海洋模式中垂直混合参数化方案介绍

介绍了海洋垂直混合过程参数化方案的发展,以及不同参数化方案在海洋模式中的应用情况。首先,介绍不同垂直混合参数化方案的物理问题、理论依据、数学表达和特征,并对不同参数化方案进行了比较。其次,针对中尺度涡、亚中尺度涡以及波浪、潮流混合参数化的最新研究进展进行了总结并对垂直混合参数化的未来发展提出了一些建议。     

The Global Distribution of Diapycnal Mixing and Mixing Coefficient Tensor in the Upper 2000m Ocean from Argo Observations

Diapycnal Mixing (DM) within the upper 2000m of the global ocean is calculated by a fine-scale parameterization using the multiyear-mean density gridded product that created by employing all the Argo float observations to date through the recently published equation of seawater TEOS-10. The geographic distribution of Argo-derived DM derived in this study is spatial-dependent and varies with latitude and depth. The magnitude and pattern of DM is favorably validated by comparisons with previous studies. Furthermore, the mixing coefficient tensor K is calculated and analyzed. Components of the tensor fitting for the geopotential coordinate models are also presented. It is found that the tensor components in horizontal direction, K_xx and K_yy, have similar magnitude and distribution pattern. In the vertical, K_zz is enhanced over regions with rough topography and strong wind (e.g., Westerly region), suggesting agreement with previous estimates. This work presents a scheme to estimate the DM and mixing coefficient tensor using Argo observations, and offers a useful Argo-based mixing product for the purpose of promoting the study and modeling of ocean circulation and other processes.     

Sensitive study of the long and short surface wave-induced vertical mixing in a wave-circulation coupled model

The previous studies by the MASNUM research team have shown the effectiveness of the wave-induced mixing (Bv) in improving the simulation of upper-ocean thermal structure. The mechanisms of Bv are further investigated by incorporating different Bv products into the MASNUM wave-circulation coupled model. First, experiments were designed to explore the effects of Bv, which contain the contributions at different wave lengths (l). The results of three experiments, the non-Bv case, the short-wave case (l <300 m), and the long-wave case (l >300 m) are compared, and it is found that the long waves are the most important component for Bv to generate mixing in the upper ocean. As the swell plays dominant role in mixing, the parameterization of Bv into wind may be not a proper way. Second, Bv effects at different time-scales, including daily and monthly, were examined. The results show that the monthly averaged Bv has larger impact than the daily averaged Bv, especially in summer.     

The role of surface waves in the ocean mixed layer

Previously, most ocean circulation models have overlooked the role of the surface waves. As a result, these models have produced insufficient vertical mixing, with an under - prediction of the ,nixing layer （ML） depth and an over - prediction of the sea surface temperature （SST）, particularly during the summer season. As the ocean surface layer determines the lower boundary conditions of the atmosphere, this deficiency has severely limited the performance of the coupled ocean - atmospheric models and hence the climate studies. To overcome this shortcoming, a new parameterization for the wave effects in the ML model that will correct this systematic error of insufficient mixing. The new scheme has enabled the mixing layer to deepen, the surface excessive heating to be corrected, and an excellent agreement with observed global climatologic data. The study indicates that the surface waves are essential for ML formation, and that they are the primer drivers of the upper ocean dynamics; therefore, they are critical for climate studies.     

Differential diffusion: an oceanographic primer

Even where mean gradients of temperature T and salinity S are both stabilizing, it is possible for the larger molecular diffusivity of T to result in differential diffusion, the preferential transfer of T relative to S. The present note reviews existing evidence of differential diffusion as provided by laboratory experiments, numerical simulations, and oceanic observations. Given potentially serious implications for proper interpretation of estimates of diapycnal density diffusivity from both ocean microscale measurements and ocean tracer release experiments, as well as the sensitivity of predictive global ocean models to this diffusive parameter, it is essential that this process be better understood in the oceanographic context.     

On the role of wave breaking in ocean dynamics under typhoon Matsa in the Bohai Sea,China

The role of wave breaking (WB) in the ocean dynamics in the Bohai Sea, China under typhoon condition is systematically investigated utilizing a coupled wave-current model. The influences of WB on ocean dynamics and processes (mixing coefficient, temperature, mixed layer depth, and current) during the entire typhoon period (including the pre-typhoon, during-typhoon and after-typhoon stages) are comprehensively detected and discussed. Experimental results show that WB greatly enhances the turbulent mixing at about top 10 m depth under typhoon condition, the increase can be up to 10 times that of the normal weather. At the same time, WB generally strengthens the sea surface cooling by ~1.2°C at the during-typhoon stage, about 3 times that in normal weather. The mixed layer depth, is rapidly increased by ~1.6–3.6 m during typhoon due to WB, particularly, the deepening is stronger in the region from 120.5°E to 121.0°E on account of close to the typhoon eye. In addition, WB renders the current speed more uniformly within the entire depth in the Bohai Sea, the change in speed is ~0.2 m/s, whereas the alternation in current vector is generally opposite to the wind direction except for the typhoon eye region, reflecting that WB has an inhibitory effect on the typhoon-forced current change. The effects of WB on vertical mixing coefficient response to the typhoon rapidly, while the impacts of WB on temperature, and mixed layer depth present hysteretic responses to typhoon. Finally, the mechanisms and distribution characteristics of WB-induced mixing and tidal mixing are compared under typhoon condition.     

Types of mixing and heterogeneities in siliciclastic-carbonate sediments

Mixed siliciclastic-carbonate deposits consist of a suite of different types of mixing between the two components, from bed (core-plug) to stratigraphic (seismic) scales, producing a high vertical and lateral lithological variability. Mixed deposits results from the interaction of siliciclastic input and coeval carbonate production controlled by temporal and/or spatial factors. Although mixed deposits are very diffuse in the geological record, studies about these deposits are scrappy and not well encoded. Accordingly, mixed deposits represent a labyrinth for researchers who want to investigate them for the first time.In this paper, different types of mixing (compositional versus strata) controlled by different allocyclic (e.g. sea-level, climate) and/or autocyclic (e.g. depositional processes) factors that operate at different scale are documented. Mixing is recognized and described at three main scales of observation: bed/core-plug scale; lithofacies/well-log scale; and stratigraphic/seismic scale. (i) Compositional mixing reflects the contemporaneous accumulation of the two heterolithic fraction in space and time. This type of mixing is observable at lamina to bed scale, locally producing depositional structures diagnostic for particular depositional environments. (ii) Strata mixing results from the alternation of the two heterolithic fraction in time. This type of mixing is observable at lithofacies to stratigraphic scale and can be related to depositional processes, climatic variations and/or relative sea-level changes.A correct identification of these different types of mixing and the scale of their occurrence is crucial in revealing (i) physical processes that control the sedimentation, (ii) environmental factors that influence the carbonate factory related to the siliciclastic dispersal mechanisms, and (iii) internal heterogeneity of the resulting sedimentary deposit. Furthermore, the petroleum industry is interested to unravel new insights about internal properties of mixed siliciclastic-carbonate systems (e.g., porosity, permeability) and to reconstruct predictive 3D models for the related reservoirs. The correct prediction of internal heterogeneity and the recognition of lateral and vertical compartmentalization have an important impact on hydrocarbon exploration and exploitation.     

Progress in the determination of Ac in sea water

²²⁷Ac is a naturally occurring radioisotope with a unique combination of properties that make it suitable for the determination of deep ocean mixing and upwelling rates. Here, we present a method for the determination of ²²⁷Ac in sea water on sample sizes of 20–80 L. The measurement is based on co-precipitation of ²²⁷ Ac with MnO₂, followed by chemical isolation of actinium in the presence of an artificial Ac isotope. Actinium is then electrodeposited onto silver discs. In two alpha-spectrometric counting periods, first the artificial ²²⁵Ac isotope is counted, then after > 100 days five daughters of ²²⁷Ac. The first counting period gives a total yield for the procedure, integrating chemical recovery and detector efficiency. The total yield was found here to be on average 15 ± 5%, the chemical yield on average about 50%. The counting of five decay products of ²²⁷Ac in the second period makes the method particularly sensitive. Using appropriate decay corrections, the initial ²²⁷Ac activity can be determined to better than 10% relative error for concentrations < 10,000 atoms/L. We compare data acquired by the new method to a data set from in-situ pumps, from a parallel sampling campaign in the Eastern Weddell Gyre, and we can show excellent agreement. Repeated determinations of ²²⁷Ac in a uranium reference material (UREM-11) demonstrate the accuracy of the method.     

一个考虑潮汐、中尺度涡和地形影响的南海底部环流诊断模型

本文构造了一个考虑潮汐、中尺度涡和地形影响下的南海底部环流诊断模型。在该模型中,潮汐混合和涡致混合引起的垂直速率用一个类似的改进参数化方案来表示。该模型结果显示在南海深层吕宋海峡"深水瀑布"和斜压影响最大,潮汐作用和中尺度涡影响次之,风场的影响最小。斜压影响的整体效应与其他因素相反。潮汐混合与涡致混合具有明显的地形依赖性。潮汐混合主要集中在南海北部海盆地形较为陡峭的陆坡区和南海中部海山区,而涡致混合主要集中在海盆西边界区以及中部海山区。在不考虑吕宋海峡"深水瀑布"、潮汐和中尺度涡的情况下(对应吕宋海峡关闭),南海底部环流为反气旋式环流。考虑吕宋海峡"深水瀑布"后,南海底层环流为气旋式环流,而潮汐混合和涡致混合起到加强整个气旋式环流强度的作用。此外,该模型还给出了南海底部环流量级大小与地形坡度之间的密切关系,即地形坡度较大的地方,其流速也大。这对于现场观测有着一定的参考意义。最后,本文用尺度分析的方法从理论上分析了该模型的适用性,证实了该模型具有一定的可靠性。     

浪致混合对亚热带冬季海洋混合强度的影响

上层海洋在全球气候系统中起着至关重要的作用。对上层海洋层结及混合的模拟偏差一直是海洋和气候数值模式发展中悬而未决的问题。本文首先评估了CMIP5中45个模式对上层海洋层结模拟的偏差,确认了冬季亚热带地区海洋模式垂向混合偏强。随后,基于自然资源部第一海洋研究所地球系统模式(FIO-ESM v1.0),分别开展了1986-2005年期间包含和关闭海浪垂向混合情况下的数值实验,分析浪致混合对亚热带冬季海洋混合强度模拟的影响及机制。发现浪致混合使得气候模式中亚热带海域冬季的海洋层结增强,增强的层结使上层海洋更加稳定。首次揭示了增加浪致混合反而降低了海洋总体的垂向混合率:浪致混合使北半球冬季亚热带海域混合率从无浪实验的227 cm²/s降低到有浪实验的178 cm²/s,降低了21.6%;南半球冬季亚热带海域混合率从无浪实验的189 cm²/s降低到有浪实验的165 cm²/s,降低了12.7%。进一步分析发现,浪致混合主要是通过增加冬季亚热带海域上层海洋的热含量从而强化了海洋的层结,最终改善了气候模式对上层...     

Conservative behaviour of boron in the Tamar Estuary

Boron is found to be conservative in the Tamar Estuary in a series of surveys conducted over an 18-month period. Dissolved boron in the estuary derives almost entirely from seawater.     

Numerical study of the meridional overturning circulation with “mixing hotspots” in the Pacific Ocean

Using an idealized ocean general circulation model, we examine the effect of “mixing hotspots” (localized regions of intense diapycnal mixing) predicted based on internal wave-wave interaction theory (Hibiya et al., 2006) on the meridional overturning circulation of the Pacific Ocean. Although the assumed diapycnal diffusivity in the mixing hotspots is a little larger than the predicted value, the upwelling in the mixing hotspots is not sufficient to balance the deep-water production; out of 17 Sv of the downwelled water along the southern boundary, only 9.2 Sv is found to upwell in the mixing hotspots. The imbalance as much as 7.8 Sv is compensated by entrainment into the surface mixed layer in the vicinity of the downwelling region. As a result, the northward transport of the deep water crossing the equator is limited to 5.5 Sv, much less than estimated from previous current meter moorings and hydrographic surveys. One plausible explanation for this is that the magnitude of the meridional overturning circulation of the Pacific Ocean has been overestimated by these observations. We raise doubts about the validity of the previous ocean general circulation models where diapycnal diffusivity is assigned ad hoc to attain the current magnitude suggested from current meter moorings and hydrographic surveys.     

冲绳海槽沉积物混合作用的研究

介绍了利用方向性同位素＾２１０Ｐａ活度在岩芯中的垂直分布，根据沉积作用的数学模式，结合有关资料对冲绳海槽海水－海底界面沉积物的混合作用强度进行了了探讨。＾２１０Ｐｂ资料表明冲绳海槽表层沉积物中，＾２１０Ｐｂ放射性活度存在空间差异，介于２６．２６－９２．４０ｄｐｍ／ｇ，沉积物混合系数个于０．５１－６．８８ｃｍ＾２．ａ，这反映了研究海区不同的混合作用强度和沉积环境。     

Elucidating the dynamics and mixing agents of a shallow fjord through age tracer modelling

The mixing agents and their role in the dynamics of a shallow fjord are elucidated through an Eulerian implementation of artificial tracers in a three-dimensional hydrodynamic model. The time scales of vertical mixing in this shallow estuary are short, and the artificial tracers are utilized in order to reveal information not detectable in the temperature or salinity fields. The fjord's response to external forcing is investigated through a series of model experiments in which we quantify vertical mixing, transport time scales of fresh water runoff and estuarine circulation in relation to external forcing.Using age tracers released at surface and bottom, we quantify the time scales of downward mixing of surface water and upward mixing of bottom water. Wind is shown to be the major agent for vertical mixing at nearly all depth levels in the fjord, whereas the tide or external sea level forcing is a minor agent and only occasionally more important just close to the bottom. The time scale of vertical mixing of surface water to the bottom or ventilation time scale of bottom water is estimated to be in the range 0.7 h to 9.0 days, with an average age of 2.7 days for the year 2004.The fjord receives fresh water from two streams entering the innermost part of the fjord, and the distribution and age of this water are studied using both ageing and conservative tracers. The salinity variations outside this fjord are large, and in contrast to the salinity, the artificial tracers provide a straight forward analysis of river water content. The ageing tracer is used to estimate transport time scales of river water (i.e. the time elapsed since the water left the river mouth). In May 2004, the typical age of river water leaving the fjord mouth is 5 days. As the major vertical mixing agent is wind, it controls the estuarine circulation and export of river water. When the wind stress is set to zero, the vertical mixing is reduced and the vertical salinity stratification is increased, and the river water can be effectively exported out of the fjord.We also analyse the river tracer fields and salinity field in relation to along estuary winds in order to detect signs of wind-induced straining of the along estuary density gradient. We find that events of down estuary winds are primarily associated with a reduced along estuary salinity gradient due to increased surface salinity in the innermost part of the fjord, and with an overall decrease in vertical stratification and river water content at the surface. Thus, our results show no apparent signs of wind-induced straining in this shallow fjord but instead they indicate increased levels of vertical mixing or upwelling during down estuary wind events.     

依据水文强化观测计算胶州湾水体混合参数

根据2001年夏季胶州湾连续站观测资料,从分析潮汐、潮流特征和温度、盐度变化趋势入手,计算了湍动能耗散率和密度扩散系数.借助调和分析、谱分析、线性回归和经验正交分析方法,得到各观测要素的统计特征,并讨论其成因.研究表明,胶州湾以半日潮振荡占优,水平潮流表现为往复流的形式;由于观测前台风过境,降水骤增,径流量变大,温度、盐度层化显著,并伴随湾内与外海水交换的进行,温/盐与潮位分别表现为显著的负/正相关,盐度逐日升高并趋于新的平衡;湍动能耗散率和密度扩散系数的量级分别为10-5w·m-3和10-6m2·s-1,受强潮流剪切影响该值明显高于陆架边缘海域,其空间主要模态表现为表层小、底层大,相应时间系数存在大约6h的显著周期.     

Sediment-activation depth values for gentle and steep beaches

This paper analyzes methods and terminologies used in literature for the determination and characterization of vertical distribution of sediment-activation depth, which is bottom sediment layer affected by hydrodynamic processes. Studies on this topic include assessments carried out during short time spans, from minutes to few hours or longer periods, from a tidal cycle to several days. In the first case, activation is generally named “mixing depth” and is calculated by evaluating vertical distribution of fluorescent tracers. In the second case, it is referred to as “disturbance depth” and is generally evaluated using plugs of marked sand and rods, or rods with a loose-fitting washer. Vertical cross and longshore distribution of mixing and disturbance depth values, recorded in different works with different techniques, were also analyzed highlighting the conceptual differences between used methods and obtained results. In a further step, a data set from literature on this topic was gathered to obtain new formulations between disturbance depth and beach and wave characteristics as well as morphodynamic beach state, expressed throughout the surf scaling parameter and the surf similarity index. Good linear regressions were observed between these variables, obtaining expressions that can be easily used in a wide range of beach states, from dissipative and intermediate to reflective ones.     

20世纪南极菲尔德斯半岛毛皮海豹食谱比例变化的同位素混合模型比较分析

稳定同位素比值分析已成功用于示踪动物的食物来源研究,近年来已发展了多个同位素混合模型用于计算混合物中多来源物质的组成比例。本文以过去百年来南极菲尔德斯半岛南极毛皮海豹(Arctocephalus gazella)毛发的C、N同位素数据为基础,运用同位素混合模型中的欧几里得距离法、线性模型和贝叶斯模型进行计算、分析和比较,得到毛皮海豹食谱中南极磷虾(Euphausia superb)、南极电灯鱼(Electrona antarctica)和尼氏裸灯鱼(Gymnoscopelus nicholsi)所占比例随时间的变化趋势。各模型计算得出的结果可分为两组,分别是欧几里得距离法公式(1)和(2)的结果,表现为近百年来毛皮海豹食谱中Euphausia superb比例与其毛发稳定N同位素比值呈同步上升的趋势;第二组为欧几里得距离法公式(3)、线性模型和贝叶斯模型的结果,Euphausia superb比例与毛皮海豹毛稳定N同位素比值呈反相关关系。综合对比研究区域气候、海冰环境变化背景和已有的南极磷虾种群密度调查资料分析,第二组模型结果与实际情况较为相符,可用于讨论食物比例的时间变化趋势。近百年来,菲尔德斯半岛毛皮海豹食谱中Euphausia superb的比例显著下降,很可能是由于区域气候快速变暖和海冰的消退造成的。本文的计算结果指示西南极半岛气候变暖已显著影响到区域海洋食物链变化。     

增强自由紊动射流卷吸、掺混性能的研究

射流作为流体运动的一种特殊形式,在工程中具有非常广泛的应用。其中,卷吸和掺混效应是射流的两大重要特征,其影响贯穿射流发展的整个过程。长期以来研究人员从理论分析、实验测量和数值模拟等几个方面对射流的卷吸、掺混性能进行了大量的研究工作,并探索出多种提高射流卷吸、掺混性能的方法。文中从喷嘴形状,喷嘴数目,喷嘴间距,喷嘴矩形截面宽高比和射流流速比等几个方面,对增强射流卷吸、掺混性能的研究方法、研究现状和研究结果进行了整理、分析,讨论了该领域存在的问题,并对该领域的研究方向进行了展望。     

A generalized coordinate ocean model and a comparison of the bottom boundary layer dynamics in terrain-following and in z-level grids

Sensitivity studies with a new generalized coordinate ocean model are performed in order to compare the behavior of bottom boundary layers (BBLs) when terrain-following (sigma or combined sigma and z-level) or z-level vertical grids are used, but most other numerical aspects remain unchanged. The model uses a second-order turbulence closure scheme that provides surface and BBL mixing and results in a quite realistic climatology and deep water masses after 100 year simulations with a coarse resolution (1° × 1°) basin-scale terrain-following grid. However, with the same turbulence scheme but using a z-level grid, the model was unable to produce dense water masses in the deep ocean. The latter is a known problem for coarse resolution z-level models, unless they include highly empirical BBL schemes.A set of dense water overflow experiments with high-resolution grids (10 and 2.5 km) are used to investigate the influence of model parameters such as horizontal diffusivity, vertical mixing, horizontal resolution, and vertical resolution on the simulation of bottom layers for the different coordinate systems. Increasing horizontal diffusivity causes a thinner BBL and a bottom plume that extends further downslope in a sigma grid, but causes a thicker BBL and limited downslope plume extension in a z-level grid. A major difference in the behavior of the BBL in the two grids is due to the larger vertical mixing generated by the turbulence scheme over the step-like topography in the z-level grid, compared to a smaller vertical mixing and a more stably stratified BBL in the sigma grid. Therefore, the dense plume is able to maintain its water mass better and penetrates farther downslope in the sigma grid than in the z-level grid. Increasing horizontal and vertical resolution in the z-level grid converges the results toward those obtained by a much coarser resolution sigma coordinate grid, but some differences remain due to the basic differences in the mixing process in the BBL.     

The mixing mechanism in the formation of ocean shear waves

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