北极平流层冬季早期变暖的GCM模拟研究

观测表明北极平流层自20世纪70年代末以来在冬季早期 (11～12月) 存在变暖的趋势。为了验证该趋势是否是由于海面温度 (SST) 升高造成的, 我们使用观测的全球SST强迫一个全球大气环流模式 (AGCM)。集合模拟的结果表明, 在SST强迫下, 北极平流层呈现统计显著的变暖趋势, 极地对流层也有相对较弱的变暖趋势, 但统计显著性较低。通过对模拟的位势高度进行经验正交函数 (EOF) 分析, 我们发现北半球位势高度第一模态 (EOF1) 的空间结构非常类似于北极涛动 (AO) 或北半球环状模 (NAM), 其平流层主分量时间序列在冬季早期呈现统计显著的负趋势。与负的AO趋势相对应的是, 对流层高纬度和平流层中高纬度波动增强, 说明极区变暖是由于波动增强产生的极区绝热加热增强造成的。另外, 模拟的结果还表明北极平流层不仅在冬季早期出现变暖的趋势, 在冬季晚期 (2～3月) 北极平流层低层也出现弱的变暖趋势。SST强迫导致北极平流层冬季变暖不利于异相臭氧化学反应的发生, 这对极地平流层臭氧恢复有着重要意义。     

利用一个海气耦合模式对台风Krovanh的模拟

采用中尺度大气模式MM5和区域海洋模式POM构造了中尺度海气耦合模式, 模拟了Krovanh (0312) 台风过程中台风－海洋相互作用, 分析了台风引起的海面降温影响台风强度的机制和海洋对台风响应的特征。试验结果显示: 考虑台风引起的海面降温使台风强度模拟有了较大改进, 模拟的台风中心气压和近中心最大风速均与实况较符合。POM模拟的海表面温度与TRMM/TMI观测的海表面温度也较为一致, 台风Krovanh在其路径右侧95 km处引起较大的海面降温, 最大降温幅度达5.8℃。与海表面温度降低相对应的是混合层深度的增加, 较大的海面降温对应较大的混合层加深, 表明大风夹卷在海表面温度的降低中起主要作用。分析表明, 台风引起的海面降温降低海洋向大气输送的潜热通量和感热通量, 特别是在台风内核区, 平均总热通量减少了32.1%。热通量的减少使得湿静力能及湿静力能径向梯度减小, 削弱了台风强度。     

NUMERICAL MODELING STUDY OF EFFECTS OF EASTERN PACIFIC WARM POOL ON ENSO

In this study, sensitivity experiments were conducted with the Zebiak-Cane ocean-atmosphere coupled model forced by the wind stress anomaly from the U.S. National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data to study the impacts of eastern Pacific warm pool on the formation and development of ENSO events. The effects of climatological mean sea surface temperature of the warm pool on forecast skill during the ENSO events of 1982–1999 are more considerable that those of climatological mean meridional winds and ocean currents. The forecast skill for the 1997/1998 El Ni?o event is characterized by sensitivity to climatological mean sea surface temperature and anomalies of northerly winds and currents. The forecast skill is found insensitive to climatological mean northerly meridional winds and currents.     

Drought reconstruction in the Qilian Mountains over the last two centuries and its implications for large-scale moisture patterns

We present a composite tree-ring chronology from two sites of Qilian Juniper(Sabina przewalskii)in the northwestern Qilian Mountains(QM),Northwestern China.Precipitation in June was found to be the main limiting factor for tree-growth.The tree rings are also significantly and positively correlated with June precipitation over large areas of the northern Tibetan Plateau(TP).The authors thus consider that the tree- ring based drought reconstruction from 1803–2006 is representative of a large area drought hist...     

The ENSO’s Effect on Eastern China Rainfall in the Following Early Summer

ENSO’s effect on the rainfall in eastern China in the following early summer is investigated by using station precipitation data and the ERA-40 reanalysis data from 1958 to 2002. In June, after the El Nino peak, the precipitation is significantly enhanced in the Yangtze River valley while suppressed in the Huaihe River-Yellow River valleys. This relationship between ENSO and the rainfall in eastern China is established possibly through two teleconnections: One is related to the western North Pacific (WNP) ...     

Latitudinal and downcore (0–750 ka) changes in n-alkane chain lengths in the eastern equatorial Pacific

The n-alkane C₃₁/(C₂₉ + C₃₁) ratios from surface sediments in the eastern equatorial Pacific (EEP) exhibit higher values to the north and lower values to the south across the southern edge (2–4°N) of the Intertropical Convergence Zone (ITCZ). Since plants tend to synthesize longer chain length n-alkanes in response to elevated temperature and/or aridity, the higher C₃₁/(C₂₉ + C₃₁) ratios at northern sites suggest a higher contribution of vegetation under hot and/or dry conditions. This is consistent with the observation that northern sites receive higher levels of plant waxes transported by northeasterly trade winds from northern South America, where hot and dry conditions prevail. Furthermore, from a sediment core covering the past 750 ka (core HY04; 4°N, 95°W) we found that C₃₁/(C₂₉ + C₃₁) ratios exhibit a long-term decrease from MIS (marine oxygen isotope stage) 17 to 13. During this period, the zonal SST (sea-surface temperature) gradient in the equatorial Pacific increased, suggesting an increase in Walker circulation. Such intensified Walker circulation may have enhanced moisture advection from the equatorial Atlantic warm pool to the adjacent northern South America, causing arid regions in northern South America to contract, which may explain long-term decrease in n-alkane chain lengths.     

Biogenic Sediments and Their Paleoceanographic Implication in West Pacific Warm Pool during the Pleistocene

Primary productivity has played an important role in the global carbon cycle during the Quaternary. The average Corg/N ratio was 7.0 from the core MD97-2140 in the West Pacific Warm Pool (WPWP) over the last 1 755 ka, which indicates a main contribution of marine organic carbon to the organic matter in the sediments. Large fluctuations in mass accumulation rates (MARs) of biogenics and lithogenics from this core since the marine isotope stage (MIS) 6 reflected the moderate variability in oceanographic condi...     

The Effect of Regional Ocean-Atmosphere Coupling on the Long-term Variability in the Pacific Ocean

A fully coupled ocean-atmosphere model is applied to highlight the mechanism of the long-term variability (including decadal and longer time scales) in the Pacific Ocean. We are interested in the effect of ocean-atmosphere coupling of different regions during these processes. The control run successfully simulates the Pacific long-term variability, whose leading modes are the Pacific (inter) Decadal Oscillation (PDO) and the North Pacific mode (NPM). Furthermore, three numerical experiments are conducted, s...     

北太平洋次表层海温异常对中国夏季降水影响的可能途径

利用Godas月平均次表层海温资料, 分析了冬、春季和夏季北太平洋次表层海温层际相似性特征, 据此对次表层海温进行分层。在此基础上研究了500 hPa位势高度场、北太平洋次表层海温、中国夏季降水三者之间的时滞相关关系, 发现春季北太平洋次表层海温场是联系前、后期大气环流的关键因素。前期冬季大气环流对春季北太平洋次表层海温场影响最显著, 春季北太平洋次表层海温场又持续影响同期及后期夏季大气环流异常。异常的夏季大气环流与同期表层、次表层海温相互作用, 共同造成夏季长江流域与华北、华南降水出现相反异常的分布型式。     

东亚夏季风环流和雨带的季节内变化

基于常规气象要素资料及变差度方法, 分析了东亚夏季风环流的演变特征, 发现东亚地区在夏季期间存在两次明显的次季节突变, 主要表现为西太平洋副热带高压 (副高) 的两次东退北跳, 第一次是在6月中旬, 第二次是在7月下旬。由于副高与雨带密切相关, 雨带在演进过程中也呈现出两次明显的突跳, 分别对应于江淮流域至日本一带梅雨期以及中国华北和东北雨季的开始。较第一次北跳而言, 副高的第二次北跳更为明显。副高的第一次北跳主要受南海地区对流活动加强的影响, 而第二次北跳则是暖池对流活动与高纬地区环流共同作用的结果。暖池地区向东北方向传播的Rossby波列以及高纬地区东传的Rossby波通过锁相作用使得副高强烈北跳。此外, 副高与其西部边缘凝结潜热的相互作用导致副高发生季节内的低频振荡。 风场变差度的分析表明, 高纬地区对流层中低层环流的调整随着夏季季节进程逐渐减弱, 这与中高纬地区温差的变化有关。而高纬地区高层环流的调整在夏季后半期随着高度的增加却逐渐增强, 这与高层环流从夏到冬的季节变化有关。从风场相似度的变化上还可以看到, 副高第二次北跳后东亚地区呈现出明显不同的环流状态。 南半球环流对于南海及暖池地区对流活动的增强有重要影响。6月中旬, 南海与暖池地区对流活动的增强是由于南海西边界西风加强并向东扩展造成的, 这与马斯克林高压 (马高) 的加强密切相关。而在7月中旬, 澳大利亚高压 (澳高) 的增强使其东北部的越赤道气流加强, 南半球大量冷空气侵入到暖池地区, 加强了暖池地区的不稳定性以及低层的辐合, 从而使暖池地区的对流活动增强。但在夏季前半期, 暖池对流活动也可调制澳高强度与其东北部越赤道气流强弱的关系, 使得二者呈现出相反的变化趋势。南半球冬季期间, 澳高在振荡中减弱, 这与澳洲大陆下垫面温度及上游马高的能量频散有关, 前者影响澳高的变化趋势 (减弱), 而后者影响澳高的低频振荡。