北欧海比容高度及其与卫星高度计海表面高度异常的比较

In this study the steric height anomaly which is calculated from the hydrological data(EN3) is compared with the sea level anomaly derived from satellite altimetry in the Nordic Seas. The overall pattern of steric height is that it is higher in the margin area and lower in the middle area. The extreme values of steric height linear change from1993 to 2010 occur in the Lofoten Basin and off the Norwegian coast, respectively. Such a distribution may be partly attributed to the freshening trend of the Nordic Seas. The correlation between SLA(sea level anomaly) and SHA(steric height anomaly) is not uniform over the Nordic Seas. The time series of SLA and SHA agree well in the Lofoten Basin and northern Norwegian Basin, and worse in the northern Norwegian Sea, implying that the baroclinic effect plays a dominant role in most areas in the Norwegian Sea and the barotropic effect plays a dominant role in the northern Norwegian Sea. The weaker correlations between SLA and SHA in the Greenland and Iceland Seas lead a conclusion that the barotropic contribution is significant in these areas. The area-mean SHA over the entire Nordic Seas has similar amplitudes compared with the SLA during 1996–2002, but SHA has become lower than SLA, being less than half of SLA since 2006.     

利用气候态水文数据Hydrobase II分析北欧海深层水的分布与输运

Deep water in the Nordic seas is the major source of Atlantic deep water and its formation and transport play an important role in the heat and mass exchange between polar and the North Atlantic. A monthly hydrological climatology—Hydrobase II—is used to estimate the deep ocean circulation pattern and the deep water distribution in the Nordic seas. An improved P-vector method is applied in the geostrophic current calculation which introduces sea surface height gradient to solve the issue that a residual barotropic flow cannot be recognized by traditional method in regions where motionless level does not exist. The volume proportions, spatial distributions and seasonal variations of major water masses are examined and a comparison with other hydrological dataset is carried out. The variations and transports of deep water are investigated based on estimated circulation and water mass distributions. The seasonal variation of deep water volume in the Greenland Basin is around 22×103 km3 whereas significantly weaker in the Lofoten and Norwegian Basins. Annual downstream transports of about 1.54×103 and 0.64×103 km3 are reported between the Greenland/Lofoten and Lofoten/Norwegian Basins. The deep water transport among major basins is generally in the Greenland-Lofoten-Norwegian direction.     

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.     

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.     

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.     

Export of Greenland Sea water across the Mohn Ridge induced by summer storms

The Mohn Ridge separates the Greenland Sea and the Lofoten Basin. Previous studies identified the export across the Mohn Ridge(EMR) from the Greenland Basin into the Lofoten Basin using water mass analysis and the tracer diffusion method, but there is still lack of direct current measurements. A surface mooring with four current meters was deployed on the Mohn Ridge from June 5 to June 18 in 2015, when three cyclones passed in the adjacent area. In the absence of cyclones, the flow on the Mohn R...     

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.     

Water mass properties and fluxes in the Rockall Trough, 1975–1998

A time series of a standard hydrographic section in the northern Rockall Trough spanning 23 yr is examined for changes in water mass properties and transport levels. The Rockall Trough is situated west of the British Isles and separated from the Iceland Basin by the Hatton and Rockall Banks and from the Nordic Seas by the shallow (500 m) Wyville–Thompson ridge. It is one pathway by which warm North Atlantic upper water reaches the Norwegian Sea and is converted into cold dense overflow water as part of the thermohaline overturning in the northern North Atlantic and Nordic Seas. The upper water column is characterised by poleward moving Eastern North Atlantic Water (ENAW), which is warmer and saltier than the subpolar mode waters of the Iceland Basin, which also contribute to the Nordic Sea inflow. Below 1200 m the deep Labrador Sea Water (LSW) is trapped by the shallowing topography to the north, which prevents through flow but allows recirculation within the basin. The Rockall Trough experiences a strong seasonal signal in temperature and salinity with deep convective winter mixing to typically 600 m or more and the formation of a warm fresh summer surface layer. The time series reveals interannual changes in salinity of ±0.05 in the ENAW and ±0.04 in the LSW. The deep water freshening events are of a magnitude greater than that expected from changes in source characteristics of the LSW, and are shown to represent periodic pulses of newer LSW into a recirculating reservior. The mean poleward transport of ENAW is 3.7 Sv above 1200 dbar (of which 3.0 Sv is carried by the shelf edge current) but shows a high-level interannual variability, ranging from 0 to 8 Sv over the 23 yr period. The shelf edge current is shown to have a changing thermohaline structure and a baroclinic transport that varies from 0 to 8 Sv. The interannual signal in the total transport dominates the observations, and no evidence is found of a seasonal signal.     

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.     

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.     

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。     

1972-2013年北欧海深层水增暖

The warming of deep waters in the Nordic seas is identified based on observations during Chinese 5th Arctic Expedition in 2012 and historical hydrographic data. The most obvious and earliest warming occurrs in the Greenland Basin(GB) and shows a coincident accelerated trend between depths 2 000 and 3 500 m. The observations at a depth of 3 000 m in the GB reveal that the potential temperature had increased from-1.30°C in the early 1970 s to-0.93°C in 2013, with an increase of about 0.37°C(the maximum spatial deviation is 0.06°C) in the past more than 40 years. This remarkable change results in that deep waters in the center of the Lofton Basin(LB) has been colder than that in the GB since the year 2007. As for the Norwegian Basin(NB), only a slight trend of warming have been shown at a depth around 2 000 m since the early 1980 s, and the warming amplitude at deeper waters is just slightly above the maximum spatial deviation, implying no obvious trend of warming near the bottom. The water exchange rate of the Greenland Basin is estimated to be 86% for the period from 1982 to 2013, meaning that the residence time of the Greenland Sea deep water(GSDW) is about 35 years. As the weakening of deep-reaching convection is going on, the abyssal Nordic seas are playing a role of heat reservoir in the subarctic region and this may cause a positive feedback on the deep-sea warming in both the Arctic Ocean and the Nordic seas.     

Volume flux and fresh water transport associated with the East Icelandic Current

The importance of the circulation of fresh water within the Nordic Seas has frequently been pointed out, especially its effect on deep water formation and therefore possibly on the thermohaline circulation. The main source of fresh water is the East Greenland Current entering the Nordic Seas through Fram Strait. The Jan Mayen Polar Current and the East Icelandic Current (EIC) carry a part of the fresh water into the Greenland and Iceland Seas respectively. As a part of the EU project VEINS, Aanderaa current meters were deployed on two moorings within the EIC from June 1997 to June 1998 on a standard CTD section from Langanes, Northeast Iceland, to the central Iceland Sea in the direction towards Jan Mayen. The current was mainly concentrated along the slope where it was baroclinic, while over the deeper part a weak barotropic flow was observed. Geostrophic calculations, referenced to the current meter data, were used for estimating the volume flux and fresh water transport with the current. The total transport over the section towards the east was found to be 2.5 Sv. The fresh water transport relative to a salinity of 34.93, above 170 m, amounted to 5.5 mSv. This is roughly 4% of the fresh water transport through Fram Strait. This transport is put into a long-term perspective using hydrographic data from the Langanes section.     

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。     

Current estimates of freshwater flux through Arctic and subarctic seas

As the world warms, the expectation is that the freshwater outflows from the Arctic Ocean to the North Atlantic will strengthen and may act to suppress the rate of the climatically-important Atlantic meridional overturning circulation. Hitherto, however, we have lacked the system of measurements required to estimate the totality of the freshwater flux through subarctic seas. Though observations remain patchy and rudimentary in places, we piece-together the results from recent large-scale observational programmes together with associated modelling, to establish preliminary maps of the rates and pathways of freshwater flux through subarctic seas. These fluxes are calculated according to two reference salinities, S = 34.8 to conform with the majority of estimates reported in the literature, and S = 35.2, the salinity of the inflowing Atlantic water, to calculate the freshwater balance of the ‘Arctic Mediterranean’. We find that 148 mSv of freshwater enters the Nordic Seas across its northern boundary. There it is supplemented by around 54 mSv of freshwater from Baltic runoff, Norwegian runoff, P − E and Greenland ice melt, so that the total freshwater contribution to the Nordic Seas from all sources is 202 mSv. Of this, around 51 mSv of freshwater is estimated to pass south to the deep Atlantic in the dense water overflows leaving an assumed balance of 151 mSv to leave the Nordic Seas in the upper water export through Denmark Strait. The corresponding estimate for the freshwater outflow west of Greenland is 103 mSv relative to 35.2 so that the total freshwater flux reaching the North Atlantic through subarctic seas is around 300 mSv.     

Circulation patterns in the lower Arctic Ocean derived from geochemical data

Climatological water-mass structures were identified in the Arctic Ocean using the geochemical dataset in the Hydrochemical Atlas of the Arctic Ocean (HAAC) as well as data on a geochemically conserved parameter, PO₄*, based on phosphate and dissolved oxygen. In the upper ocean above a depth of 500 m, the HAAC was found to reliably depict the boundary between Pacific-Origin Water (P-Water) and Atlantic-Origin Water (A-Water), which is aligned 135°E–45°W near the surface but rotates counterclockwise with depth. Thus, the Arctic and Atlantic oceans exchange high-silicate P-Water and low-silicate A-Water. The PO₄* field in the lower ocean below a depth of 1500 m was analyzed statistically, and the results indicated that the Eurasian Basin receives low-PO₄* Nordic Seas Deep Water, which flows along the bottom from the Greenland Sea. The routes from the upper ocean to the lower ocean were determined. Only the southern portion of the Canada Basin, which receives water from the Chukchi and Beaufort Seas, has high PO₄* levels; the rest of the Amerasian Basin receives low-PO₄* water from the Laptev Sea and/or the Barents Sea. The Eurasian Basin receives moderate levels of PO₄* from the Fram Strait and from the intermediate layer. The intermediate-layer water gradually travels up from the lower ocean and returns to the Atlantic, entraining the subsurface portion. It is likely that high-PO₄* water occasionally flows down from the upper ocean along Greenland, making the Eurasian Basin heterogeneous.     

Upper layer cooling and freshening in the Norwegian Sea in relation to atmospheric forcing

Several time series in the Norwegian Sea indicate an upper layer decrease in temperature and salinity since the 1960s. Time series from Weather Station “M”, from Russian surveys in the Norwegian Sea, from Icelandic standard sections, and from Scottish and Faroese observations in the Faroe–Shetland area have similar trends and show that most of the Norwegian Sea is affected. The reason is mainly increased freshwater supply from the East Icelandic Current. As a result, temperature and salinity in some of the time series were lower in 1996 than during the Great Salinity Anomaly in the 1970s. There is evidence of strong wind forcing, as the NAO winter index is highly correlated with the lateral extent of the Norwegian Atlantic Current. Circulation of Atlantic water into the western Norwegian and Greenland basins seems to be reduced while circulation of upper layer Arctic and Polar water into the Norwegian Sea has increased. The water-mass structure is further affected in a much wider sense by reduced deep-water formation and enhanced formation of Arctic intermediate waters. A temperature rise in the narrowing Norwegian Atlantic Current is strongest in the north.     

Interannual thermohaline changes in the northern North Atlantic 1991–1996

In the period 1991–1996 the WOCE hydrographic section A1E/AR7E between Greenland and Ireland was repeated five times. The observed thermohaline changes altered the baroclinic structure along the eastern margin of the subpolar gyre significantly. Between June 1995 and August 1996 an overall increase of the temperature and thickness and a decrease of the density of the Subpolar Mode Water (SPMW) layer were observed, accompanied by an increase of its salinity east of the Reykjanes Ridge and a decrease of its salinity in the Irminger Sea. The changes were most pronounced in the Iceland Basin, where the Subarctic Front retreated westwards, coinciding with a strong weakening of the Westerlies as determined by the North Atlantic Oscillation. They are related to a local reduction of the Ekman upwelling and the ocean-to-atmosphere heat flux on the one hand and to the advection of anomalies from the subtropics on the other hand.The eastward spreading of the different Labrador Sea Water (LSW) vintages led to a corresponding cooling of the LSW in the Irminger Sea and in the Iceland Basin in the period 1991–1996. The renewal of the LSW in the Rockall Trough occurred more sporadically, indicating that the North Atlantic Current (NAC) impedes the southward spreading of LSW in the eastern Atlantic. The changes in 1996 seem to have also counteracted this spreading.     

Aeromagnetic survey of the Arctic Ocean: Techniques and interpretations

Approximately 147000 km of low-level (450 m) aeromagnetic tracks were flown over the Arctic Ocean and adjacent Greenland and Norwegian Seas, for the greater part with a digitally recording nuclear precession magnetometer designed and built by Wold (1964). The digital recording feature of the system facilitated numerous data processing and analytical techniques which are described herein. These include: noise filtering coordinate conversion, removal of the regional field, second derivatives, downward continuations, polynomial fits of varying degrees to profiles and surfaces, numerical approximations, and depth to source calculations. Using these data and interpretative techniques some inferences could be made about the geologic structure and evolution of the Arctic Ocean Basin. Salient amongst these are: both gravity and magnetic data suggest that there is a 2 1/2 km basement uplift in the eastern Chukchi Shelf associated with the Tigara structure which truncates the western end of Lisburne Peninsula. A 30–40 km wide basement root encircles the Chukchi Rise and extends over 30 km into the mantle. Within the Canda Basin there is a thickening of sediments from the Asian continental margin toward the Canadian Arctic Archipelago. Sediment thickness in the Makarov Basin is 1–1 1/2 km. There appears to be only about a 1/2 km sediment cover in the Fram and Nautilus Basins. The absence of large amplitude magnetic anomalies over these basins is attributed to a 10 km elevation of the Curie isotherm. The Alpha and Nansen ridges produce magnetic profiles that show axial symmetry and correlate with profiles in the North Atlantic. A quantitative attempt has been made to verify these correlations, which infer that the Alpha Cordillera became inactive 40 mybp when the locus of rifting shifted to the Nansen Cordillera. The absence of significant magnetic anomalies over the Lomonosov Ridge reinforces the hypothesis that it is a section of the former Eurasian continental margin that was translated into the Arctic Basin by sea-floor spreading along the Nansen Cordillera axis.