冬季北大西洋涛动对中国春季降水异常的影响

利用中国397个测站降水资料和NCEP/NCAR再分析资料,采用相关分析、合成分析等方法,研究了冬季北大西洋涛动(NAO)对我国春季降水的影响。结果表明,我国春季降水与前期冬季NAO关系密切,冬季NAO偏强(弱)时,我国东部南方地区春季降水偏多(少),北方地区春季降水偏少(多)。冬季NAO信号通过波列形式传播至东亚地区,使得春季东亚副热带急流和温带急流发生变化,冬季NAO偏强(弱)时,春季东亚副热带西风急流增强(减弱),温带急流减弱(增强)。进一步分析表明,冬季NAO异常会引起春季乌拉尔山高压脊和东亚大槽的变化,导致东亚对流层上层的温度发生变化,并由此产生经向温度梯度异常,这可能是NAO影响东亚高空急流的原因之一。春季东亚对流层上层的气温变冷(暖),使得东亚地区30°-40°N区域产生下沉(上升)运动,20°-30°N区域产生上升(下沉)运动,最终导致我国东部南方地区春季降水偏多(少)、北方地区春季降水偏少(多)。     

东亚冬季风的年代际变化及其与全球气候变化的可能联系

对近年来中外关于东亚冬季风(EAWM)年代际变化问题研究进展做了回顾和评述,主要包括以下3个方面内容:(1)东亚冬季风明显受到全球气候变化的影响,从20世纪50年代开始,中国冬季气温经历了一次冷期(从20世纪50年代延续到80年代初中期),一次暖期(从20世纪80年代初中后期延续到21世纪初)和近10-15年(约从1998年开始)出现的气候变暖趋缓期(也称气候变暖停顿期)。(2)东亚冬季风主要表现出强-弱-强3阶段的特征,即从1950年到1986/1987年,明显偏强;从1986/1987年冬季开始,东亚冬季风减弱;约2005年之后,东亚冬季风开始由弱转强。与东亚冬季风的年代际变化特征相对应,东亚冬季大气环流以及中国冬季气温和寒潮都表现出一致的年代际变化。(3)东亚冬季风的年代际变化与大气环流和太平洋海表温度(SST)的区域模态变化密切相关。当北半球环状模/北极涛动(NAM/AO)和太平洋年代际振荡(PDO)处于负(正)位相,东亚冬季风偏强(弱),中国冬季气温偏低(高)。此外,北大西洋年代尺度振荡(AMO)对东亚冬季风也有重要影响,在AMO负位相时,对应东亚冷期(强冬季风),正位相对应暖期(弱冬季风)。因而海洋的年代际变化是造成东亚冬季风气候脉动的主要自然原因,而全球气候变暖对东亚冬季风强度的减弱也有明显影响。     

Surface ozone over the Atlantic ocean

During the past decade (1977–1986) ozone has been measured on board various research vessels between 83° N and 76° S. The latitudinal distribution shows nearly twice as high an ozone concentration in the Northern Hemisphere (30 ppb) as compared to the Southern Hemisphere (15 ppb). North of 70° N the concentration rapidly drops to low values (17 ppb). In the ITCZ we frequently observed a pronounced secondary maximum which indicates that tropospheric-stratospheric exchange may be induced by deep convection. During other occasions, ozone concentrations below 10 ppb were observed in the ITCZ. Secondary maxima are also found in each hemisphere, indicating the latitudes in which stratospheric-tropospheric exchange occurs. One such pronounced maximum is found between 60 and 65° N. A comparable counterpart does not, however, occur in the respective southern latitudes.The observations, although incomplete, allow for conclusions with respect to seasonal cycles, at least in some of the latitudes or for some part of the year. In the tropics differences between continental and maritime yearly cycles are found. In mid to high northern latitudes, a sharp maximum is found which extends to the summer months over continental European sites. In the respective southern latitudes, such a spring maximum is only indicated.Comparison with occasionally concurrent Kr 85 measurements by other groups allows for the conclusion that in some cases photochemically produced ozone in polluted air masses is seen over the Atlantic ocean.     

Petrogenesis of picrites from the Caribbean Plateau and the North Atlantic magmatic province

We studied the petrography, mineralogy and geochemistry of picrites from three different regions: the island of Curaçao which forms part of a Cretaceous oceanic plateau; Iceland, an active hot spot on the mid-Atlantic ridge; and the early Tertiary volcanic margin off the coast of Greenland, which formed during the rifting that created the Atlantic ocean. Using the compositions of olivine phenocrysts and relations between MgO and FeO, Al₂O₃ and Ni, we estimated compositions of parental liquids and the proportion of accumulated olivine in each rock. Picrites from Curaçao formed mainly from a liquid with 12 wt.% MgO and they contain up to 55 wt.% excess olivine in the form of phenocrysts. A small proportion of more forsterite-rich olivine grains are xenocrysts from a more magnesian source. Picrites from Iceland formed from a slightly less magnesian liquid but one with also about 12 wt.% MgO and they contain both olivine and plagioclase in the accumulated assemblage. Picrites from the Greenland volcanic margin formed from a liquid that was distinctly more magnesian, with up to 20 wt.% MgO. In some of these rocks the proportion of accumulated olivine was minimal and in these the whole-rock composition is roughly equivalent to the liquid composition. The picrites from the three areas formed under different conditions and through contrasting melting processes. The Curaçao picrites derive from pooled liquids formed through moderate degrees of melting at moderate depths beneath a relatively old and thick oceanic lithosphere. Iceland picrites, on the other hand, formed through advanced fractional melting of mantle that ascended almost to the base of the crust at the mid-ocean spreading center. An unusual combination of relatively high concentrations of incompatible trace elements and high MgO indicates that the Greenland picrites formed by relatively low degrees of melting at greater depths in the mantle.     

北大西洋年代际振荡（AMO）气候影响的研究评述

北大西洋年代际振荡（theAtlantic Multidecadal Oscillation,AMO）是发生在北大西洋区域空间上具有海盆尺度、时间上具有多十年尺度的海表温度（sea surface temperature,SST）准周期性暖冷异常变化。它具有65～80a周期,振幅为0.4℃。AMO的形成与热盐环流的准周期性振荡有关,它是气候系统的一种自然变率。诸多研究表明,AMO在北大西洋局地气候及全球其他区域气候演变中发挥了重要影响。欧亚大陆的表面气温,美国大陆、巴西东北部、西非以及南亚的降水,北大西洋飓风等都与之密切相关。AMO对东亚季风气候的年代际变化有显著的调制作用,暖位相AMO增强东亚夏季风,减弱冬季风,冷位相则相反。本文总结了这方面的研究进展,讨论了AMO对未来气候预测的意义,认为最近20多年来我国冬季的显著增暖与AMO处于暖位相有关,是人类温室气体强迫与暖位相AMO（自然因子）两种增暖影响相叠加的结果。随着AMO逐渐转入冷位相,我国冬季变暖趋势将放慢,并有望于21世纪20年代中期逆转。     

冬季北大西洋涛动对热带印度洋海表热通量的影响

利用1979—2017年TropFlux海气热通量资料、ERA5再分析资料及HadISST资料,分析了冬季北大西洋涛动(North Atlantic Oscillation, NAO)与同期热带印度洋海气热通量的关系。结果表明,NAO指数与热带印度洋海气净热通量整体上呈负相关,意味着NAO为正位相时,海洋向大气输送热量,其显著区域主要位于热带西印度洋(50°～70°E,10°S～10°N)。净热通量的变化主要依赖于潜热通量和短波辐射的变化;潜热通量和短波辐射在NAO正(负)位相事件期间的贡献率分别为72.96%和61.48%(71.72%和57.06%)。NAO可通过Rossby波列影响印度洋地区局地大气环流,进而影响海气热通量;当NAO为正位相时,波列沿中低纬路径传播至印度洋地区,在阿拉伯海北部对流层高层触发异常反气旋环流。该异常反气旋性环流加强了阿拉伯高压,使得北印度洋偏北风及越赤道气流加强。伴随风速的加强,海面蒸发增强,同时加强的越赤道气流导致热带辐合带强度偏强,深对流加强引起对流层水汽和云量增多,进而引起海表下行短波辐射减少。     

中大西洋营养盐富集实验中浮游植物生长的营养盐限制作用

借助中国首次环球科学考察航次,在中大西洋航段现场对表层海水进行了添加N、P、Si的营养盐富集实验,通过实验过程中水体营养盐浓度、叶绿素a（Chl-a）浓度以及温度等参数的分析,探讨了实验海区浮游植物生长的营养盐限制作用。结果表明,添加N、P、Si都可造成实验水体中Chl-a浓度明显增大,且N对浮游植物生长的限制作用最明显,其次为P,Si的作用最弱。实验过程中水体N／P值的变化同叶绿素a浓度及浮游植物生长速度（R）没有可对比性,N／P值与后两者之间的相关性都差,可以认为水体中N／P值并不能单独限制浮游植物生长。实验水体温度同Chl-a浓度和R值间也缺乏相关性,表明水体温度同样不能控制浮游植物生长。     

CAM3.0模式中北大西洋风暴轴对“三核型”海温异常的响应

为了探究冬季(12~2月)北大西洋地区风暴轴与海温异常的关系,利用伴随AO(Arctic Oscillation)出现的"三核型"海温异常对大气环流模式CAM3.0进行强迫,将模拟结果与NCEP/NCAR再分析资料做对比,发现CAM3.0在该SSTA强迫下可以再现伴随AO异常出现的风暴轴与急流异常,结论可概括为:海温正(负)异常时,北大西洋风暴轴增强(减弱),北美急流出口区向北(向南)摆动;"三核型"海温异常作为外强迫源,可能通过2种途径影响风暴轴的强度:首先通过热力作用改变中纬度低层大气斜压性来直接影响风暴轴,其次通过影响北美急流出口区的南北摆动来间接影响风暴轴;另外,海温正(负)异常可以增强(减弱)瞬变波与基本气流之间的正反馈效应。     

Future changes in internal variability of the Atlantic Meridional Overturning Circulation

The response of the internal variability of the Atlantic Meridional Overturning Circulation (MOC) to enhanced atmospheric greenhouse gas concentrations has been estimated from an ensemble of climate change scenario runs. In the model, enhanced greenhouse forcing results in a weaker and shallower MOC with reduced internal variability. At the same time at 55°N between 0 and 1,000 m the overturning increases as a result of a change in the area of convection. In a warmer world, new regions of deepwater formation form further north due to the poleward retreat of the sea-ice boundary. The dominant pattern of internal MOC-variability consists of a monopole centered around 35°N. Due to anthropogenic warming this monopole shifts poleward. The shift is associated with a stronger relation between MOC-variations and heat flux variations over the subpolar gyre. In old convective sites (Labrador Sea) convection becomes more irregular which leads to enhanced heat flux variability. In new convective sites heat flux variations initially are related to sea-ice variations. When the sea-ice coverage further decreases they become associated with (irregular) deepwater formation. Both processes act to tighten the relation between subpolar surface heat flux variability and MOC-variability, resulting in a poleward shift of the latter.