全球海洋碳循环模式MOM4_L40对碳和营养物自然分布的模拟

本文介绍了国家气候中心发展的一个全球海洋碳循环环流模式,并分析评估了该模式的基本性能.该模式是在美国地球物理流体动力学实验室(GFDL,Geophysical Fluid Dynamics Laboratory)的全球海洋环流模式MOM4(Modular Ocean Model Version 4)基础上发展的一个垂直方向40层、包含生物地球化学过程的全球三维海洋碳循环环流模式,简称为MOM4_L40(Modular Ocean Model Version 4 With 40Levels).该模式在气候场强迫下长期积分1000年,结果分析表明,与观测相比,模式较好地模拟了海洋温度、盐度、总二氧化碳、总碱、总磷酸盐的表面和垂直分布特征.模拟的海洋总二氧化碳分布与观测基本相符,表层为低值区,其下为高值区,高值区域位于10°S—60°N之间,但2000m以上模拟值较观测偏小,2000m以下模拟值较观测偏大.总体来说,MOM4_L40模式是一个可信赖的海洋碳循环过程模拟研究工具.     

北京上甸子区域本底站大气CFC-11浓度在线观测

利用自组装的气相色谱系统(GC-ECD),在世界气象组织(WMO)/全球大气观测网(GAW)上甸子区域大气本底站开展CFC-11浓度在线观测.2007年4月～2008年3月期间,CFC-11浓度波动较大,非本底浓度值出现频率62%.其本底浓度中值为245.4ppt(1ppt=10-12mol/mol),10%和90%的CFC-11浓度数据百分位值分别为244.6和246.1ppt;非本底浓度中值为254.7ppt,10%和90%的CFC-11浓度数据百分位值分别为246.6和272.1ppt.夏秋季CFC-11本底浓度与非本底浓度差值较大、冬春季差值较小,主要由于夏秋季节环境温度高,储存在聚氨酯硬泡中的CFC-11更易挥发而逃逸到大气中,使CFC-11浓度水平有较明显抬升.上甸子站CFC-11各月本底浓度与北半球中高纬度爱尔兰MHD站、美国THD站和巴巴多斯RPB站接近,高于南半球澳大利亚CGO站和美属萨摩亚SMO站.四个季节CFC-11浓度高值主要在W-WSW-SW扇区,表明来自该方向气流对CFC-11浓度抬升产生正的贡献;冬春季CFC-11浓度低于夏秋季,尤其NNE-NE-ENE-E风向观测浓度与本底值接近,表明冬春季该方向气流相对清洁.     

大洋风生环流观测优化的伴随分析

提出了一种依据海洋伴随资料同化达到改进海洋观测方案的客观分析方法.针对一个"真实的海洋"进行不同空间断面(或对不同层次)的假设采样,分别将这些"不完备的观测"应用于Byran-Cox海洋环流模式的伴随系统,可以计算"不完备观测"同化以后的模式环流与"完备观测"资料同化(控制试验)得到的环流之间的距离--反演距离.由于海洋伴随资料同化所具有的局地修正效应和邻域修正效应,不同观测方案所对应的反演距离有着明显的差异.采用上述方法在一定观测代价下可以对大洋风生环流进行观测方案的优化分析.     

全球大洋混合层深度的计算及其时空变化特征分析

本文利用2005-2009年的全球网格化Argo数据,分别采用温度判据和密度判据计算了全球大洋混合层深度(Mixed Layer Depth, MLD),讨论了障碍层(Barrier Layer, BL)和补偿层(Compensated Layer, CL)对混合层深度计算的影响,得到了合成的混合层深度,并研究了其时空变化特征. 研究表明:(1)在赤道西太平洋(10°S -5°N,150°E-150°W),孟加拉湾,热带西大西洋(10°N-20°N,30°W-60°W)是障碍层高发区域. 冬季的北太平洋副热带区域(30°N附近)以及东北大西洋(40°N-60°N,0°-30°W)是补偿层发生的区域. (2) 在各个半球的夏季MLD都比较浅,在各个半球的冬季MLD则普遍比较深. 北太平洋和北大西洋的MLD的分布和变化比较相似,印度洋MLD受季风影响显著,呈现半年周期变化. 太平洋和大西洋的MLD 的经向分布大致呈现出"两端深,中间浅"的拱形特点. (3)混合层深度距平场EOF第一模态时间变化为周期的年信号,北太平洋和北大西洋、南大洋(尤其是南极绕流区)都是MLD变化剧烈的海域,第二模态显示全球大洋混合层深度距平存在着一个半年的变化周期.     

南海海面高度和输运流函数: 全球 变网格模式结果

为了研究南海环流结构和变异及其与外部水域的关系, 我们建立了一个嵌套于全球大洋环流模式中的高分辨率中国近海环流数值模式. 给出模拟所得南海月平均以及年平均的海面高度和流函数分布, 与TOPEX/POSEIDON资料比较表明, 所得海面高度距平与观测十分一致. 基于这些结果, 讨论了南海的环流结构, 尤其是上层环流结构. 结果表明: 对于表层海水来说, 黑潮在冬、春和秋季均通过吕宋海峡入侵南海, 夏季则表层没有入侵. 但对于整个海水而言, 全年均有海水从太平洋通过吕宋海峡进入南海. 这一差异表明, 在夏季, 太平洋的海水是在次表层和中层入侵南海的.南海北部陆坡附近全年受气旋式环流控制. 夏季的南海南部反气旋流圈、越南东南离岸流和冬季的南海南部气旋流圈都得到了很好的再现. 南海海面高度和海面高度距平之间的差异明显. 表明, 在利用卫星高度计资料研究南海的上层环流时, 长期平均海面高度的空间分布有重要意义.     

海洋中尺度涡与内波的地震图像

海洋反射地震通常用于调查、研究海底地质构造,勘探油气与天然气水合物资源.近期研究表明多道反射地震方法也可以对水柱的热盐细结构成像.中尺度涡与内波是重要的物理海洋现象,但是常规的物理海洋调查是在间隔若干公里的离散测站上进行的,水平分辨率较低,因此对中尺度涡的结构与内波的横向分布了解较差.本文利用在大西洋东部、南海采集的地震数据给出了低频反射地震可以对中尺度涡与内波清晰成像的新的证据.反射地震方法较传统海洋观测手段,具有明显的优势,主要体现在高的水平分辨率和短时间内对整个海水剖面进行成像方面．从地震剖面上,能够清楚地观测到中尺度涡、内波造成的反射特征变化,从而有助于改进对能量在不同尺度的海水运动之间传递过程的认识.     

大西洋经向翻转环流的演变特征及其对北半球地表气温的热动力影响

综合现代观测、历史代用资料和气候模式模拟,本文综述了大西洋经向翻转环流(AMOC)的过去、现在和未来的演变特征,以及AMOC对区域和半球尺度地表气温(SAT)的影响.代用资料重建结果表明, AMOC从19世纪末开始减弱, 20世纪整体减弱,但信度较低.直接观测结果显示, 2004～2016年间AMOC减弱,但无法区分出其年代际变化和长期趋势.气候模式预测可知,若温室气体排放持续增加, AMOC未来将会减弱,但在2100年前不会出现突然的崩溃. AMOC的热力作用表现为,强AMOC导致北大西洋海表面热通量释放和经向热输送增强,造成半球尺度SAT增加,千年尺度的气候冷(暖)期对应AMOC减弱(增强).强AMOC的经向热输送能够影响北极增暖,并通过北极海冰和AMOC之间的互反馈作用影响区域SAT异常和极端温度.在动力方面,强AMOC调制源自北大西洋沿北半球中高纬度地区传播的罗斯贝波活动,引起乌拉尔和西伯利亚地区的环流异常变率增加,对欧亚大陆中高纬地区极端冷、暖事件发生频率均有显著影响.此外, AMOC也可以通过对行星尺度环流的动力调整影响区域和全球地表气温异常.多年代际AMOC变率与大西洋多...     

考虑真实能量来源的大洋垂直湍流混合系数气候数据集

利用ERA Interim再分析风和海浪资料、SODA三维温盐流资料、WOA09海洋温盐客观分析资料计算了风和潮汐引起的大洋垂直湍流混合系数,并从外部风能量输入、混合引起的重力位能改变、人为限定混合系数上限、不考虑外部能量输入的K理论参数化方案角度分别讨论了大洋垂直混合的能量守恒问题.研究表明:平均化资料和混合系数限定都会造成显著的能量不守恒,二者可造成高达90%的能量损失及能量输入周期律的显著改变.风在南大洋有巨大的能量输入至海洋上层湍流中,潮汐在深海山脉附近也有巨大的能量输入.本文认为严格考虑外部风和潮汐能量输入对于海洋长期气候模拟是必要的,在目前模式技术水平下恰当的使用本文计算出的混合系数集是一个快速经济的选择.     

历史水文观测数据反演的热带太平洋西边界流多年代变化趋势

大尺度海洋环流是海洋能量再分配的基本物理过程之一,研究西太平洋海洋环流在全球气候变暖背景下的多年代变化趋势对理解和预测未来西太平洋气候变化具有重要意义.本文利用最新发布的世界大洋数据集(WOA18)的年代平均水文观测时间序列,计算了1955～2017年间热带西太平洋北赤道流、北赤道逆流、棉兰老流、源地黑潮和新几内亚沿岸...     

FGOALS－g快速耦合模式模拟的北太平洋年代际变率

本文分析了由中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室（LASG/IAP）最新发展的FGOALS_－g快速耦合模式300 a积分模拟结果,通过与多种观测资料的对比分析,讨论了北太平洋年代际变率的时空结构、主要年代际模态的演变特征以及与ENSO的联系等研究内容. 结果表明：该模式能成功模拟出北太平洋年代际变率的主要空间分布特征;模拟的年代际模态具有多时间尺度性,其中最显著的是周期约为10～20 a左右的准20年振荡模态,该模态上层海洋热容量异常的演变过程主要表现为大致沿副热带海洋涡旋做海盆尺度顺时针旋转的特征,相应的大气异常不仅与阿留申低压的变异有关,而且与太平洋-北美PNA）遥相关型以及上游的欧亚大气环流异常有密切关系;模拟的北太平洋年代际变率对年际ENSO循环的发生频率和强度有明显的调制作用. 但模拟的KOE区和阿拉斯加湾SST异常振幅比观测偏强,这与模式海冰偏多、高纬度SST偏冷的误差有关.     

Relationship between sea surface salinity and ocean circulation and climate change

Based on Argo sea surface salinity(SSS) and the related precipitation(P), evaporation(E), and sea surface height data sets, the climatological annual mean and low-frequency variability in SSS in the global ocean and their relationship with ocean circulation and climate change were analyzed. Meanwhile, together with previous studies, a brief retrospect and prospect of seawater salinity were given in this work. Freshwater flux(E-P) dominated the mean pattern of SSS, while the dynamics of ocean circulation modulated the spatial structure and low-frequency variability in SSS in most regions. Under global warming, the trend in SSS indicated the intensification of the global hydrological cycle, and featured a decreasing trend at low and high latitudes and an increasing trend in subtropical regions. In the most recent two decades, global warming has slowed down, which is called the"global warming hiatus". The trend in SSS during this phase, which was different to that under global warming, mainly indicated the response of the ocean surface to the decadal and multi-decadal variability in the climate system, referring to the intensification of the Walker Circulation. The significant contrast of SSS trends between the western Pacific and the southeastern Indian Ocean suggested the importance of oceanic dynamics in the cross-basin interaction in recent decades. Ocean Rossby waves and the Indonesian Throughflow contributed to the freshening trend in SSS in the southeastern Indian Ocean, while the increasing trend in the southeastern Pacific and the decreasing trend in the northern Atlantic implied a long-term linear trend under global warming. In the future, higher resolution SSS data observed by satellites, together with Argo observations, will help to extend our knowledge on the dynamics of mesoscale eddies, regional oceanography, and climate change.     

Long-term ocean simulations in FESOM: evaluation and application in studying the impact of Greenland Ice Sheet melting

The Finite Element Sea-ice Ocean Model (FESOM) is formulated on unstructured meshes and offers geometrical flexibility which is difficult to achieve on traditional structured grids. In this work, the performance of FESOM in the North Atlantic and Arctic Ocean on large time scales is evaluated in a hindcast experiment. A water-hosing experiment is also conducted to study the model sensitivity to increased freshwater input from Greenland Ice Sheet (GrIS) melting in a 0.1-Sv discharge rate scenario. The variability of the Atlantic Meridional Overturning Circulation (AMOC) in the hindcast experiment can be explained by the variability of the thermohaline forcing over deep convection sites. The model also reproduces realistic freshwater content variability and sea ice extent in the Arctic Ocean. The anomalous freshwater in the water-hosing experiment leads to significant changes in the ocean circulation and local dynamical sea level (DSL). The most pronounced DSL rise is in the northwest North Atlantic as shown in previous studies, and also in the Arctic Ocean. The released GrIS freshwater mainly remains in the North Atlantic, Arctic Ocean and the west South Atlantic after 120 model years. The pattern of ocean freshening is similar to that of the GrIS water distribution, but changes in ocean circulation also contribute to the ocean salinity change. The changes in Arctic and sub-Arctic sea level modify exchanges between the Arctic Ocean and subpolar seas, and hence the role of the Arctic Ocean in the global climate. Not only the strength of the AMOC, but also the strength of its decadal variability is notably reduced by the anomalous freshwater input. A comparison of FESOM with results from previous studies shows that FESOM can simulate past ocean state and the impact of increased GrIS melting well.     

Long-term ocean simulations in FESOM: evaluation and application in studying the impact of Greenland Ice Sheet melting

The Finite Element Sea-ice Ocean Model (FESOM) is formulated on unstructured meshes and offers geometrical flexibility which is difficult to achieve on traditional structured grids. In this work, the performance of FESOM in the North Atlantic and Arctic Ocean on large time scales is evaluated in a hindcast experiment. A water-hosing experiment is also conducted to study the model sensitivity to increased freshwater input from Greenland Ice Sheet (GrIS) melting in a 0.1-Sv discharge rate scenario. The variability of the Atlantic Meridional Overturning Circulation (AMOC) in the hindcast experiment can be explained by the variability of the thermohaline forcing over deep convection sites. The model also reproduces realistic freshwater content variability and sea ice extent in the Arctic Ocean. The anomalous freshwater in the water-hosing experiment leads to significant changes in the ocean circulation and local dynamical sea level (DSL). The most pronounced DSL rise is in the northwest North Atlantic as shown in previous studies, and also in the Arctic Ocean. The released GrIS freshwater mainly remains in the North Atlantic, Arctic Ocean and the west South Atlantic after 120 model years. The pattern of ocean freshening is similar to that of the GrIS water distribution, but changes in ocean circulation also contribute to the ocean salinity change. The changes in Arctic and sub-Arctic sea level modify exchanges between the Arctic Ocean and subpolar seas, and hence the role of the Arctic Ocean in the global climate. Not only the strength of the AMOC, but also the strength of its decadal variability is notably reduced by the anomalous freshwater input. A comparison of FESOM with results from previous studies shows that FESOM can simulate past ocean state and the impact of increased GrIS melting well.

     

The roles of surface heat flux and ocean heat transport convergence in determining Atlantic Ocean temperature variability

The temperature variability of the Atlantic Ocean is investigated using an eddy-permitting (1/4°) global ocean model (ORCA-025) forced with historical surface meteorological fields from 1958 to 2001. The simulation of volume-averaged temperature and the vertical structure of the zonally averaged temperature trends are compared with those from observations. In regions with a high number of observations, in particular above a depth of 500 m and between 22° N and 65° N, the model simulation and the dataset are in good agreement. The relative contribution of variability in ocean heat transport (OHT) convergence and net surface heat flux to changes in ocean heat content is investigated with a focus on three regions: the subpolar and subtropical gyres and the tropics. The surface heat flux plays a relatively minor role in year-to-year changes in the subpolar and subtropical regions, but in the tropical North Atlantic, its role is of similar significance to the ocean heat transport convergence. The strongest signal during the study period is a cooling of the subpolar gyre between 1970 and 1990, which subsequently reversed as the mid-latitude OHT convergence transitioned from an anomalously weak to an anomalously strong state. We also explore whether model OHT anomalies can be linked to surface flux anomalies through a Hovmöller analysis of the Atlantic sector. At low latitudes, increased ocean heat gain coincides with anomalously strong northward transport, whereas at mid-high latitudes, reduced ocean heat loss is associated with anomalously weak heat transport.     

Impact of GODAE products on nested HYCOM simulations of the West Florida Shelf

Nested non-assimilative simulations of the West Florida Shelf for 2004–2005 are used to quantify the impact of initial and boundary conditions provided by Global Ocean Data Assimilation Experiment ocean products. Simulations are nested within an optimum interpolation hindcast of the Atlantic Ocean, the initial test of the US Navy Coupled Ocean Data Assimilation system for the Gulf of Mexico, and a global ocean hindcast that used the latter assimilation system. These simulations are compared to one that is nested in a non-assimilative Gulf of Mexico model to document the importance of assimilation in the outer model. Simulations are evaluated by comparing model results to moored Acoustic Doppler Current Profiler measurements and moored sea surface temperature time series. The choice of outer model has little influence on simulated velocity fluctuations over the inner and middle shelf where fluctuations are dominated by the deterministic wind-driven response. Improvement is documented in the representation of alongshore flow variability over the outer shelf, driven in part by the intrusion of the Loop Current and associated cyclones at the shelf edge near the Dry Tortugas. This improvement was realized in the simulation nested in the global ocean hindcast, the only outer model choice that contained a realistic representation of Loop Current transport associated with basin-scale wind-driven gyre circulation and the Atlantic Meridional Overturning Circulation. For temperature, the non-assimilative outer model had a cold bias in the upper ocean that was substantially corrected in the data-assimilative outer models, leading to improved temperature representation in the simulations nested in the assimilative outer models.     

Numerical simulation of ice-ocean variability in the Barents Sea region

A dynamic–thermodynamic sea ice model has been coupled to a three-dimensional ocean general circulation model for the purpose of conducting ocean climate dynamical downscaling experiments for the Barents Sea region. To assess model performance and suitability for such an application, the coupled model has been used to conduct a hindcast for the period 1990–2002. A comparison with available observations shows that the model successfully tracks seasonal and inter-annual variability in the ocean temperature field and that the simulated horizontal and vertical distribution of temperature are in good agreement with observations. The model results follow the seasonal and inter-annual variability in sea ice cover in the region, with the exception that the model results show too much ice melting in the northern Barents Sea during summer. The spatial distribution of the winter simulated sea ice cover is in close agreement with observations. Modelled temperatures and ice concentrations in the central Barents Sea are biased too high and too low, respectively. The probable cause is too high inflow of Atlantic Water into the Barents. The seasonal and inter-annual fluctuations in temperature and sea ice cover in the central Barents are, however, in excellent agreement with observations. Salt release during the freezing process in the numerical simulation exhibits considerable inter-annual variability and tends to vary in an opposite manner to the net inflow volume flux at the western entrance of the Barents Sea. Overall, the model produces realistic ice-ocean seasonal and inter-annual variability and should prove to be a useful tool for dynamical downscaling applications.     

South Atlantic circulation in a world ocean model

The circulation in the South Atlantic Ocean has been simulated within a global ocean general circulation model. Preliminary analysis of the modelled ocean circulation in the region indicates a rather close agreement of the simulated upper ocean flows with conventional notions of the large-scale geostrophic currents in the region. The modelled South Atlantic Ocean witnesses the return flow and export of North Atlantic Deep Water (NADW) at its northern boundary, the inflow of a rather barotropic Antarctic Circumpolar Current (ACC) through the Drake Passage, and the inflow of warm saline Agulhas water around the Cape of Good Hope. The Agulhas leakage amounts to 8.7 Sv, within recent estimates of the mass transport shed westward at the Agulhas retroflection. Topographic steering of the ACC dominates the structure of flow in the circumpolar ocean. The Benguela Current is seen to be fed by a mixture of saline Indian Ocean water (originating from the Agulhas Current) and fresher Subantarctic surface water (originating in the ACC). The Benguela Current is seen to modify its flow and fate with depth; near the surface it flows north-westwards bifurcating most of its transport northward into the North Atlantic Ocean (for ultimate replacement of North Atlantic surface waters lost to the NADW conveyor). Deeper in the water column, more of the Benguela Current is destined to return with the Brazil Current, though northward flows are still generated where the Benguela Current extension encounters the coast of South America. At intermediate levels, these northward currents trace the flow of Antarctic Intermediate Water (AAIW) equatorward, though even more AAIW is seen to recirculate poleward in the subtropical gyre. In spite of the model’s rather coarse resolution, some subtle features of the Brazil-Malvinas Confluence are simulated rather well, including the latitude at which the two currents meet. Conceptual diagrams of the recirculation and interocean exchange of thermocline, intermediate and deep waters are constructed from an analysis of flows bound between isothermal and isobaric surfaces. This analysis shows how the return path of NADW is partitioned between a cold water route through the Drake Passage (6.5 Sv), a warm water route involving the Agulhas Current sheeding thermocline water westward (2.5 Sv), and a recirculation of intermediate water originating in the Indian Ocean (1.6 Sv).     

Ross ice shelf cavity circulation,residence time,and melting: Results from a model of oceanic chlorofluorocarbons

Despite their harmful effects in the upper atmosphere, anthropogenic chlorofluorocarbons dissolved in seawater are extremely useful for studying ocean circulation and ventilation, particularly in remote locations. Because they behave as a passive tracer in seawater, and their atmospheric concentrations are well-mixed, well-known, and have changed over time, they are ideal for gaining insight into the oceanographic characteristics of the isolated cavities found under Antarctic ice shelves, where direct observations are difficult to obtain. Here we present results from a modeling study of air–sea chlorofluorocarbon exchange and ocean circulation in the Ross Sea, Antarctica. We compare our model estimates of oceanic CFC-12 concentrations along an ice shelf edge transect to field data collected during three cruises spanning 16 yr. Our model produces chlorofluorocarbon concentrations that are quite similar to those measured in the field, both in magnitude and distribution, showing high values near the surface, decreasing with depth, and increasing over time. After validating modeled circulation and air–sea gas exchange through comparison of modeled temperature, salinity, and chlorofluorocarbons with field data, we estimate that the residence time of water in the Ross Ice Shelf cavity is approximately 2.2 yr and that basal melt rates for the ice shelf average 10 cm yr⁻¹. The model predicts a seasonal signature to basal melting, with highest melt rates in the spring and also the fall.     

Regional patterns of observed sea level change: insights from a 1/4° global ocean/sea-ice hindcast

A global eddy-admitting ocean/sea-ice simulation driven over 1958–2004 by daily atmospheric forcing is used to evaluate spatial patterns of sea level change between 1993 and 2001. In the present study, no data assimilation is performed. The model is based on the Nucleus for European Models of the Ocean code at the 1/4° resolution, and the simulation was performed without data assimilation by the DRAKKAR project. We show that this simulation correctly reproduces the observed regional sea level trend patterns computed using satellite altimetry data over 1993–2001. Generally, we find that regional sea level change is best simulated in the tropical band and northern oceans, whereas the Southern Ocean is poorly simulated. We examine the respective contributions of steric and bottom pressure changes to the total regional sea level changes. For the steric component, we analyze separately the contributions of temperature and salinity changes as well as upper and lower ocean contributions. Generally, the model results show that most regional sea level changes arise from temperature changes in the upper 750 m of the ocean. However, contributions of salinity changes and deep steric changes can be locally important. We also propose a map of ocean bottom pressure changes. Finally, we assess the robustness of such a model by comparing this simulation with a second simulation performed by MERCATOR-Ocean based on the same core model, but differing by its short length of integration (1992–2001) and its surface forcing data set. The long simulation presents better performance over 1993–2001 than the short simulation, especially in the Southern Ocean where a long adjustment time seems to be needed. In memory of my little brother Jean-Eudes, whose thirst for science filled out the rich discussions we had about my investigations and his job as user-service provider for MERCATOR-Ocean.