Simulation of meridional overturning in the upper layer of the South China Sea with an idealized bottom topography

The South China Sea (SCS) is the largest marginal sea in the northwestern Pacific. Several important water gateways exist around the SCS, such as Luzon Strait, Taiwan Strait, etc., that lie in the northeastern SCS, and others like Kalimantan Strait, Balawan Strait, etc., lie in the south. Luzon Strait connects the SCS and Pacific via the Philippine Sea, and its water exchange has significant impact on the interior circulations of the SCS[1]. Since direct observation data about the …     

Simulation or meridional overturning in the upper layer of the South China Sea with an idealized bottom topography

The large-scale upper circulations and meridional overturning in the upper layer of the South China Sea (SCS) with idealized bottom topography in winter and summer are investigated. Simulations with the GFDL general circulation model are carried out under the conditions of open or enclosed boundary regarding transport in the Luzon Strait. The intrusion area of Kuroshio, its impact on the meridional overturning in the upper layer of the SCS and seasonal characteristic of this impact are explored, respectively. The model is forced by climatological wind stress and relaxed to monthly mean climatological temperature and salinity. The resultant meridional overturning is non-enclosed, wit htransporting from north to south in the surface and returning to north at the depth of about 500 m in winter, about 200 m in summer, with amplitudes of 10^5 m^3/s. It shows the transporting path of intermediate water of the SCS and offers an idealized reference for further study on dynamics of wind-driven and thermohaline circulation of the SCS.     

Decline of the marine ecosystem caused by a reduction in the Atlantic overturning circulation

Reorganizations of the Atlantic meridional overturning circulation were associated with large and abrupt climatic changes in the North Atlantic region during the last glacial period. Projections with climate models suggest that similar reorganizations may also occur in response to anthropogenic global warming. Here I use ensemble simulations with a coupled climate-ecosystem model of intermediate complexity to investigate the possible consequences of such disturbances to the marine ecosystem. In the simulations, a disruption of the Atlantic meridional overturning circulation leads to a collapse of the North Atlantic plankton stocks to less than half of their initial biomass, owing to rapid shoaling of winter mixed layers and their associated separation from the deep ocean nutrient reservoir. Globally integrated export production declines by more than 20 per cent owing to reduced upwelling of nutrient-rich deep water and gradual depletion of upper ocean nutrient concentrations. These model results are consistent with the available high-resolution palaeorecord, and suggest that global ocean productivity is sensitive to changes in the Atlantic meridional overturning circulation.     

全球地形影响大气和海洋经圈环流的耦合模式研究

利用气候系统模式(CESM1.0)研究陆地地形改变对大气?海洋经圈环流的影响。模式首先给出真实海陆分布及陆地地形情况下的大气?海洋气候态, 然后给出平板陆地情况下(陆地海拔均匀10 m)的气候态。与真实世界相比, 平板陆地情形下大气?海洋经圈环流发生重大改变: 首先, 年平均大气对流中心南移到赤道附近, 使得大气哈德雷环流相对于赤道对称; 其次, 海洋的经向翻转流变强, 大西洋经向翻转流完全消失, 取而代之的是在太平洋出现强大的经向翻转流及热盐环流。在平板陆地情形下, 北半球中高纬度大气抬升减弱, 向北的大气热量输送减少, 北半球温度降低, 大气对流中心因而向赤道迁移; 同时, 海洋向极地的热量输送也减弱, 中高纬度海洋变冷, 北太平洋海水密度增加很多, 北大西洋海水密度降低, 导致海洋经向翻转流从大西洋转移到太平洋。     

Break-up of the Atlantic deep western boundary current into eddies at 8 degrees S

The existence in the ocean of deep western boundary currents, which connect the high-latitude regions where deep water is formed with upwelling regions as part of the global ocean circulation, was postulated more than 40 years ago. These ocean currents have been found adjacent to the continental slopes of all ocean basins, and have core depths between 1,500 and 4,000 m. In the Atlantic Ocean, the deep western boundary current is estimated to carry (10-40) x 10(6) m3 s(-1) of water, transporting North Atlantic Deep Water--from the overflow regions between Greenland and Scotland and from the Labrador Sea--into the South Atlantic and the Antarctic circumpolar current. Here we present direct velocity and water mass observations obtained in the period 2000 to 2003, as well as results from a numerical ocean circulation model, showing that the Atlantic deep western boundary current breaks up at 8 degrees S. Southward of this latitude, the transport of North Atlantic Deep Water into the South Atlantic Ocean is accomplished by migrating eddies, rather than by a continuous flow. Our model simulation indicates that the deep western boundary current breaks up into eddies at the present intensity of meridional overturning circulation. For weaker overturning, continuation as a stable, laminar boundary flow seems possible.     

Cenozoic stratigraphy of Taiwan: Window into rifting, stratigraphy and paleoceanography of South China Sea

Shallow marine sequences of the northern South China Sea (SCS) are uplifted and exposed by plate convergence in the Taiwan mountain belt. These deposits provide detailed geological information about the rifting event, stratigraphy, sedimentology, paleoclimate and paleoceanography of the shallow SCS to compare with what are recorded in the ODP 1148 deep-sea core. Seismic surveys and marine micropalentological studies show that Eocene sequences in the offshore Taiwan Strait and onland Taiwan mountain belt are all deposited in rifting basins and are covered unconformably by the Late Oligocene-Neogene post-rifting strata. Between syn-rifting and post-rifting sequences, there is a regional break-up unconformity throughout the island. Early Oligocene and Late Eocene strata are missing along the break-up unconformity equivalent to the T7 unconformity in the Pearl River Mouth Basin off south China. This may suggest that the SCS oceanic crust could have initiated between 33 and 39 Ma. Neither obvious stratigraphic gap nor slumping features are found in the Oligocene-Miocene transition interval of Taiwan. This observation highly contrasts with what has been documented from the ODP 1148 deep-sea core. This suggests that the stratigraphic gap and slumping features could only be recorded in the SCS deep sea region, but not in the shallow shelf near Taiwan. Compared to the Middle Miocene paleoceanographic re-organization events in the SCS deep sea, the geological history of the Taiwan shallow sequence shows changes of in sedimentation and faunal composition. Due to the Antarctic glacial expansion at~14 Ma, Middle to late Miocene strata of the Western Foothills show progressive regression sedimentations associated with a decrease of benthic foraminif-eral abundance and a sharp faunal turnover event. Many Early-Middle Miocene endemic benthic foraminifers were extinct in 14-13 Ma and new benthic foraminifers of the Kuroshio Current fauna appeared from 10.2 Ma, comparable with new occurrence of Modern benthic foraminifers at 9 Ma in the Java Sea area. This reveals that the Western Boundary Kuroshio Current in the North Pacific could initiate from 10-9 Ma due to closures of the Indo-Pacific seaways by convergent tectonics between the Australian Continent and the Indonesian Arc in 12-8 Ma. Subduction of the SCS oceanic lithosphere since the Middle Miocene resulted in formation of the Hengchun Ridge accretionary prism and the North Luzon Arc. Occurrence of these two bathymetric highs ( 2400 m) since the Middle Miocene and closures of the inter-arc passages in the North Luzon arc in the last 3.5 Ma would control the water exchanges between the West Pacific and the deep SCS. Accordingly, the tectonic evolution in the Central Range-Hengchun Peninsula accretionary prism and the arc-forearc Coastal Range not only control directly the route for water exchanges between the West Pacific and the SCS, but also indirectly shows a great influence on the geochemistry of deep SCS waters. The latter is best shown by much negative carbon isotope values of benthic foraminifers in the ODP 1148 deep-sea core than the West Pacific records in the last 14 Ma.     

Rapid subtropical North Atlantic salinity oscillations across Dansgaard-Oeschger cycles

Geochemical and sedimentological evidence suggest that the rapid climate warming oscillations of the last ice age, the Dansgaard-Oeschger cycles, were coupled to fluctuations in North Atlantic meridional overturning circulation through its regulation of poleward heat flux. The balance between cold meltwater from the north and warm, salty subtropical gyre waters from the south influenced the strength and location of North Atlantic overturning circulation during this period of highly variable climate. Here we investigate how rapid reorganizations of the ocean-atmosphere system across these cycles are linked to salinity changes in the subtropical North Atlantic gyre. We combine Mg/Ca palaeothermometry and oxygen isotope ratio measurements on planktonic foraminifera across four Dansgaard-Oeschger cycles (spanning 45.9-59.2 kyr ago) to generate a seawater salinity proxy record from a subtropical gyre deep-sea sediment core. We show that North Atlantic gyre surface salinities oscillated rapidly between saltier stadial conditions and fresher interstadials, covarying with inferred shifts in the Tropical Atlantic hydrologic cycle and North Atlantic overturning circulation. These salinity oscillations suggest a reduction in precipitation into the North Atlantic and/or reduced export of deep salty thermohaline waters during stadials. We hypothesize that increased stadial salinities preconditioned the North Atlantic Ocean for a rapid return to deep overturning circulation and high-latitude warming by contributing to increased North Atlantic surface-water density on interstadial transitions.     

Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes

The Atlantic meridional overturning circulation is widely believed to affect climate. Changes in ocean circulation have been inferred from records of the deep water chemical composition derived from sedimentary nutrient proxies, but their impact on climate is difficult to assess because such reconstructions provide insufficient constraints on the rate of overturning. Here we report measurements of 231Pa/230Th, a kinematic proxy for the meridional overturning circulation, in a sediment core from the subtropical North Atlantic Ocean. We find that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500 yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago. Following these cold events, the 231Pa/230Th record indicates that rapid accelerations of the meridional overturning circulation were concurrent with the two strongest regional warming events during deglaciation. These results confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate changes.     

Short-circuiting of the overturning circulation in the Antarctic Circumpolar Current

The oceanic overturning circulation has a central role in the Earth's climate system and in biogeochemical cycling, as it transports heat, carbon and nutrients around the globe and regulates their storage in the deep ocean. Mixing processes in the Antarctic Circumpolar Current are key to this circulation, because they control the rate at which water sinking at high latitudes returns to the surface in the Southern Ocean. Yet estimates of the rates of these processes and of the upwelling that they induce are poorly constrained by observations. Here we take advantage of a natural tracer-release experiment-an injection of mantle helium from hydrothermal vents into the Circumpolar Current near Drake Passage-to measure the rates of mixing and upwelling in the current's intermediate layers over a sector that spans nearly one-tenth of its circumpolar path. Dispersion of the tracer reveals rapid upwelling along density surfaces and intense mixing across density surfaces, both occurring at rates that are an order of magnitude greater than rates implicit in models of the average Southern Ocean overturning. These findings support the view that deep-water pathways along and across density surfaces intensify and intertwine as the Antarctic Circumpolar Current flows over complex ocean-floor topography, giving rise to a short circuit of the overturning circulation in these regions.     

南海海盆演变与深部海流

 南海在距今34 Ma之前的始新世从陆地变为海洋,古水深不断加深,至距今24 Ma之前的中新世/渐新世之交,由于T60构造运动,南海海盆整体进入深海环境。但是,自中新世以来随着吕宋岛弧向欧亚板块碰撞,南海海盆的半封闭程度在距今10.0、6.5、3.0和1.2 Ma之前加剧,导致南海深部海水只能来自巴士海峡海槛深度2600 m以浅的太平洋。此后,巴士海峡两侧的南海与太平洋深部海水交换,由于全球海平面变化,呈现冰期/间冰期模式。     

Slowing of the Atlantic meridional overturning circulation at 25 degrees N

The Atlantic meridional overturning circulation carries warm upper waters into far-northern latitudes and returns cold deep waters southward across the Equator. Its heat transport makes a substantial contribution to the moderate climate of maritime and continental Europe, and any slowdown in the overturning circulation would have profound implications for climate change. A transatlantic section along latitude 25 degrees N has been used as a baseline for estimating the overturning circulation and associated heat transport. Here we analyse a new 25 degrees N transatlantic section and compare it with four previous sections taken over the past five decades. The comparison suggests that the Atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004. Whereas the northward transport in the Gulf Stream across 25 degrees N has remained nearly constant, the slowing is evident both in a 50 per cent larger southward-moving mid-ocean recirculation of thermocline waters, and also in a 50 per cent decrease in the southward transport of lower North Atlantic Deep Water between 3,000 and 5,000 m in depth. In 2004, more of the northward Gulf Stream flow was recirculating back southward in the thermocline within the subtropical gyre, and less was returning southward at depth.     

2016年物理海洋学热点回眸

 物理海洋学是由海洋观测、理论分析和数值模拟共同推动的科学。得益于海洋观测和数值模拟能力的提高,2016年物理海洋学取得了一系列重要进展。简述了2016年大西洋经向翻转环流变异、西边界流与中尺度涡相互作用、温跃层湍流混合等重要研究成果,回顾了中国在热带西太平洋及邻近海域海洋观测网的建成、新型Argo浮标投入使用等海洋学热点事件。     

吕宋海峡黑潮表层形态EOF模态分析

应用Argos表层漂流浮标资料所指示的浮标轨迹图以及基于卫星高度计资料和三维水动力模型海面高度场数据的经验正交函数(EOF)模态分析,探讨了黑潮在吕宋海峡形变的时空分布特征.结果表明:吕宋海峡黑潮的形态呈现明显的季节变化,其中跨隙形态为其最基本的形态;秋冬两季,部分黑潮水会以流套和分支的形式入侵南海,流套最西可延伸至118°E;春夏季节则儿乎不出现流套或者分支的结构,相反会有一部分南海水汇入黑潮主干.模型数据模态分析结果还再现了冬季黑潮水沿着南海北部陆坡向西入侵的形态.     

Polar ocean stratification in a cold climate

The low-latitude ocean is strongly stratified by the warmth of its surface water. As a result, the great volume of the deep ocean has easiest access to the atmosphere through the polar surface ocean. In the modern polar ocean during the winter, the vertical distribution of temperature promotes overturning, with colder water over warmer, while the salinity distribution typically promotes stratification, with fresher water over saltier. However, the sensitivity of seawater density to temperature is reduced as temperature approaches the freezing point, with potential consequences for global ocean circulation under cold climates. Here we present deep-sea records of biogenic opal accumulation and sedimentary nitrogen isotopic composition from the Subarctic North Pacific Ocean and the Southern Ocean. These records indicate that vertical stratification increased in both northern and southern high latitudes 2.7 million years ago, when Northern Hemisphere glaciation intensified in association with global cooling during the late Pliocene epoch. We propose that the cooling caused this increased stratification by weakening the role of temperature in polar ocean density structure so as to reduce its opposition to the stratifying effect of the vertical salinity distribution. The shift towards stratification in the polar ocean 2.7 million years ago may have increased the quantity of carbon dioxide trapped in the abyss, amplifying the global cooling.     

Deep convective clouds with sustained supercooled liquid water down to -37.5 degrees C

In cirrus and orographic wave clouds, highly supercooled water has been observed in small quantities (less than 0.15 g m(-3)). This high degree of supercooling was attributed to the small droplet size and the lack of ice nuclei at the heights of these clouds. For deep convective clouds, which have much larger droplets near their tops and which take in aerosols from near the ground, no such measurements have hitherto been reported. However, satellite data suggest that highly supercooled water (down to -38 degrees C) frequently occurs in vigorous continental convective storms. Here we report in situ measurements in deep convective clouds from an aircraft, showing that most of the condensed water remains liquid down to -37.5 degrees C. The droplets reach a median volume diameter of 17 microm and amount to 1.8 gm(-3), one order of magnitude more than previously reported. At slightly colder temperatures only ice was found, suggesting homogeneous freezing. Because of the poor knowledge of mixed-phase cloud processes, the simulation of clouds using numerical models is difficult at present. Our observations will help to understand these cloud processes, such as rainfall, hail, and cloud electrification, together with their implications for the climate system.     

光照和盐度对8种底栖硅藻生长及其生理生化成分的影响

研究了不同光照强度和盐度条件对8种海洋底栖硅藻生长及其生理生化成分的影响．结果表明：8种底栖硅藻生长最适光照强度和盐度不完全相同,其范围分别为1500—5500lx和25-40;硅藻的理化成分受光照和盐度条件影响也较大,其含量均呈现低-高-低的变化趋势．光照低于1500lx、盐度低于25或光照、盐度高于最适值时均不利于理化成分的积累,但高于最适光强时利于胞内外总糖的产生;光强高于5500lx,盐度高于40时,总脂肪含量几乎不增加甚至急剧下降．     

Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation

The covariation of carbon dioxide (CO(2)) concentration and temperature in Antarctic ice-core records suggests a close link between CO(2) and climate during the Pleistocene ice ages. The role and relative importance of CO(2) in producing these climate changes remains unclear, however, in part because the ice-core deuterium record reflects local rather than global temperature. Here we construct a record of global surface temperature from 80 proxy records and show that temperature is correlated with and generally lags CO(2) during the last (that is, the most recent) deglaciation. Differences between the respective temperature changes of the Northern Hemisphere and Southern Hemisphere parallel variations in the strength of the Atlantic meridional overturning circulation recorded in marine sediments. These observations, together with transient global climate model simulations, support the conclusion that an antiphased hemispheric temperature response to ocean circulation changes superimposed on globally in-phase warming driven by increasing CO(2) concentrations is an explanation for much of the temperature change at the end of the most recent ice age.     

Temporal and spatial variabilities of sea surface heights in the northeastern South China Sea

Analyses of sea surface height anomalies (SSHA) from Jan. 1993 to Dec. 2001 measured by TOPEX/Poseidon (T/P) satellite altimetry indicate that there are two energy highs of SSHA fluctuations on both sides of the Luzon Strait. The highs centered at (19.5°N, 119.5°E) and (22.0°N, 124.0°E) respectively, are separated by a low energy band in the Strait. Empirical Orthogonal Function (EOF) analysis on the "Far Field" suggests that fluctuations of these two areas are mutually independent. EOF analysis on the "Near Field" shows that SSHA oscillation in the northeastern South China Sea (SCS) is dominated by seasonal variations accompanied with significant intra-seasonal and inter-annual fluctuations. Among which, EOFI and EOF2 reflect 66.7% of the total variance. They are both seasonal but showing three months difference in phase. The basic pattern of EOFI appears to be the seasonal SSHA oscillation centered at (18.0°N, 119.0°E) northwest off Luzon Island, which shows maximums (minimums) in August/Septemb     

四筒基平台拖航试验分析-水深影响研究

为了研究不同水深下筒型基础拖航中的运动响应,以海军某筒型基础平台为研究对象,在拖航干舷、筒的位置以及波浪条件一定的前提下,通过模型试验测定筒型基础平台在不同水深下随浪和顶浪拖航时在横荡、纵荡和垂荡方向上的加速度、筒内气压力和筒底水压力以及拖航中的拖缆力．试验结果表明,在筒型基础由浅水进入深水拖航的过程中,平台的稳定性能和操纵性能下降,且出现倾覆现象,必须降低船速和设置护航船防止漂移,且15m水深时该平台的航速不能超过1．5节（0．771m／s）,20m水深时航速不能超过1．0节（0．514m／s）．     

Multi-spatial variability modes of the Atlantic Meridional Overturning Circulation

The multi-spatial variability modes of the Atlantic Meridional Overturning Circulation (MOC) are identified in the natural coupled simulation of two climate models, the MOC either oscillates at decadal scales with strong cross- equatorial flow or fluctuates locally at interannual scales with weaker cross-equatorial flow. Former studies mainly emphasize the paleo-environmental and paleo-climatic impacts of the meridional overturning states transition; this analysis indicates the existence of the multi-spatial variability modes of the MOC at interannual to decadal scales. Further analysis indicates that the conventionally used MOC index, which is defined as the maximum zonal mean meridional stream-function of the North Atlantic, cannot properly describe the multi-spatial variability characteristics of the MOC.