Sensitivity of the Atlantic Ocean circulation to a hydraulic overflow parameterisation in a coarse resolution model: Response of the subpolar gyre

We investigate the sensitivity of a coarse resolution coupled climate model to the representation of the overflows over the Greenland–Scotland ridge. This class of models suffers from a poor representation of the water mass exchange between the Nordic Seas and the North Atlantic, a crucial part of the large-scale oceanic circulation. We revisit the explicit representation of the overflows using a parameterisation by hydraulic constraints and compare it with the enhancement of the overflow transport by artificially deepened passages over the Greenland–Scotland ridge, a common practice in coarse resolution models. Both configurations increase deep water formation in the Nordic Seas and represent the large-scale dynamics of the Atlantic realistically in contrast to a third model version with realistic sill depths but without the explicit overflow transport. The comparison of the hydrography suggests that for the unperturbed equilibrium the Nordic Seas are better represented with the parameterised overflows. As in previous studies, we do not find a stabilising effect of the overflow parameterisation on the Atlantic meridional overturning circulation but merely on the overflow transport. As a consequence the surface air temperature in the Nordic Seas is less sensitive to anomalous surface fresh water forcing.Special attention is paid to changes in the subpolar gyre circulation. We find it sensitive to the overflow transport and the density of these water masses through baroclinic adjustments. The analysis of the governing equations confirms the presence of positive feedbacks inherent to the subpolar gyre and allows us to isolate the influence of the overflows on its dynamics.     

大西洋热盐环流年代际变化机制研究Ⅲ.北大西洋海气要素对热盐环流年代际振荡的影响

基于该系列文章前文研究中构建的海气耦合气候模式和所揭示的北大西洋热盐环流年代际振荡机制,针对海气要素对该振荡机制的影响问题进行了重点的探讨。为细致准确的研究北大西洋海洋要素同北大西洋热盐环流年代际振荡的关系,有针对性的定义了副极地海区表层密度指数和北大西洋暖流强度指数并对模式结果进行了全面分析。分析结果表明副极地海区表层密度变化领先大西洋径向翻转环流(MOC)变化7 a,北大西洋暖流的变化领先 MOC变化4 a,格陵兰－苏格兰海脊溢流水强度(包括丹麦海峡溢流水和法鲁海峡溢流水,是北大西洋深层水的重要来源)的变化领先 MOC的变化3 a;北大西洋大气要素变化对北大西洋热盐环流年代际振荡有非常重要的调制作用,当副极地流环和北大西洋暖流(NAC)达到最强的2 a之前,高纬度地区大气为气旋式环流异常,中纬度地区大气为反气旋式环流异常,海表热通量在大西洋副极地海区是负异常,这都有利于副极地流环和NAC的加强,更多高盐度的北大西洋水进入格陵兰-冰岛-挪威海(GIN)海域,由此可以导致GIN海域表层密度上升,使水体的层结稳定性减弱,有利于深层对流的发生,同时大气变化通过风应力旋度和海表热通量也直接影响GIN海域深层水的生成,进而导致格陵兰－苏格兰海脊溢流水的强度增加。     

大西洋热盐环流年代际变化机制研究Ⅲ.北大西洋海气要素对热盐环流年代际振荡的影响

基于该系列文章前文研究中构建的海气耦合气候模式和所揭示的北大西洋热盐环流年代际振荡机制,针对海气要素对该振荡机制的影响问题进行了重点的探讨.为细致准确的研究北大西洋海洋要素同北大西洋热盐环流年代际振荡的关系,有针对性的定义了副极地海区表层密度指数和北大西洋暖流强度指数并对模式结果进行了全面分析.分析结果表明副极地海区表...     

On freshwater-dependent bifurcations in box models of the interhemispheric thermohaline circulation

Conceptual box models of the interhemispheric thermohaline circulation are studied with respect to bifurcations. Freshwater fluxes are the main control parameters of the system: they determine the stable states and transitions between stable states of the large-scale thermohaline circulation. In this study of interhemispheric box models both numerical and analytical methods are used to investigate transition mechanisms of the thermohaline circulation. The box model examined first is an interhemispheric four-box model. It is shown that the two bifurcations where the present THC can become unstable, the saddle-node and the Hopf bifurcation, depend in a different way on hemispheric freshwater fluxes. A reduction of the model variables leads to the conclusion that two fixed freshwater fluxes between three surface boxes are the model feature responsible for the bifurcation behaviour found. The significance of the Hopf bifurcation for the stability of the thermohaline circulation is discussed.     

On gyre interactions

The principal meeting point of the subtropical and subpolar gyres of the North Atlantic is at the Tail of the Grand Banks where the two western boundary currents, the Gulf Stream and Labrador Current, join forces as the North Atlantic Current, which flows northeast almost 10° in latitude before turning east as the Subpolar Front, ultimately feeding the Labrador and Nordic Seas and the thermohaline overturning. After the Gulf Stream turns into the North Atlantic Current at the Grand Banks, its role shifts from a wind-driven current to a link in the large-scale thermohaline circulation. The processes governing this transition, in particular the continued transport north of mass and heat, are questions of considerable climatic importance. The North Atlantic Current is a very unusual western boundary current in that its mass transport decreases in the downstream direction.The mean path and annual shifting of the eastward flowing Gulf Stream is conjectured to result from a time-varying shelf-Slope Water overflow of waters from the Labrador shelf. As the volume transport increases in fall and deepens the Slope Water pycnocline, it forces the Gulf Stream south and deepens the Sargasso Sea thermocline as well. The timing of these steps governs the June maximum in baroclinic transport. There is some evidence that this ‘back-door’ gyre interaction may operate on interannual time scales as well. The question then arises whether the shelf-to-Slope Water Sea transport also plays a role in governing the separation of the Gulf Stream.The widely observed robustness of the width of the Gulf Stream appears to result from a tight balance between the release of available potential energy and the kinetic energy of the current. A broader current would release more energy than can be ‘disposed of’, while a narrower current requires more kinetic energy than is available to sustain it. It is shown that for plausible dissipation rates in the recirculation gyres, the amount of energy that needs to be expelled from the Gulf Stream is such a small fraction of that advected through as to be vitually undetectable, hence the stiffness of the current.     

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.     

Thermohaline circulation in the North Atlantic and its simulation with a box model

Features of the North Atlantic thermohaline circulation response to periodic, stochastic, and instantaneous forcing are studied using a four-box model. The present-day circulation is shown to be characterized by a stable quasi-periodic oscillatory mode that manifests itself as the Atlantic Multidecadal Oscillation. The thermohaline catastrophe is unlikely in the modern climate epoch.     

基于地球系统模式FIO-ESM v2.0对1850~2014年大西洋经向翻转环流变化的研究

大西洋经向翻转环流(Atlantic meridional overturning circulation,AMOC)作为全球大洋的极向热量输送带,对大西洋附近区域的天气及全球气候变化都存在至关重要的影响。采用自然资源部第一海洋研究所研发的地球系统模式FIO-ESM v2.0(First Institute of Oceanography-earth system model version 2.0)分析了1850~2014年AMOC的空间分布特征及时间变化规律,并进一步讨论造成该变化的可能因素。研究结果表明:1850~2014年AMOC最大值出现在40°N、1 000 m深度附近,其时间序列总体呈现-0.079 1×10⁶ m³/(s·a)的减弱趋势,该期间伴随着Labrador、Irminger海域冬季混合层深度的变浅。通过将模式计算的AMOC强度与RAPID (rapid climate change programme)和OSNAP (overturning in the subpolar North Atlantic program)观测资料进行对比,结合模式间并行比较结果显示该模式能较好地再现观测数据期间的AMOC变化规律。FIO-ESM v2.0模式模拟的AMOC具有55 a左右的年代际周期,Labrador、Irminger海域冬季混合层深度变化揭示的对流变化以及Labrador、GIN海域表层海水密度变化造成的海水下沉对AMOC强度的周期性振荡贡献较明显,其周期性变化与海表盐度(sea surface salinity,SSS)、海表温度(sea surface temperature,SST)、蒸发与降水的差值、北大西洋涛动(North Atlantic oscillation,NAO)等要素的变化密切相关。     

Causes of a fresher, colder northern North Atlantic in late 20th century in a coupled model

Observational evidence indicates that in the northern North Atlantic, especially in the Labrador Sea, almost the whole column of the ocean water is fresher, and colder in late 20th century than in 1950–1960s. Here we analyze a four-member ensemble of the 20th century simulations from a coupled climate model to examine the possible causes for these observed changes. The model simulations resemble the observed changes in the northern North Atlantic. The simulated results show that a decreased meridional freshwater divergence and an increased meridional heat divergence associated with a weaker thermohaline circulation in the North Atlantic are the primary causes for the freshening and cooling in the northern North Atlantic. The increased precipitation less evaporation tends to enforce the freshening, but the reduced sea ice flux into this region tends to weaken it. On the other hand, the surface warming induced by a higher atmospheric CO₂ concentration tends to heat up the northern North Atlantic, but is overcome by the cooling from increased meridional heat divergence.     

末次盛冰期以来巴伦支海-喀拉海古海洋环境及海冰研究进展

巴伦支海-喀拉海是北冰洋最大的边缘海,能够对环境变化做出快速的响应和反馈,是全球气候变化最为敏感的区域之一,其古海洋环境演变及海冰变化研究是全球气候变化研究的重要组成部分。末次盛冰期以来,该区域的古海洋环境受到太阳辐射、海流强度、海平面变化、温盐环流和河流输入等因素影响发生了一系列不同尺度的波动。巴伦支海受到北大西洋暖水和极地冷水两大水团相互作用的影响,在水团交界处 （极锋) 由于不同水团性质的差异,导致其海水温度、盐度及海冰发生剧烈变化。而喀拉海则受到叶尼塞河和鄂毕河大量淡水输入影响,海流系统较巴伦支海相对复杂,沉积物主要来源于河流输入的陆源物质,并可以通过磁化率的分析明确区分两条河流的陆源物质。由于受到冷水和暖水的相互作用,巴伦支海-喀拉海海冰变化迅速,并且在全新世中晚期存在 0.4 ka 和 0.95 ka 的变化周期,但海冰变化的影响因素并不是单一的,而是气候系统内部各因子相互作用的结果。目前古海冰重建研究工作主要为定性研究,定量研究相对较少,所选用的重建指标也相对单一,另外存在年代框架差、分辨率低等不足。本文以巴伦支海和喀拉海为中心,总结了其快速气候突变事件、古温度盐度、海平面及海冰的变化,对影响因素进行了探讨,并通过分析末次盛冰期以来古海洋环境研究的不足,提出了相应的展望。     

Three-dimensional numerical prediction of vertical temperature structure of the Huanghai and the Bohai Seas

In the paper, the sea is divided into two layers with density jumping, assuming that the physical parameters in each layer are independent of depth. Two-layer flow field with tide and wind currents is calculated with extended ADI method, after the calculation for flow field is stable , coupled with temperature diffusion equations and thermohaline depth prediction equation, a four-day time prediction of the surface, bottom temperature and thermohaline depth of the Huanghai and the Bohai Seas. At the same time, three dimensional temperature field of sea water is predicted through vertical temperature distribution function. The result indicates that the prediction quality of the whole model and the fitting degree between the predicted result and the measured values are satisfactory.     

渤、黄、东海夏季环流的三维斜压模型

基于拉格朗日时均观点描述环流，建立起潮流与准定常流共同占优势系统中的陆浅海环流模型，并诊断计算了夏季渤、黄、东海的三维环流图。模拟结果较好地再现了渤、黄、东海主要流系的特征。对照冬季结果，对渤、黄、东海环流的季节变化做了阐述。从环流垂向分量的分布图上，可发现渐闽近海、长江口外存在较明显的上升流区。另外，对夏季渤、黄、东海的热盐环流和潮致余流分别进行了模拟，发现它们均能在黄海构成一逆时针向的五流系统，这对形成和维持夏季黄海冷水团的存在有重要作用。热盐环流的模拟结果表明，黄海冷水团环流含有“热成流”的成分；通过Lagrange余流的计算发现环绕黄河冷水团的环流还含有“潮成流”的成分。     

The Max-Planck-Institute global ocean/sea ice model with orthogonal curvilinear coordinates

The Hamburg Ocean Primitive Equation model has undergone significant development in recent years. Most notable is the treatment of horizontal discretisation which has undergone transition from a staggered E-grid to an orthogonal curvilinear C-grid. The treatment of subgridscale mixing has been improved by the inclusion of a new formulation of bottom boundary layer (BBL) slope convection, an isopycnal diffusion scheme, and a Gent and McWilliams style eddy-induced mixing parameterisation. The model setup described here has a north pole over Greenland and a south pole on the coast of the Weddell Sea. This gives relatively high resolution in the sinking regions associated with the thermohaline circulation. Results are presented from a 450 year climatologically forced integration. The forcing is a product of the German Ocean Model Intercomparison Project and is derived from the European Centre for Medium Range Weather Forecasting reanalysis. The main emphasis is on the model’s representation of key quantities that are easily associated with the ocean’s role in the global climate system. The global and Atlantic northward poleward heat transports have peaks of 1.43 and 0.84 PW, at 18° and 21° N respectively. The Atlantic meridional overturning streamfunction has a peak of 15.7 Sv in the North Atlantic and an outflow of 11.9 Sv at 30° S. Comparison with a simulation excluding BBL shows that the scheme is responsible for up to a 25% increase in North Atlantic heat transport, with significant improvement of the depths of convection in the Greenland, Labrador and Irminger Seas. Despite the improvements, comparison with observations shows the heat transport still to be too weak. Other outstanding problems include an incorrect Gulf Stream pathway, a too strong Antarctic Circumpolar Current, and a too weak renewal of Antarctic Intermediate Water. Nevertheless, the model has been coupled to the atmospheric GCM ECHAM5 and run successfully for over 250 years without any surface flux corrections.     

On the role of bottom roughness in overflows

Overflows play an important role in the downwelling limb of the oceanic thermohaline circulation. In light of the recognition that some overflows are not homogenous but exhibit a vertical density structure, and details of topography influence the pathways of some overflows, the impact of topographic roughness on the product property distribution is explored using the 3D non-hydrostatic spectral element model Nek5000. Numerical experiments are carried out by varying bottom roughness amplitude and ambient stratification parameters, in a regime where equilibrated product water masses are formed in a non-rotating environment.Our main finding is that bottom roughness can influence the overflow product distribution such that the highest salinity classes are removed and neutral buoyancy level is attained higher up in the stratified ambient water column. It is also shown that the form drag coefficients in overflows over rough bottom can be much larger than the skin drag coefficient over smooth bottom. To our knowledge, form drag has never been measured in oceanic overflows. As such, these numerical experiments imply that such measurements would be useful for a better understanding of overflow dynamics. It is also found that the ratio of source and product overflow mass transports is robust to changes in bottom roughness. This appears to happen because the distribution of entrainment is totally different in the case of rough bottom. Entrainment tends to initiate earlier (due to vertical motion induced by topography) and terminate earlier (due to development of form drag) than that over smooth topography.     

Freshwater forcing as a booster of thermohaline circulation

Making use of a simple two‐layer model, we analyze the impact of freshwater forcing on the thermohaline circulation. We consider the forward‐type circulation dominated by thermal forcing, implying that the freshwater forcing acts to reduce the density contrast associated with the equator‐to‐pole temperature contrast (prescribed in the model). The system is described by two variables: the depth of the upper layer ( H ) and the density contrast between the upper and lower layer (Δρ), which decreases with salinity contrast. The rate of poleward flow of light surface water and the diapycnal flow (i.e., upwelling) driven by widespread small‐scale mixing are both modeled in terms of H and Δρ. Steady states of thermohaline circulation are found when these two flows are equal. The representation of the diapycnal flow ( M_D ) is instrumental for the dynamics of the system. We present equally plausible examples of a physically based representation of M_D for which the thermohaline circulation either decreases or increases with density contrast. In the latter case, contrary to the traditional wisdom, the freshwater forcing amplifies the circulation and there exists a thermally dominated equilibrium for arbitrary intensity of freshwater forcing. Here, Stommel's famous feedback between circulation and salinity contrast is changed from a positive to a negative feedback. The interaction of such a freshwater boosted thermohaline circulation with the climate system is fundamentally different from what is commonly assumed, an issue which is briefly addressed.     

印度尼西亚海潮致混合研究现状与展望

印度尼西亚海(简称印尼海)位于热带太平洋和印度洋交汇的海域,是全球最大的内潮生成海域.内潮耗散导致强烈的潮致混合,一方面将温跃层以下的海水卷入上层,降低印尼海海表温度,之后通过海气相互作用产生显著的天气和气候效应;另一方面对穿越印尼海的印度尼西亚贯穿流的物质与能量输运也有着重要影响.自Ar-lindo计划以来,人们对印...     

On the correlation between oscillations of the Caspian Sea level and the North Atlantic climate

Correlations between the changes in the climate of the Caspian Sea region and in its level and the variations in the North Atlantic climate are studied. The indices of North Atlantic oscillation (NAO), Atlantic multidecadal oscillation (AMO), the intensity of Atlantic thermohaline circulation (ATHC), and the air humidity above the North Atlantic are used as basic indicators of climatic variations that influence the Caspian Sea. Results of an experiment for reproducing the World Ocean circulation and the parameterization of cyclic climate peculiarities made it possible to reveal their impact on the formation of Eurasian climatic variability and on the level regime of the Caspian Sea. This impact is studied through the variability of ATHC, the NAO index, and a composite index of moisture transport (CIMT) that is proposed as a result of the studies.     

北欧海大气低空辐合辐散的东西振荡模态及其相应的大气遥相关异常环流

北欧海作为连接北冰洋和北大西洋的重要海域,其由热效应产生的辐合场值得关注.本文利用NECP/NCAR的速度势函数(Velocity Potential,VP)数据表征北欧海辐合辐散场,发现在秋冬季北欧海上空大气低层存在一个独立的辐合区域.经验正交函数分解结果表明,北欧海上空的VP显示出除了表征北极涛动的全区一致型模态外...     

On the relationship between dense water formation and the “Meridional Overturning Cell” in the North Atlantic Ocean

We analyze the three-dimensional structure of the Meridional Overturning Cell (MOC) in a numerical model of the North Atlantic and Arctic Oceans. The MOC is found to consist of several downward branches: some associated with dense water formation driven by surface buoyancy forcing (Labrador Sea, northern Norwegian Sea and Barents Sea), and some associated with interior mixing processes in the model (most notably entrainment of thermocline waters into the dense overflows at the Greenland–Scotland Ridge). Two experiments with altered surface buoyancy conditions are performed, both resulting in a change in the dense water formation rate in the Labrador Sea. A change of the same sign is observed in the strength of the MOC (and thus in the meridional heat flux); however, the change is less (20–50%) than would be expected from the change in the dense water formation rate. The MOC and the rate of dense water formation at higher latitudes seem not to be tightly linked on the decadal time scale. Instead, significant changes in the volume of water masses may take place. Taking this phenomenon into account may improve the interpretation of the chain of events that constitutes a climate oscillation, whether it be observed in models or in nature.     

海洋模式中潮致混合调和分析参数化方案的建立

潮致混合对海洋环流的调整起着重要作用。陆架环流的数值模拟中如果不考虑潮汐作用,往往不能得到与观测相符的垂向温盐结构。本文基于调和分析方法,建立了一套潮致混合参数化方案。该方案通过对垂向混合系数进行调和分析,从而得到随时间变化的潮致混合系数。将该方案用于黄海冷水团数值模拟的结果显示,其能够得到与在数值模式开边界直接加入潮汐强迫相当的冷水团温盐结构。和直接引入潮汐强迫相比,这一潮致混合参数化方案的优势在于,它能够大大节省数值模拟计算机时,因此有望显著提高大规模高分辨率的海洋环流及气候模式的模拟能力和计算效率。