of an 80-Year Integration of a 1/3-Degree Ocean Model of the Subpolar North Atlantic

Previous work had examined an ocean model of the subpolar gyre of the North Atlantic Ocean that used the Gent and McWilliams parameterization with a variable eddy-transfer coefficient, and showed significant improvements to the model’s circulation and hydrography. This note examines an extended (80-year-long) integration of the same model and focuses on the adjustment of the intermediate and deep waters as well as on model stability. It is shown that the model is able to retain a good representation of the water masses, especially in the Labrador Sea, through the full integration. Labrador Sea Water dispersal is well simulated by the model in the western basin, with a good correspondence between the model and observational salinities on the σ₂ = 36.95 isopycnal surface. Labrador Sea Water dispersal to the eastern basin is not nearly as well represented, as this water mass has trouble passing over the Mid-Atlantic Ridge in the model. The variable eddy-transfer coefficient significantly improves the model representation of the Cold Intermediate Layer on the Labrador shelf by reducing spurious diapycnal mixing. Finally, the evidence in this note suggests that open boundary conditions do not generate significant model drift, even for integrations approaching a century in length.     

Warming and salinification of Labrador Sea Water and deep waters in the subpolar North Atlantic at 60°N in 1997–2006

A quantitative estimate of the temperature and salinity variations in the Labrador Sea Water (LSW), the Iceland-Scotland Overflow Water (ISOW), and the Denmark Strait Overflow Water (DSOW) is given on the basis of the analysis of repeated observations over a transatlantic section along 60°N in 1997, 2002, 2004, and 2006. The changes distinguished in the research evidence strong warming and salinification in the layers of the Labrador Sea Water and deep waters at the latitude of the section. The maximum increments of the temperature (+0.35°C) and salinity (+0.05 psu) were found in the Irminger Basin in the core of the deep LSW, whose convective renewal in the Labrador Sea stopped in the mid-1990s. The long-term freshening of the ISOW, which started in the mid-1960s, changed in the mid-1990s to a period of intense stable warming and salinification of this water. By 2005, the salinity in the core of the ISOW in the Iceland Basin increased to the values (～34.99 psu) characteristic of the mid-1970s. In 2002, the warming “signal” of the ISOW reached the Irminger Basin. From 1997 to 2006, the warming and salinification of the columns of the Labrador Sea Water and deep waters became as high as 0.2°C and 0.03 psu, respectively. The character of the long-term variations in the thermohaline properties of the LSW and ISOW from the 1950s evidence that these variations were nearly in-phase and correlated with the low-frequency component of the North Atlantic Oscillation.     

Deep western boundary current dynamics and associated sedimentation on the Eirik Drift, Southern Greenland Margin

Growing interest in the dynamics and temporal variability of the deep western boundary current (DWBC) in the northern North Atlantic has led to numerous studies of the modern hydrography and palaeoceanography of this current system. The DWBC is fed by the two dense water-masses that spill over the Greenland–Iceland–Scotland Ridge; Denmark Strait Overflow Water (DSOW) and Iceland Scotland Overflow Water (ISOW). These overflows entrain ambient water masses, primarily Labrador Sea Water (LSW), as they cross the Iceland and Irminger Basins before merging in the vicinity of south-east Greenland. A number of studies have been performed around the Eirik Drift, located off the southern Greenland margin, downstream of this main merging point. However, the relationship between the DWBC and the associated sedimentation at this location has yet to be fully elucidated. New hydrographic data show that the current's main sediment load is carried by only one of its components, the DSOW. Seismic surveys and sediment cores confirm that Holocene sedimentation is limited to areas underlying the most offshore part of the current, where the hydrographic data show the highest concentration of DSOW. Active sedimentation through the Holocene therefore appears to have been controlled by proximity to the sediment-laden DSOW.Our interpretation of new and historic geostrophic transport and tracer data from transects around the southern Greenland margin also suggests that the DWBC undergoes significant growth through entrainment as it flows around the Eirik Drift. We attribute this to multiple strands of ISOW following different depth-dependent pathways between exiting the Charlie Gibbs Fracture Zone and joining the DWBC. Comparison of our new data with other modern hydrographic datasets reveals significant temporal variability in the DWBC, associated with variations in the position, structure and age since ventilation of the current in the vicinity of Eirik Drift. The complexity of the current dynamics in this area has implications for the interpretation of hydrographic and palaeoceanographic data.     

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.     

Cessation and partial reversal of deep water freshening in the northern North Atlantic: observation-based estimates and attribution

Recent decadal salinity changes in the Greenland-Scotland overflow-derived deep waters are quantified using CTD data from repeated hydrographic sections in the Irminger Sea. The Denmark Strait Overflow Water salinity record shows the absence of any net change over the 1980s–2000s; changes in the Iceland–Scotland Overflow Water (ISOW) and in the deep water column (σ₀ > 27.82), enclosing both overflows, show a distinct freshening reversal in the early 2000s. The observed freshening reversal is a lagged consequence of the persistent ISOW salinification that occurred upstream, in the Iceland Basin, after 1996 in response to salinification of the northeast Atlantic waters entrained into the overflow. The entrainment salinity increase is explained by the earlier documented North Atlantic Oscillation (NAO)-induced contraction of the subpolar gyre and corresponding northwestward advance of subtropical waters that followed the NAO decline in the mid-1990s and continued through the mid-2000s. Remarkably, the ISOW freshening reversal is not associated with changes in the overflow water salinity. This suggests that changes in the NAO-dependent relative contributions of subpolar and subtropical waters to the entrainment south of the Iceland–Scotland Ridge may dominate over changes in the Nordic Seas freshwater balance with respect to their effect on the ISOW salinity.     

西北冰洋表层沉积物中的底栖有孔虫组合及其古环境意义

通过对中国第1~4次北极考察在西北冰洋采集的表层沉积物中底栖有孔虫丰度及其优势种分布特征与环境因素关系的综合研究发现,楚科奇海区低的底栖有孔虫丰度主要受较高的陆源物质输入的稀释作用影响;楚科奇海台和阿尔法脊较高的底栖有孔虫丰度主要受到暖而咸的大西洋中层水的影响;受碳酸钙溶解作用影响的门捷列夫深海平原和加拿大海盆底栖有孔虫丰度较低,并且水深3 597 m的站位出现了似瓷质壳的Pyrgo williamsoni和Quinqueloculina orientalis,说明该区的CCD深度大于3 600 m。根据底栖有孔虫7个优势属种的百分含量分布特征可以划分出5个区域组合:南楚科奇海陆架-白令海峡组合以优势种Elphidium excavatum和Buccella frigida为特征,可能反映受白令海陆架水影响的浅水环境;阿拉斯加沿岸-波弗特海组合以优势种Florilus scaphus和Elphidium albiumbilicatum为特征,可能反映受季节性海冰融化,低盐的阿拉斯加沿岸流以及河流淡水输入的低盐环境;大西洋中层水组合以优势种Cassidulina laevigata为特征,可能反映高温高盐的大西洋中层水影响的环境;北极深层水组合以优势种Cibicides wuellerstorfi为特征,可能反映水深大于1 500 m低温高盐的北极深层水环境;门捷列夫深海平原组合以优势种Oridorsalis umbonatus为特征,可能反映低营养的底层水环境。     

Recent changes in the Labrador Sea Water within the Deep Western Boundary Current southeast of Cape Cod

Water properties measured by the central mooring in the Line W mooring array southeast of Cape Cod document a large character shift during the period of November 2001 to April 2008. The observed temperature, salinity and planetary potential vorticity (PPV) anomalies manifest changes in the formation region of the water masses present at Station W, specifically upper Labrador Sea Water (uLSW), deep Labrador Sea Water (dLSW) and Overflow Water (OW). During the observation period, the minimum in the PPV anomaly field relative to the record mean PPV profile migrated from 1500 m, where it was originally found, to 700 m. Temporal changes in the vertical distribution of temperature and salinity were correlated with the PPV changes. This suggests a dLSW-dominated first half of the record versus an uLSW-dominated second half. The structure of these anomalies is consistent with observations within the Labrador Sea, and their transit time to Line W agrees well with tracer-derived times for signals spreading along the western boundary. In that context, the observed water properties at Line W in the early 2000s reflected the intense deep convection in the Labrador Sea in the mid-1990s, with less intense convection subsequently affecting lighter isopycnals. The observed velocity field is dominated by high-frequency (periods of days to months) fluctuations, however, a fraction of the velocity variability is correlated with changes in water mass properties, and indicate a gradual acceleration of the southwestward flow, with a corresponding increase in Deep Western Boundary Current transport.     

The oxygen isotope composition of water masses in the northern North Atlantic

The ratio of oxygen-18 to oxygen-16 (expressed as per mille deviations from Vienna Standard Mean Ocean Water, δ¹⁸O) is reported for seawater samples collected from seven full-depth CTD casts in the northern North Atlantic between 20° and 41°W, 52° and 60°N. Water masses in the study region are distinguished by their δ¹⁸O composition, as are the processes involved in their formation. The isotopically heaviest surface waters occur in the eastern region where values of δ¹⁸O and salinity (S) lie on an evaporation–precipitation line with slope of 0.6 in δ¹⁸O–S space. Surface isotopic values become progressively lighter to the west of the region due to the addition of ¹⁸O-depleted precipitation. This appears to be mainly the meteoric water outflow from the Arctic rather than local precipitation. Surface samples near the southwest of the survey area (close to the Charlie Gibbs Fracture Zone) show a deviation in δ¹⁸O–S space from the precipitation mixing line due to the influence of sea ice meltwater. We speculate that this is the effect of the sea ice meltwater efflux from the Labrador Sea. Subpolar Mode Water (SPMW) is modified en route to the Labrador Sea where it forms Labrador Sea Water (LSW). LSW lies to the right (saline) side of the precipitation mixing line, indicating that there is a positive net sea ice formation from its source waters. We estimate that a sea ice deficit of ≈250 km³ is incorporated annually into LSW. This ice forms further north from the Labrador Sea, but its effect is transferred to the Labrador Sea via, e.g. the East Greenland Current. East Greenland Current waters are relatively fresh due to dilution with a large amount of meteoric water, but also contain waters that have had a significant amount of sea ice formed from them. The Northeast Atlantic Deep Water (NEADW, δ¹⁸O=0.22‰) and Northwest Atlantic Bottom Waters (NWABW, δ¹⁸O=0.13‰) are isotopically distinct reflecting different formation and mixing processes. NEADW lies on the North Atlantic precipitation mixing line in δ¹⁸O–salinity space, whereas NWABW lies between NEADW and LSW on δ¹⁸O–salinity plots. The offset of NWABW relative to the North Atlantic precipitation mixing line is partially due to entrainment of LSW by the Denmark Strait overflow water during its overflow of the Denmark Strait sill. In the eastern basin, lower deep water (LDW, modified Antarctic bottom water) is identified as far north as 55°N. This LDW has δ¹⁸O of 0.13‰, making it quite distinct from NEADW. It is also warmer than NWABW, despite having a similar isotopic composition to this latter water mass.     

Oceanic vertical mixing of the lower halocline water in the Chukchi Borderland and Mendeleyev Ridge

Oceanic vertical mixing of the lower halocline water (LHW) in the Chukchi Borderland and Mendeleyev Ridge was studied based on in situ hydrographic and turbulent observations. The depth-averaged turbulent dissipation rate of LHW demonstrates a clear topographic dependence, with a mean value of 1.2×10–9 W/kg in the southwest of Canada Basin, 1.5×10–9 W/kg in the Mendeleyev Abyssal Plain, 2.4×10–9 W/kg on the Mendeleyev Ridge, and 2.7×10–9 W/kg on the Chukchi Cap. Correspondingly, the mean depth-averaged vertical heat flux of the LHW is 0.21 W/m2 in the southwest Canada Basin, 0.30 W/m2 in the Mendeleyev Abyssal Plain, 0.39 W/m2 on the Mendeleyev Ridge, and 0.46 W/m2 on the Chukchi Cap. However, in the presence of Pacific Winter Water, the upward heat released from Atlantic Water through the lower halocline can hardly contribute to the surface ocean. Further, the underlying mechanisms of diapycnal mixing in LHW—double diffusion and shear instability—was investigated. The mixing in LHW where double diffusion were observed is always relatively weaker, with corresponding dissipation rate ranging from 1.01×10–9 W/kg to 1.57×10–9 W/kg. The results also show a strong correlation between the depth-average dissipation rate and strain variance in the LHW, which indicates a close physical linkage between the turbulent mixing and internal wave activities. In addition, both surface wind forcing and semidiurnal tides significantly contribute to the turbulent mixing in the LHW.     

Dissolved organic matter cycling in the confluence of the Atlantic and Indian oceans south of Africa

The boundary between the Atlantic and Indian sectors of the Southern Ocean is a key spot of the thermohaline circulation, where the following water masses mix up: Indian Central water (ICW), South Atlantic Central Water (SACW), Antarctic Intermediate Water (AAIW), Circumpolar Deep Water (CDW), North Atlantic Deep Water (NADW), Weddell Sea Deep Water (WSDW) and Antarctic Winter Water (WW). An optimum multiparameter analysis based on the distributions of potential temperature, salinity, NO (=O₂+9.3×NO₃) and silicate during the GoodHope 2004 (GH04) cruise allowed us to (i) define the realms of these water masses; (ii) obtain the water mass proportion weighted-average (archetypal) apparent oxygen utilization (AOU) and dissolved organic carbon (DOC) concentrations of each water mass; and (iii) estimate the contribution of DOC to the oxygen demand of the study area. WW represented only 5.2% of the water volume sampled during GH04, followed by WSDW with 10.8%, NADW with 12.7%, SACW with 15.3%, AAIW with 23.1% and CDW with 32.8%. The distributions of DOC and AOU were mainly explained by the mixing of archetypal concentrations of these variables, 75±5% and 65±3% respectively, which retained the variability due to the basin-scale mineralization from the formation area to the barycentre of each water mass along the GH04 line. DOC accounted for 26±2% and 12±5% of the oxygen demand of the meso- and bathypelagic ocean, respectively. Conversely, local mineralization processes, retained by the residuals of the archetypal concentrations of DOC and AOU, did not contribute to improve significantly the mixing model of DOC.     

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。     

Labrador Sea convection and subpolar North Atlantic Deep Water export in the SODA assimilation model

Labrador Sea convection was most intense and reached the greatest depths in the early 1990s, followed by weaker, shallower, and more variable convection after 1995. The Simple Ocean Data Assimilation (SODA) version 2.0.2/2.0.4 assimilation model is used to explore convective activity in the North Atlantic Ocean for the period from 1992 to 2007. Hydrographic conditions, which are relatively well observed during this period, are used to compare modeled and observed winter mixed-layer depths and water mass anomalies in relation to Deep Western Boundary Current transports and meridional overturning circulation (MOC) changes at the exit of the subpolar basin. The assimilation differs markedly from local observations in the March mixed-layer depth, which represents deep convection and water mass transformation. However, mean MOC rates at the exit of the subpolar gyre, forced by stratification in the mid-latitudes, are similar to estimates based on observations and show no significant decrease during the 1992–2007 period. SODA reproduces the deep Labrador Sea Water formation in the western North Atlantic without any clear indication of significant formation in the Irminger Sea while the lighter upper Labrador Sea Water density range is reached in the Irminger Sea in the 1990s, in agreement with existing assumptions of deep convection in the Irminger Sea and also supported by computed lag correlations with the Labrador Sea. Deep Water transformation mainly takes place in the eastern North Atlantic. The introduction of CFC-11 into the SODA model as a tracer reproduces the mean and multiyear variations of observed distributions.