Flow of grounded abyssal ocean currents along zonally-varying topography on a rotating sphere

A steady nonlinear planetary–geostrophic model in spherical coordinates is presented describing the hemispheric-scale meridional flow of grounded abyssal currents on a zonally-sloping bottom. The model, which corresponds mathematically to a quasi-linear hyperbolic partial differential equation, can be solved explicitly for a cross-slope isopycnal field that is grounded (i.e. intersects the bottom on the up slope and down slope sides). As a consequence of the conservation of potential vorticity, the abyssal currents possess decreasing thickness in the equatorward direction while maintaining constant meridional volume transport. There is a small westward zonal transport in the interior of these currents that results in westward intensification as they flow toward the equator. Conditions for the possible formation of a shock to develop on the up slope flank of the current are derived.     

Internal dissipative boundary layers in the cross-equatorial flow of a grounded deep western boundary current

It is known that dissipative adjustment must occur in the cross-equatorial dynamics of a deep western boundary current (DWBC) that is in planetary-geostrophic balance away from the equator. Theoretical modelling and numerical simulations suggest that the dissipative zones correspond to “small” isolated zonally-elongated regions within the trough and crest of a nonlinear stationary equatorial planetary wave that is formed as the DWBC flows eastward along the equator. An internal frictional boundary layer theory is advanced to describe the leading order structure of the DWBC in the dissipative regions, which asymptotically matches with the large scale inviscid flow characteristics in the equatorial region.     

Reconciling pyroclastic flow and surge: the multiphase physics of pyroclastic density currents

Two end-member types of pyroclastic density current are commonly recognized: pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows. We provide scaling relations that unify these end-members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the Stokes number (S_T), the stability factor (Σ_T) and the dense-dilute condition (D_D). We recognize five types of particle behaviors within a fluid eddy as a function of S_T and Σ_T: (1) particles sediment from the eddy, (2) particles are preferentially settled out during the downward motion of the eddy, but can be carried during its upward motion, (3) particles concentrate on the periphery of the eddy, (4) particles settling can be delayed or ‘fast-tracked’ as a function of the eddy spatial distribution, and (5) particles remain homogeneously distributed within the eddy. We extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current. This stratification may favor particle interactions in the basal part of the flow and D_D determines whether the flow is dense or dilute. Using only intrinsic characteristics of the current, our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents.     

Energy distributions of the large-scale horizontal currents caused by wind in the baroclinic ocean

Wind is the main energy source for the generation of the internal waves and the ocean mixing. Wunsch[1] estimated that about 1 TW (1 TW = 1012 W) energy was transported into the ocean from the winds by us-ing the altimeter data. Watanabe et al.[2] numerically calculated that the mixing processes obtained 0.7 TW energy from the global wind, which afforded most of the energy needed by the maintenance of the Merid-ional Overturning Circulation (MOC). During the past 50 years, in the Norther…     

青藏高原东南缘深部地壳流与地磁日变化短时畸变源电流的联系

在青藏高原东南缘,前人使用大地电磁探测和地震学方法得出的结果都揭示了可能存在部分熔融状态的地壳流,而这种地下熔融体与周围物质的作用可能引起了地下强电流异常,进一步导致地表地磁响应.基于连续的地磁观测,发现2018年7月31日在川滇块体周边出现大范围的地磁Z分量日变化短时畸变,畸变发生后100天内发生了4次5级以上地震.为了定量研究这一现象,本文基于Biot-Savart定理和采用SVD（Singular Value Decomposition,奇异值分解）的阻尼最小二乘法对地磁日变化短时畸变数据开展反演.结果显示：（1）以大地电磁测深给出的电性模型作为初始条件,反演得到的电流强度为3700~5000 A,有效深度为25~60 km;（2）地下畸变电流的空间分布位置和深度和地下电性高导体分布一致,与前人给出的地壳流位置吻合;（3）地壳流偶然微小运动可能引起了大范围的强电流,这种短时存在的高强度电流沿高导带分布,可能是地磁日变化短时畸变的源电流;（4）推测深部地壳流的运动具有传递应力作用,参与诱发了100天内发生多次5级以上地震.对源电流进行反演的定量化工作,以地下电流的方式佐证了可能存在地壳流.

     

The closing of a seaway: ocean water masses and global climate change

The Late Neogene witnessed various major paleoceanographic changes that culminated in intense Northern Hemisphere Glaciation (NHG). The cause and effects of these changes are still debated. We use a multiproxy approach to determine the relative timing of the closure of the Panama gateway, changes in Atlantic circulation, global cooling and ice sheet growth. Benthic foraminiferal Mg/Ca records from a Pacific and an Atlantic Site have been produced and are interpreted in terms of bottom water temperatures. These Mg-temperature records are combined with published benthic δ¹³C, δ¹⁸O and erosion records to reconstruct the flow of proto-North Atlantic Deep Water (proto-NADW) over the past 12 Ma. The results suggest that between 12.5 and 10.5 Ma, and again between about 8.5 and 6 Ma, a nutrient-depleted water mass that was colder (by 1–2°C) and fresher than the intervening deep water mass filled the Atlantic basin. This proto-NADW became warmer (by 1°C) and saltier between 6 and 5 Ma, coincident with the restriction of surface water flow through the Central American Seaway. The Mg-temperature records define a subsequent global cooling trend of 3.5°C between 5 Ma and today. Early NHG in the late Miocene was perhaps related to the formation of the relatively cold, fresh proto-NADW. The formation of the warmer and saltier proto-NADW in the early Pliocene may have initially limited Northern Hemisphere ice growth. However, the increased moisture released at high northern latitudes associated with formation of ‘warm’ proto-NADW, coupled with the global temperature decrease of deep (and hence polar surface) waters, likely helped initiate the intense NHG of the Plio–Pleistocene.     

青藏高原东南缘深部地壳流与地磁日变化短时畸变源电流的联系

在青藏高原东南缘,前人使用大地电磁探测和地震学方法得出的结果都揭示了可能存在部分熔融状态的地壳流,而这种地下熔融体与周围物质的作用可能引起了地下强电流异常,进一步导致地表地磁响应.基于连续的地磁观测,发现2018年7月31日在川滇块体周边出现大范围的地磁Z分量日变化短时畸变,畸变发生后100天内发生了4次5级以上地震.为了定量研究这一现象,本文基于Biot-Savart定理和采用SVD （Singular Value Decomposition,奇异值分解）的阻尼最小二乘法对地磁日变化短时畸变数据开展反演.结果显示：（1）以大地电磁测深给出的电性模型作为初始条件,反演得到的电流强度为3700~5000 A,有效深度为25~60 km;（2）地下畸变电流的空间分布位置和深度和地下电性高导体分布一致,与前人给出的地壳流位置吻合;（3）地壳流偶然微小运动可能引起了大范围的强电流,这种短时存在的高强度电流沿高导带分布,可能是地磁日变化短时畸变的源电流;（4）推测深部地壳流的运动具有传递应力作用,参与诱发了100天内发生多次5级以上地震.对源电流进行反演的定量化工作,以地下电流的方式佐证了可能存在地壳流.     

Reflection of equatorial Kelvin waves at eastern ocean boundaries Part I: hypothetical boundaries

A baroclinic shallow-water model is developed to investigate the effect of the orientation of the eastern ocean boundary on the behavior of equatorial Kelvin waves. The model is formulated in a spherical polar coordinate system and includes dissipation and non-linear terms, effects which have not been previously included in analytical approaches to the problem. Both equatorial and middle latitude response are considered given the large latitudinal extent used in the model. Baroclinic equatorial Kelvin waves of intraseasonal, seasonal and annual periods are introduced into the domain as pulses of finite width. Their subsequent reflection, transmission and dissipation are investigated. It is found that dissipation is very important for the transmission of wave energy along the boundary and for reflections from the boundary. The dissipation was found to be dependent not only on the presence of the coastal Kelvin waves in the domain, but also on the period of these coastal waves. In particular the dissipation increases with wave period. It is also shown that the equatorial β-plane approximation can allow an anomalous generation of Rossby waves at higher latitudes. Nonlinearities generally have a small effect on the solutions, within the confines of this model.     

准地转大洋风生环流的格子Boltzmann数值模拟

建立了求解准地转相当正压涡度方程的格子Boltzmann (LB)模型. 该模型将准地转相当正压涡度方程作为一个平流-扩散-反应方程来加以处理,在整体二阶精度下,通过Chapman_Enskog多尺度分析法,可将格子Boltzmann方程还原到相当正压涡度方程. 在不同Reynolds数、不同边界条件以及不同风应力驱动下的数值解表明,该模型正确反映了风生环流的基本结构和不同边界的耗散特征,并得到风生环流的多平衡态解等非线性特征. 此外,不同Rossby变形半径下的实验证明,小Rossby变形半径更容易激发环流的非线性模态. 通过与同等类型有限差方案的比较,表明本文的LB模型具有稳定性好、精度高等优点.     

Understanding flow dynamics and density currents in a river-reservoir system under upstream reservoir releases

ABSTRACT

A three-dimensional flow and temperature model was applied for a 124 km river-reservoir system from Lewis Smith Dam tailrace to Bankhead Lock & Dam, Alabama. The model was calibrated against measured water levels, temperatures, velocities and flow rates from 4 May to 3 September 2011 under small constant release (2.83 m³/s) and large intermittent releases (~140 m³/s) from an upstream reservoir. Distributions of simulated flow and temperatures and particle tracking at various locations were analyzed which revealed the complex interactions of density currents, dynamic surface waves and solar heating. Flows in the surface and bottom layers moved in both upstream and downstream directions. If there was small constant release only from Smith Dam, simulated bottom temperatures at Cordova were on average 4.8°C higher than temperatures under actual releases. The momentum generated from large releases pushed bottom density currents downstream, but the released water took several days to reach Cordova.

Editor D. Koutsoyiannis; Associate editor B. Dewals     

Tidal currents,energy flux and bottom boundary layer thickness in the Clyde Sea and North Channel of the Irish Sea

A high-resolution three-dimensional model of the Clyde Sea and the adjacent North Channel of the Irish Sea is used to compute the major diurnal and semidiurnal tides in the region, the associated energy fluxes and thickness of the bottom boundary layer. Initially, the accuracy of the model is assessed by performing a detailed comparison of computed tidal elevations and currents in the region, against an extensive database that exists for the M₂, S₂, N₂, K₁ and O₁ tides. Subsequently, the model is used to compute the tidal energy flux vectors in the region. These show that the major energy flux is confined to the North Channel region, with little energy flux into the Clyde Sea. Comparison with the observed energy flux in the North Channel shows that its across-channel distribution and its magnitude are particularly sensitive to the phase difference between elevation and current. Consequently, small changes in the computed values of these parameters due to slight changes of the order of the uncertainty in the open-boundary values to the model, can significantly influence the computed energy flux. The thickness of the bottom boundary layer in the region is computed using a number of formulations. Depending upon the definition adopted, the empirical coefficient C used to determine its thickness varies over the range 0.1 to 0.3, in good agreement with values found in the literature. In the North Channel, the boundary layer thickness occupies the whole water depth, and hence tidal turbulence produced at the sea bed keeps the region well mixed. In the Clyde Sea, the boundary layer thickness is a small fraction of the depth, and hence the region stratifies.Responsible Editor: Phil Dyke     

Reflection of equatorial Kelvin waves at eastern ocean boundaries Part II: Pacific and Atlantic Oceans

The effect of viscosity, non linearities, incident wave period and realistic eastern coastline geometry on energy fluxes are investigated using a shallow water model with a spatial resolution of 1/4 degree in both meridional and zonal directions. Equatorial and mid-latitude responses are considered. It is found that (1) the influence of the coastline geometry and the incident wave period is more important for the westward energy flux than for the poleward flux, and (2) the effect of the inclination of the eastern ocean boundary on the poleward energy flux, for the Pacific and Atlantic Oceans, decline as the period of the incident wave increases. Furthermore, the model simulations suggest that the poleward energy fluxes from meridional boundaries give plausible results for motions of seasonal and annual periods. For comparatively shorter periods, a realistic coastline geometry has to be included for more accurate results. It is recommended that any numerical model involving the reflection of baroclinic Rossby waves (of intraseasonal, seasonal or annual periods) on the eastern Pacific or Atlantic Oceans, should consider the effect of the coastline geometry in order to improve the accuracy of the results.     

The importance of ocean temperature to global biogeochemistry

Variations in the mean temperature of the ocean, on time scales from millennial to millions of years, in the past and projected for the future, are large enough to impact the geochemistry of the carbon, oxygen, and methane geochemical systems. In each system, the time scale of the temperature perturbation is key. On time frames of 1-100 ky, atmospheric CO₂ is controlled by the ocean. CO₂ temperature-dependent solubility and greenhouse forcing combine to create an amplifying feedback with ocean temperature; the CaCO₃ cycle increases this effect somewhat on time scales longer than ∼5-10 ky. The CO₂/T feedback can be seen in the climate record from Vostok, and a model including the temperature feedback predicts that 10% of the fossil fuel CO₂ will reside in the atmosphere for longer than 100 ky. Timing is important for oxygen, as well; the atmosphere controls the ocean on short time scales, but ocean anoxia controls atmospheric pO₂ on million-year time scales and longer. Warming the ocean to Cretaceous temperatures might eventually increase pO₂ by approximately 25%, in the absence of other perturbations. The response of methane clathrate to climate change in the coming century will probably be small, but on longer time scales of 1-10 ky, there may be a positive feedback with ocean temperature, amplifying the long-term climate impact of anthropogenic CO₂ release.     

基于SWARM双星观测的场向电流的经度变化

本文利用SWARM A和C双星高精度的矢量磁场数据研究了不同季节高纬地区场向电流(FACs)随地磁经度和地方时的变化情况.研究发现:在南北半球,FACs存在明显的经度变化,南半球FACs的变化强度大约是北半球的1.2～3.2倍.利用潮汐谱分析法我们发现FACs中占主导的非迁移潮汐分量为DW2和D0.在春秋和夏季半球,DW2波更为明显.D0波可用太阳光照的经度变化来解释,向阳侧靠近磁极的经度带比远离磁极的经度带有更强的太阳光照射.DW2波则与地磁场强度和地磁倾角等因素有关.全球电离层与热层模型计算的FACs中D0波占主导,且中性风和对流电场对D0波的贡献几乎相当.     

海洋声层析应用与观测研究综述

综述了自20世纪80年代以来国际上开展的主要海洋声层析观测实验.对它在海洋观测里的应用和取得的成果进行了系统总结和回顾,主要包括:(1)海洋中尺度现象观测;(2)海流,潮流,涡度的测量;(3)对流监测;(4)流量和热量输运测量;(5)气候声学测温;(6)内波观测;(7)沿海声学层析;(8)中国学者的工作.对海洋声层析的相关问题作了评述,指出了海洋声学层析研究的重大意义、广阔的应用前景和存在的难点,对我国在这一领域的研究进行了展望并提出一些建议.     

Inferences of ocean currents from seismic streamer position data

This paper describes the first attempt to infer ocean currents from the shapes of seismic streamers using real data. It demonstrates that it is feasible to infer the ocean currents, using a total least‐squares solution at each shotpoint, when there is no lateral steering. There are some artefacts in the inferred currents when there is lateral steering; this is believed to be caused by errors in estimating the streamer velocity. This paper describes the residual equations that form the cost function and discusses how to choose weights in the cost function based on physical criteria. Correctly choosing weights is something of an art and requires further research to make automatic and robust.     

地磁感应电流（GIC）的作用与评估

地磁感应电流（GIC）可能对各种人工长距离导电体造成影响与危害.地磁扰动产生的感应电场的强度与地磁场强度、地下电阻率结构相关,在导电系统内生成的GIC的强度则同时与导电系统的内在结构有关.计算了加拿大Manitoba省三个典型地区在2000年7月15日的一个强烈磁暴期间产生的感应电场.通过对地磁活动性的统计分析,估计加拿大魁北克电网可能经受的最大GIC达每相78A（一年一次）和234A（每十年一次）.     

极区Hall电流经度差异特征的统计学研究:CHAMP卫星观测

本文利用2001至2010年间CHAMP（CHallenging Minisatellite Payload）卫星标量磁力仪（Overhauser Magnetometer）观测的磁场数据,反演得到电离层霍尔（Hall）电流,并且对极区电离层Hall电流的特征进行了统计学研究.研究主要关注平静期,即重联电场小于2 mV·m^-1条件下,在磁纬60°至90°范围内的Hall电流在不同太阳活动、季节、磁经度、磁地方时等条件下的变化特征.研究发现：Hall电流具有明显的经度差异,在南北半球呈现显著一波经度结构,而且南北半球反相,即北半球电流密度呈现峰-谷-峰结构,而南半球呈现谷-峰-谷结构.Hall电流密度的经度结构与太阳活动紧密相关.太阳活动高年经度差异最大,太阳活动中年经度差异次之,太阳活动低年经度差异最小.研究发现,在磁地方时为10-14MLT的白天,影响Hall电流的因素主要是太阳辐射;而磁地方时为21-03MLT的夜晚,除了电导率的影响之外,可能存在其他的重要的物理过程影响着Hall电流的经度分布.本文还研究了与电流相关的焦耳热的经度分布情况,发现其在南北半球分别呈现单峰、单谷结构,经度差异亦十分明显.

     

中国低纬地区地磁变化与赤道电射流

根据资料分析了中国低纬地区地磁场Ｈ分量的静日变化规律。结果表明，Ｈ分量日变幅在中午１２点左右达到最大值，在夜间达到最小值，而且，这种受赤道电射流影响的现象还随着季节和太阳活动变化而变化。     

海底淤泥层对海洋工程地震作用环境的影响问题

本提出了海底淤泥的流变性质对海洋工程的地震 作用环境的影响问题,并进行了有关的讨论。