Reproduction of the large-scale state of water and sea ice in the arctic ocean in 1948–2002: Part I. Numerical model

Calculations were performed using a model of the combined circulation of the Atlantic Ocean (from 20° S), the Arctic Ocean, and the Bering Sea with a resolution of 0.25° by latitude and longitude for 1958–2006. The results are compared with observational data and results obtained by other models. Model estimates were obtained for the evolution of the Atlantic water inflow into the Arctic basin through the Fram Strait and the Barents Sea. Increased transports of Atlantic water inflow into the Arctic basin were found for the first half of the 1990s and 2004–2006. The relation between Atlantic water transports into the Arctic basin and variations in the North Atlantic oscillation is shown. A positive trend of Atlantic water inflow into the Arctic basin through the Fram Strait (0.061 Sv per year) was revealed. The evolution of the freshwater-layer thickness in the Beaufort Circulation (BC) is considered. There are three periods of its increased values combined with the increased anticyclonic vorticity of BC currents: the 1960s, the 1980s, and from 1999 until now. The model estimate for a statistical mean timescale of the cycle of freshwater concentration and sink from the BC is 16 years, which is close to currently existing estimates. The evolution of anticyclonic vorticity of currents leads the variations in the freshwater-layer thickness of the BC by 1.75 years. Since the mid-1970s, there have been long positive trends of both the freshwater-layer thickness and anticyclonic vorticity of currents in the BC. In the same time period, there has been a satellite-registered negative trend in the ice area in the Arctic, which was reproduced by the model.     

Some thoughts on the freezing and melting of sea ice and their effects on the ocean

The high-latitude freezing and melting cycle can variously result in haline convection, freshwater capping or freshwater injection into the interior ocean. An example of the latter process is a secondary salinity minimum near 800 m-depth within the Arctic Ocean that results from the transformation on the Barents Sea shelf of Atlantic water from the Norwegian Sea and its subsequent intrusion into the Arctic Ocean. About one-third of the freshening on the shelf of that initially saline water appears to result from ice melt, although the actual sea ice flux is small, only about 0.005 Sv. A curious feature of this process is that water distilled at the surface of the Arctic Ocean by freezing ends up at mid-depth in the same ocean. This is a consequence of the ice being exported southward onto the shelf, melted, and then entrained into the northward Barents Sea throughflow that subsequently sinks into the Arctic Ocean. Prolonged reduction in sea ice in the region and in the concomitant freshwater injection would likely result in a warmer and more saline interior Arctic Ocean below 800 m.     

Influence that interannual variations in Siberian river discharge have on redistribution of freshwater fluxes in Arctic Ocean and North Atlantic

A numerical simulation with a coupled sea-ice model of the Arctic and North Atlantic oceans is used to study the influence that the interannual variations in the Siberian river discharge have on the distribution and propagation of freshwater in this region. In numerical experiments we compared simulations with the use of observational data on the discharge of the most significant Siberian rivers (Ob, Yenisei, and Lena) against the results of climatic seasonally average variations of their discharges. This comparison showed that the interannual variations may have significant consequences despite their smallness when compared with oceanic-scale water transport. These consequences include (1) the intensification of either cyclonic or anticyclonic components of motion of the subsurface Arctic Ocean waters and, as a result, the redistribution of freshwater fluxes from Arctic regions between the Fram Strait and the straits of the Canadian Archipelago. A change in the store of fresh Arctic Ocean waters due to interannual variations in the Ob, Yenisei, and Lena discharges is approximately ±400 km³, whereas the volume of water redirected in this regard, which forms a link between some straits, reaches 15 thousand km³. On the other hand, (2) insignificant changes in the propagation direction of freshwaters are multiply enhanced in the process of their motion in the North Atlantic as part of the subpolar gyre because of their smaller or larger involvement in the processes of vertical mixing. As a result of this, anomalies of freshwater develop considerably far from the river mouths, like in the region of the Azores islands, and are 5–6 times larger than the maximum values of the accumulated variability volumes of the river discharge.     

Arctic salinity anomalies and their link to the North Atlantic during a positive phase of the Arctic Oscillation

Many of the changes observed during the last two decades in the Arctic Ocean and adjacent seas have been linked to the concomitant abrupt decrease of the sea level pressure in the central Arctic at the end of the 1980s. The decrease was associated with a shift of the Arctic Oscillation (AO) to a positive phase, which persisted throughout the mid 1990s. The Arctic salinity distribution is expected to respond to these dramatic changes via modifications in the ocean circulation and in the fresh water storage and transport by sea ice. The present study investigates these different contributions in the context of idealized ice-ocean experiments forced by atmospheric surface wind-stress or temperature anomalies representative of a positive AO index.Wind stress anomalies representative of a positive AO index generate a decrease of the fresh water content of the upper Arctic Ocean, which is mainly concentrated in the eastern Arctic with almost no compensation from the western Arctic. Sea ice contributes to about two-third of this salinification, another third being provided by an increased supply of salt by the Atlantic inflow and increased fresh water export through the Canadian Archipelago and Fram Strait. The signature of a saltier Atlantic Current in the Norwegian Sea is not found further north in both the Barents Sea and the Fram Strait branches of the Atlantic inflow where instead a widespread freshening is observed. The latter is the result of import of fresh anomalies from the subpolar North Atlantic through the Iceland-Scotland Passage and enhanced advection of low salinity waters via the East Icelandic Current. The volume of ice exported through Fram Strait increases by 20% primarily due to thicker ice advected into the strait from the northern Greenland sector, the increase of ice drift velocities having comparatively less influence. The export anomaly is comparable to those observed during events of Great Salinity Anomalies and induces substantial freshening in the Greenland Sea, which in turn contributes to increasing the fresh water export to the North Atlantic via Denmark Strait. With a fresh water export anomaly of 7 mSv, the latter is the main fresh water supplier to the subpolar North Atlantic, the Canadian Archipelago contributing to 4.4 mSv.The removal of fresh water by sea ice under a positive winter AO index mainly occurs through enhanced thin ice growth in the eastern Arctic. Winter SAT anomalies have little impact on the thermodynamic sea ice response, which is rather dictated by wind driven ice deformation changes. The global sea ice mass balance of the western Arctic indicates almost no net sea ice melt due to competing seasonal thermodynamic processes. The surface freshening and likely enhanced sea ice melt observed in the western Arctic during the 1990s should therefore be attributed to extra-winter atmospheric effects, such as the noticeable recent spring-summer warming in the Canada-Alaska sector, or to other modes of atmospheric circulations than the AO, especially in relation to the North Pacific variability.     

A Coupled Ice-Ocean Model in the Pan-Arctic and North Atlantic Ocean: Simulation of Seasonal Cycles

A coupled ice-ocean model is configured for the pan-Arctic and northern North Atlantic Ocean with a 27.5 km resolution. The model is driven by the daily atmospheric climatology averaged from the 40-year NCEP reanalysis (1958–1997). The ocean model is the Princeton Ocean Model (POM), while the sea ice model is based on a full thermodynamical and dynamical model with plastic-viscous rheology. A sea ice model with multiple categories of thickness is utilized. A systematic model-data comparison was conducted. This model reasonably reproduces seasonal cycles of both the sea ice and the ocean. Climatological sea ice areas derived from historical data are used to validate the ice model performance. The simulated sea ice cover reaches a maximum of 14 × 10⁶ km² in winter and a minimum of 6.7 × 10⁶ km² in summer. This is close to the 95-year climatology with a maximum of 13.3 × 10⁶ km² in winter and a minimum of 7 × 10⁶ km² in summer. The simulated general circulation in the Arctic Ocean, the GIN (Greenland, Iceland, and Norwegian) seas, and northern North Atlantic Ocean are qualitatively consistent with historical mapping. It is found that the low winter salinity or freshwater in the Canada Basin tends to converge due to the strong anticyclonic atmospheric circulation that drives the anticyclonic ocean surface current, while low summer salinity or freshwater tends to spread inside the Arctic and exports out of the Arctic due to the relaxing wind field. It is also found that the warm, saline Atlantic Water has little seasonal variation, based on both simulation and observations. Seasonal cycles of temperature and salinity at several representative locations reveals regional features that characterize different water mass properties.     

21世纪格陵兰冰川融化速率对海平面变化的影响

本文利用大洋环流模式POP研究RCP4.5情景下21世纪格陵兰冰川不同的融化速率对全球及区域海平面变化的影响。结果显示:当格陵兰冰川的融化速率以每年1%增加时,全球大部分海域的动力和比容海平面变化基本不变,主要是由于格陵兰冰川在低速融化时并不会导致大西洋经向翻转流减弱。当格陵兰冰川的融化速率以每年3%和每年7%增加时,动力海平面在北大西洋副极地、大西洋热带、南大西洋副热带和北冰洋海域呈现出显著的上升趋势,这是因为格陵兰冰川快速融化导致大量的淡水输入附近海域,造成该上层海洋层化加强和深对流减弱,导致大西洋经向翻转流显著减弱;与此同时,热比容海平面在北冰洋、格陵兰岛南部海域和大西洋副热带海域显著下降,而在热带大西洋和湾流海域明显上升;此时盐比容海平面的变化与热比容海平面是反相的,这是由于大量的低温低盐水的输入,造成北大西洋副极地海域变冷变淡、大西洋经向翻转流和热盐环流显著减弱,引起了太平洋向北冰洋的热通量和淡水通量减少,导致了北冰洋海水变冷变淡,同时热带大西洋滞留了更多的高温高盐水,随着湾流被带到北大西洋,北大西洋副极地海域低温低盐的海水,被风生环流输运到副热带海域。     

Certain results of numerical simulation of processes in the Arctic Ocean

Variations in hydrophysical parameters in the Arctic Ocean and the North Atlantic are studied on the basis of numerical simulation with the use of an ocean circulation model (including ice formation and drift). The main circulation and ice-drift modes have been ascertained depending on atmospheric cycles. The possibilities of the parameterization of intermediate and deep water formation in numerical models of polar ocean dynamics are considered. The effect of the interannual variability of the discharge of Siberian rivers on the distribution and propagation of fresh water in this region are estimated from numerical experiments. The simulation results of the propagation of the dissolved methane from Siberian rivers are presented.     

北极径流增加对大西洋经向翻转环流影响的模拟研究

观测显示过去几十年间北极入海径流呈现增加趋势,CMIP5耦合模式预测表明21世纪北极入海径流仍会增加,在RCP8.5路径下,21世纪末北极入海径流量将会是1950年的1.4倍。本文利用冰-海耦合数值模式研究了北极径流增加对大西洋经向翻转环流的影响。基于两个数值实验的结果表明,如果北极入海径流按每年0.22%的速度（与RCP8.5路径下的速度相当）增加,大西洋经向翻转环流的强度在100、150和200年后将会分别减弱0.6（3%）、1.2（7%）和1.8（11%） Sv。北极入海径流增加导致大西洋经向翻转环流减弱的主要原因是,北极入海径流增加的淡水被输运到北大西洋后,会抑制北大西洋深层水的生成,这也会导致北大西洋深层水海水年龄的增加。     

北极河流径流对北冰洋环流的影响

北极河流径流是北冰洋淡水的最大来源,其变化会对北冰洋中的诸多过程有重要影响。本文基于全球高分辨率海洋?海冰耦合模式的模拟结果,研究北冰洋温盐、海冰以及环流对北极河流径流的敏感性。通过对比有气候态北极河流径流输入的控制实验结果和径流完全关闭的敏感性实验结果,研究发现北极径流对北冰洋温度、盐度、海冰以及海洋环流等有显著的影响。关闭北极河流径流后,在河口附近的陆架上温度降低、盐度升高,且导致500 m深度处温度下降以及盐度升高;河口附近的陆架处,海冰密集度与海冰厚度增加。关闭北极河流径流也对北冰洋内的环流有影响:由于缺少来自欧亚大陆的北极径流的输入,穿极漂流与东格陵兰流流速减小且盐度增加;关闭北极径流导致近岸海表面高度降低,沿欧亚陆架的北冰洋边界流减弱,白令海入流增强。通过对比关闭北极径流实验与控制实验的温度和盐度剖面,发现关闭北极径流后大西洋层温度降低,各陆架海盐跃层的梯度减小,盐跃层厚度减小。     

The effect of tides on the volume of sea ice in the Arctic Ocean

Tides are believed to drive vertical mixing in the Arctic Ocean, thereby helping heat to reach the bottom of the sea ice layer, especially in regions with thick ice covers. However, tides are usually not included in ocean models. We investigated the effect of tides on sea ice in the Arctic Ocean using an ice-coupled ocean model that includes tides simultaneously. We found that with tidal forcing, the volume of sea ice increased by 8.5% in Baffin Bay, whereas it decreased by 17.8% in the Canadian Arctic Archipelago. The increase in sea ice volume in Baffin Bay results from the convergence of sea ice, driven by tidal residual currents. In contrast, the decrease in ice volume in the Canadian Archipelago is due to the suppression of ice formation in winter, especially in areas with steep topography, where the vertical mixing of temperature is enhanced by tides. Our results imply that tides should be directly included into the oceanic general circulation model (OGCM) to realistically reproduce the distribution of sea ice in the Arctic Ocean.     

西北冰洋中更新世以来黏土矿物变化特征及其反映的洋流和冰盖演化

本文通过对中国第七次北极考察在西北冰洋阿尔法脊南部钻取的ARC7-LIC岩芯沉积物的XRF Ca/Al比值、冰筏碎屑和黏土矿物等研究,重建了中更新世以来研究区沉积物源和周边冰盖的演化历史。ARC7-LIC岩芯黏土矿物组合类型变化显示:深海氧同位素(MIS)29～13期黏土矿物组合主要以西伯利亚物源区为主,而MIS 12期以来以北美物源为主。黏土矿物组合的变化,反映中布容期前后洋流模式的改变。同时,MIS 12期高含量的蒙脱石可能来自北美物源。物源指标对比显示,劳伦冰盖在MIS 16期首次向西北冰洋大规模排泄冰山,并且从MIS 12期开始,西北冰洋周围冰盖的进退幅度增大。在MIS 6期、4期和3期,Ca/Al和高岭石含量的不协同变化指示北美冰盖的发育具有区域差异性,位于阿拉斯加北部和麦肯齐河流域的冰盖较班克斯岛?维多利亚岛一侧更发育,崩解的冰山能将高岭石带到研究区沉积下来。     

Numerical simulation of influence of the anomalies of the Central-eastern Equatorial Pacific SST and Arctic seaice cover in summer on the atmospheric circulation

A series of numerical experiments have been conducted with a perpetual July, nine-level general circulation spectral model to determine the effect of variation of the Arctic sea ice cover extent and the joint effect of anomalies of both the Arctic sea ice cover and the Central-eastern Equatorial Pacific sea surface temperature on the summer general circulation. Results show that the two factors,anomalously large extent of the Arctic sea ice cover and anomalously warm sea surface temperature over the Central-eastern Equatorial Pacific Ocean, play substantially the equal role in the effect on the summer general circulation, and either of them can notably induce the atmospheric anomalies. The main dynamical processes determining the effect of the Arctic sea ice and the equatorial SST anomalies are associated with two leading teleconnection patterns, i. e. the Asia North/American and Eurasian patterns observed in atmosphere. The results presented in this paper again prove that the general circulation is fun     

An assessment of Arctic Ocean freshwater content changes from the 1990s to the 2006-2008 period

Unprecedented summer-season sampling of the Arctic Ocean during the period 2006-2008 makes possible a quasi-synoptic estimate of liquid freshwater (LFW) inventories in the Arctic Ocean basins. In comparison to observations from 1992 to 1999, LFW content relative to a salinity of 35 in the layer from the surface to the 34 isohaline increased by 8400±2000 km³ in the Arctic Ocean (water depth greater than 500 m). This is close to the annual export of freshwater (liquid and solid) from the Arctic Ocean reported in the literature.Observations and a model simulation show regional variations in LFW were both due to changes in the depth of the lower halocline, often forced by regional wind-induced Ekman pumping, and a mean freshening of the water column above this depth, associated with an increased net sea ice melt and advection of increased amounts of river water from the Siberian shelves. Over the whole Arctic Ocean, changes in the observed mean salinity above the 34 isohaline dominated estimated changes in LFW content; the contribution to LFW change by bounding isohaline depth changes was less than a quarter of the salinity contribution, and non-linear effects due to both factors were negligible.     

北极海冰密集度预报对大气强迫敏感性的个例研究

A regional Arctic configuration of the Massachusetts Institute of Technology general circulation model （MIT-gcm） is used as the coupled ice-ocean model for forecasting sea ice conditions in the Arctic Ocean at the Na-tional Marine Environmental Forecasting Center of China （NMEFC）, and the numerical weather prediction from the National Center for Environmental Prediction Global Forecast System （NCEP GFS） is used as the atmospheric forcing. To improve the sea ice forecasting, a recently developed Polar Weather Research and Forecasting model （Polar WRF） model prediction is also tested as the atmospheric forcing. Their forecasting performances are evaluated with two different satellite-derived sea ice concentration products as initializa-tions： （1） the Special Sensor Microwave Imager/Sounder （SSMIS） and （2） the Advanced Microwave Scanning Radiometer for EOS （AMSR-E）. Three synoptic cases, which represent the typical atmospheric circulations over the Arctic Ocean in summer 2010, are selected to carry out the Arctic sea ice numerical forecasting experiments. The evaluations suggest that the forecasts of sea ice concentrations using the Polar WRF atmo-spheric forcing show some improvements as compared with that of the NCEP GFS.     

Sensitivity of a global ocean model to increased run-off from Greenland

We study the reaction of a global ocean–sea ice model to an increase of fresh water input into the northern North Atlantic under different surface boundary conditions, ranging from simple restoring of surface salinity to the use of an energy balance model (EBM) for the atmosphere. The anomalous fresh water flux is distributed around Greenland, reflecting increased melting of the Greenland ice sheet and increasing fresh water export from the Arctic Ocean. Depending on the type of surface boundary condition, the large circulation reacts with a slow-down of overturning and gyre circulations. Restoring of the total or mean surface salinity prevents a large scale redistribution of the salinity field that is apparent under mixed boundary conditions and with the EBM. The control run under mixed boundary conditions exhibits large and unrealistic oscillations of the meridional overturning. Although the reaction to the fresh water flux anomaly is similar to the response with the EBM, mixed boundary conditions must thus be considered unreliable. With the EBM, the waters in the deep western boundary current initially become saltier and a new fresh water mass forms in the north-eastern North Atlantic in response to the fresh water flux anomaly around Greenland. After an accumulation period of several decades duration, this new North East Atlantic Intermediate Water spreads towards the western boundary and opens a new southward pathway at intermediate depths along the western boundary for the fresh waters of high northern latitudes.     

Heat and Freshwater Budgets and Pathways in the Arctic Mediterranean in a Coupled Ocean/Sea-ice Model

The Arctic Mediterranean is important for climate studies because of its unique thermodynamic characteristics and its potential role in freshwater export, which would influences air-sea and ice-sea interactions and may change the North Atlantic thermohaline circulation. It is difficult to obtain consistent and complete estimates of heat and freshwater budgets due to sparse observation. In this paper, we use a coupled Arctic ocean/sea-ice model with NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data, long-term gauged river runoff data, precipitation data and estimates of volume transports to examine heat and freshwater budgets and pathways in dynamically and thermodynamically consistence. The model implements Neptune effect, flux-corrected-transport algorithm and more sophisticated treatments of heat and freshwater fluxes. Uncertainties and deficiencies in the modeling were also evaluated. Results indicate that the Arctic Ocean is provided heat mainly from the Fram Strait branch of Atlantic water at about 46 TW, which is within the range in literature. The Barents Sea branch carries about 43 TW of net heat entering the Barents Sea, but only 2 TW of net heat enters the Arctic Ocean. The Atlantic water is significantly modified in the Barents Sea. About 39 TW of heat is lost, which is consistent with the range of estimates by Simonsen and Haugan (1996). The model suggests 79,422 km³ of freshwater storage mainly distributing the Canada Basin, the Beaufort Sea and the Eurasian coast, which is in a good agreement with estimate by Aagaard and Carmack (1989). Freshwater origins from river runoff, precipitation and the Bering Strait throughflow. Liquid freshwater mainly exports via the Canadian Archipelago and Fram Strait at the rates of 3100 km³/yr and 1400 km³/yr. Sea-ice is dominantly transported through Fram Strait with 1923 km³/yr. Model discrepancies exist and climate drift is clear, which require comprehensive physical treatments of mixing processes and dense water processes in the model.     

西北航道夏季气旋登陆期间海冰对波浪的影响

西北航道是指从北大西洋经加拿大北极群岛进入北冰洋,再进入太平洋的航道,是连接大西洋和太平洋的捷径。为了探讨西北航道通航期极端天气条件下强风及海冰对波浪场的影响机制,建立并验证了考虑海冰影响下的西北航道风浪演化模型,并以2012年8月北极气旋登陆期间为例探讨西北航道通航期波浪特性及波能流密度的时空演化及其对风和海冰的响应。研究结果表明,北极夏季海冰大多分布于西北航道以北海域,而风向大部分集中在SSW(南偏西22.5°)至SW(南偏西45°),西北航道海冰的存在并不会引起有效风区的明显减少,也不会引起无冰海域波能流的明显减小(不超过5%)。但是,当风向变为北向风时,无冰海域波能流减小幅度最多高达62%。最后,综合海冰和波浪要素的时空分布,提出了极端天气条件下西北航道通航期的最佳适航路线,为西北航道的夏季安全通航提供了科学依据。     

Dense water formation and circulation in the Barents Sea

Dense water masses from Arctic shelf seas are an important part of the Arctic thermohaline system. We present previously unpublished observations from shallow banks in the Barents Sea, which reveal large interannual variability in dense water temperature and salinity. To examine the formation and circulation of dense water, and the processes governing interannual variability, a regional coupled ice-ocean model is applied to the Barents Sea for the period 1948-2007. Volume and characteristics of dense water are investigated with respect to the initial autumn surface salinity, atmospheric cooling, and sea-ice growth (salt flux). In the southern Barents Sea (Spitsbergen Bank and Central Bank) dense water formation is associated with advection of Atlantic Water into the Barents Sea and corresponding variations in initial salinities and heat loss at the air-sea interface. The characteristics of the dense water on the Spitsbergen Bank and Central Bank are thus determined by the regional climate of the Barents Sea. Preconditioning is also important to dense water variability on the northern banks, and can be related to local ice melt (Great Bank) and properties of the Novaya Zemlya Coastal Current (Novaya Zemlya Bank). The dense water mainly exits the Barents Sea between Frans Josef Land and Novaya Zemlya, where it constitutes 63% (1.2 Sv) of the net outflow and has an average density of 1028.07 kg m⁻³. An amount of 0.4 Sv enters the Arctic Ocean between Svalbard and Frans Josef Land. Covering 9% of the ocean area, the banks contribute with approximately 1/3 of the exported dense water. Formation on the banks is more important when the Barents Sea is in a cold state (less Atlantic Water inflow, more sea-ice). During warm periods with high throughflow more dense water is produced broadly over the shelf by general cooling of the northward flowing Atlantic Water. However, our results indicate that during extremely warm periods (1950s and late 2000s) the total export of dense water to the Arctic Ocean becomes strongly reduced.     

On the simulation of temperature and salinity fields in the Arctic Ocean

This paper considers the formation of the thermohaline structure of the Arctic Ocean: the formation of the salinity field and a freshwater reservoir in the Beaufort Sea and the transport of warm Atlantic water into the central part of the Arctic Ocean. A new version of the Finite Element Model of the Arctic Ocean (FEMAO) with a low spatial resolution is used. The main distinctions of this version are the following features: a new equation of state, a more sophisticated parameterization of vertical turbulence, modified formulations for the boundary conditions on open boundaries (using satellite data on the sea level) and at the upper boundary of the ocean, and the use of a variable eddy diffusivity in the parameterization of the eddy transport of a scalar. Our experiments indicated that the use of the parameterization of the eddy transport of a scalar enhances the transport of warm Atlantic waters to the central part of the Arctic Ocean through the Fram Strait; the results are most realistic when a variable coefficient is used. The Neptune effect has a contradictory role and, in the future, a higher spatial resolution should be used instead of this parameterization. We revealed that a key factor in the thermohaline fields on a large time scale is the interaction with the Atlantic Ocean, which is the source of heat and saline water.