东亚冬季风的演变特征

文章主要讨论了东亚冬季风和冷涌的演变特征，并与南亚作了对比，发现在东亚地区，冬季风演变主要表现为10月中旬经向环流的突变及9月初、11月中旬和1月末对流层低层温度的3次突变；而在南亚地区，经向环流的变化不如东亚地区明显，而且高层要先于低层变化，对流层低层温度存在2次突变。在整个冬季，东亚地区冷涌的演变过程，主要表现为南海地区冷涌在12月份出现最高频率，而西太平洋冷涌在1月份出现最高频率；南亚地区冷涌在12月份出现最高频率，但远小于东亚地区且衰减速度很快。另一个不同点是东亚地区的冷涌强度是往上衰减的，而南亚地区的冷涌强度则是往上增强的。这说明东亚冬季风和南亚冬季风的性质有较大的区别。     

中世纪温暖期气候变化的花粉化石记录

麦里泥炭剖面高分辨率花粉化石记录表明,东北科尔沁沙地在相当于中世纪温暖期阶段夏季降水量曾显著增加,主要表现为沙丘上的乔木和草本植物均获得了繁茂的生长,花粉沉积速率明显上升。作者推测,在中世纪温暖期中,欧亚大陆作为一个整体夏季温度可能暖于今天,这可能通过增强夏季风环流使中国东北夏季雨量增多,为沙丘植物茂盛生长提供了有利条件。     

东亚夏季风的年际变化及其对环流和降水的影响

Using NCEP/NCAR reanalysis geopotential height (GHT) and wind at 850 hPa, GHT at 500 hPa, precipitation rate, sea level pressure (SLP) and precipitation observations from more than 600 stations nationwide in June–August from 1951 to 2006, and focusing on the East Asia-West Pacific region (10°–80°N, 70°–180°E), interannual variation of East Asian summer monsoon (EASM) and its correlations with general circulation and precipitation patterns are studied by using statistical diagnostic methods such as 9-point high pass filtering, empirical orthogonal function (EOF) analysis, composite analysis and other statistical diagnosis, etc. It is concluded as follows: (1) EOF analysis of SLP in the East Asia-West Pacific region shows the existence of the zonal dipole oscillation mode (APD) between the Mongolia depression and the West Pacific high, and APD index can be used as an intensity index of EASM. (2) EOF analysis of GHT anomalies at 500 hPa in the East Asia-West Pacific region shows that the first EOF mode is characterized with an obvious meridional East Asian pattern (EAP), and EAP index can also be used as an EASM intensity index. (3) The composite analysis of high/low APD index years reveals the close correlation of APD index with EAP at 500 hPa (or 850 hPa). The study shows an obvious opposite correlation exists between APD index and EAP index with a correlation coefficient of −0.23, which passes the confidence test at 0.10 level. (4) Both APD and EAP indexes are closely correlated with precipitation during flood-prone season in China and precipitation rate over the East Asia-West Pacific region. The significant correlation area at 5% confidence level is mainly located from the southern area of the Yangtze River valley to the ocean around southern Japan, and the former is a positive correlation and the latter is a negative one. Foundation: Cooperative Project funded by the Ministry of Science and Technology of the People’s Republic of China, No.2007DFB20210; National Natural Science Foundation of China, No.90502003; JICA China-Japan Technical Cooperative Project “China-Japanese Cooperative Research Center on Meteorological Disasters”. Author: Yu Shuqiu, Associate Professor, specialized in climate and climate change.     

REASONS FOR THE LATE ONSET AND ANOMALOUS SOUTHWARD PERSISTENCE OF THE SOUTH CHINA SEA SUMMER MONSOON IN 2005

Features of atmospheric circulation and thermal structures are discussed using the NCAR/NCEP data to reveal the reasons for the late onset and anomalous southward persistence of the South China Sea Summer Monsoon (SCSSM) in 2005. The results show that three factors are crucial. First, a strong Arabian High overlaps with a high-latitude blocking high and channels strong cold air to southern Asia. Second, the Tibetan Plateau has a bigger snow cover than usual in spring and the melting of snow cools down the surface. Third, the Somali Jet breaks out at a much later date, being not conducive to convection over Indochina. The former two factors restrict atmospheric sensible heating over the Tibetan Plateau and nearby regions while the third one limits latent heating over Indochina. All of the factors slow down atmospheric warming and postpone the onset of SCSSM. Long after the onset of SCSSM, strong cold air over India advances the Southwest Monsoon northward slowly, resulting in weaker convection and latent heating over the Tibetan Plateau and nearby areas. The negative feedback conversely inhibits further northward movement of Southwest Monsoon.     

新的高原季风指数与四川盆地夏季降水的关系

用NCEP/NCAR逐月再分析资料和中国560站月降水资料,定义了一个高原季风指数IPM2。结果表明:与原有高原季风指数相比,该指数与四川盆地夏季降水的相关性更好,能够较好地反映四川盆地夏季降水的异常变化。当高原夏季风偏弱时,巴尔喀什湖至贝加尔湖低压槽、亚洲东岸高压脊、印度低压均加强,同时西太平洋副热带高压偏北,来自孟加拉湾的西南风水汽输送和源于西太平洋的偏南风水汽输送均加强,这种环流形式有利于四川盆地西(东)部夏季降水偏多(少);当高原夏季风偏强时,情况相反。IPM2弱(强)大(小)年与四川盆地西(东)部涝年环流背景相似,表明IPM2能够很好反映四川盆地夏季降水异常的环流场特征。     

2003年江淮流域强降水过程与30—70d天低频振荡的联系

利用NCEP／NCAR再分析和地面观测站的逐日降水资料,研究了2003年夏季江淮流域强降水过程与低频振荡的联系。结果显示,主周期为30～70d的低频振荡对2003年江淮流域暴雨的形成具有重要贡献：低频涡旋在江淮地区降水期的对流层高、低层呈负、正配置,具有斜压结构,利于降水发生;850hPa上正涡度系统的传播具有明显的北传和西传特征;存在于西太平洋、西北太平洋及其以东地区的低频波列（P—J）的活动过程影响了我国2003年江淮低频强降水的形成;整层低频水汽通量显示来自副热带高压外围的西南季风对水汽输送的贡献较显著,且2003年江淮地区30-70d时间尺度上降水的水汽来源为南海而非孟加拉湾或西太平洋。     

近千年亚洲季风变化驱动力的初步研究

通过反演方法,利用已有的影响亚洲季风变化的相应资料,通过动力模型建立了近千年亚洲季风变化的驱动力方程,根据各指标的具体数值反演出方程中各项系数和对应的贡献率。分析方程中各气候代用指标对亚洲季风变化的影响程度,发现近千年亚洲季风并非只受到这些气候要素的线性作用,而是受到较为复杂的非线性驱动力作用,其中大气中CO₂含量、印度洋海表温度、前一期季风强度、南方涛动指数、CH₄气体含量、格陵兰温度、北大西洋海表温度、北半球温度等在驱动近千年亚洲季风变化中起到较为重要的作用,而在长时间尺度上对季风变化有重要影响的太阳活动,近千年中其直接影响并不十分显著,这也可能是指标精度的原因。     

过去千年3个特征时期东亚冬夏季风关系的模拟研究

使用美国大气研究中心开展的过去千年集合模拟试验（Community Earth System Model-Last Millennium Ensemble,简称CESM-LME）数据,对过去千年（公元850~2005年）3个重要的特征时期——中世纪气候异常期、小冰期和现代暖期的东亚冬、夏季风关系,尤其是年代-多年代尺度上的关系进行了对比研究。结果表明：在年代和多年代尺度上,由自然外强迫主导的中世纪气候异常期和小冰期及人类活动主导的现代暖期,东亚冬、夏季风均呈负位相变化形势,但影响二者关系的机制在3个时期并不相同。研究发现,太平洋年代际振荡（Pacific Decadal Oscillation,简称PDO）可能是造成前两个特征时期东亚冬、夏季风反位相变化的主要原因,大西洋多年代际振荡（Atlantic Multidecadal Oscillation,简称AMO）的作用相对较小。现代暖期AMO的作用有所加强,与PDO的作用相当,同时夏季风环流对PDO和AMO的响应较前两个时期强,且响应特征有所不同,这可能与人类活动有较大关系。另外在人类活动作用下,季风指数的定义方法可能会对季风关系的研究结果产生影响,这是未来预估研究中需要留意的地方。

     

东北夏季气温变异的区域差异及其与大气环流和海表温度的关系

利用1951～2000年中国东北地区23个台站资料，对东北夏季气温的时空分布进行了研究，发现其变化除具有整体的一致性外，东北南部和北部的夏季气温在年际和年代际时间尺度都表现出很大不同，其中北部区域的夏季气温在1987～1988年间发生了一次显著的气候突变. 另外，剔除夏季气温全区一致变化的年份后，南北两区夏季气温与大气环流和海表温度的关系表明：突变前，影响北部和南部冷/热夏季的大气环流形势存在显著的不同，关键海域亦有很大差异：影响南部的为中纬度西太平洋和印度洋部分海域，影响北部的主要为ENSO事件；突变后，两区的夏季气温及相应大气环流和关键海区都趋于一致. 在整个分析时段内，北部夏季气温与东亚夏季风存在显著负相关，而南部的关系则不明显.     

Monsoon Change in East Asia in the 21st Century: Results of RegCM3

The authors investigate monsoon change in East Asia in the 21st century under the Special Report on Emissions Scenarios (SRES) A1B scenario using the results of a regional climate model, RegCM3, with a high horizontal resolution. First, the authors evaluate the model’s performance compared with NCEP-NCAR reanalysis data, showing that the model can reliably reproduce the basic climatology of both winter and summer monsoons over East Asia. Next, it is found that the winter monsoon in East Asia would slightly weaken in the 21st century with spatial differences. Over northern East China, anomalous southerly winds would dominate in the mid-and late-21st century because the zonal land-sea thermal contrast is expected to become smaller, due to a stronger warming trend over land than over ocean. However, the intensity of the summer monsoon in East Asia shows a statistically significant upward trend over this century because the zonal land-sea thermal contrast between East Asia and the western North Pacific would become larger, which, in turn, would lead to larger sea level pressure gradients throughout East Asia and extending to the adjacent ocean.