1998年南海、孟加拉湾夏季风期间动能收支特征

该文采用1998年加密观测资料经同化处理后得到的客观分析格点资料, 对南海地区和孟加拉湾地区的动能收支进行了诊断分析和对比, 得出: B区夏季风爆发, 其850 hPa区域平均总动能表现为爆发性增长, C区则表现为一个逐步增长的过程.越赤道气流通过南边界的动能输送对B区夏季风建立贡献很大, 西边界动能输入对C区夏季风建立也起了十分显著的作用.季风盛行期, B区夏季风动能的发展维持主要是动能水平通量散度的贡献, 其中西边界动能的流入贡献最大, 孟加拉湾夏季风的变化主要为印度季风影响所致; C区夏季风动能主要是依靠其区域内动能制造来维持.对于850 hPa层, B区主要通过斜压过程制造动能, 正压过程破坏更多的动能, C区主要是正压过程制造动能.两区对流层高层都为动能主要流出区, 而对流层低层, B区为动能流入区, C区为动能流出区.     

An Analysis of the Characteristics of Monsoon Onset over the Bay of Bengal and the South China Sea in 2010

Based on NCEP/NCAR daily reanalysis and the Tropical Rainfall Measuring Mission data, the background atmospheric circulation and the characteristics of meteorological elements during the period of the Bay of Bengal monsoon (BOBM) and the South China Sea (SCS) monsoon (SCSM) in 2010 are studied. The impacts of the BOBM onset on the SCSM onset and the relationship between the two monsoons are also analyzed. The two main results are as follows. (1) The BOBM onset obviously occurs earlier than the SCSM onset in 2010, which is a typical onset process of the Asian monsoon. During the BOBM’s onset, northward jump, and eastward expansion, convective precipitation and southwest winds occurred over the SCS, which resulted in the onset of the SCSM. (2) The relationship among strong convection, heavy rainfall, and vertical circulation configuration is obtained during the monsoon onsets over the BOB and SCS, and it is concluded that the South Asian High plays an important role in this period.     

Effect of condensational heating over the Bay of Bengal on the onset of the South China Sea monsoon in 1998

Summary In this study, the authors analyse the observational features of the onset of the South China Sea (SCS) monsoon in 1998 shown in reanalysis data and use a numerical model to understand the mechanisms responsible for these features.The onset of SCS summer monsoon in 1998 occurred around 21 May. Prior to this period, monsoon depression activity was strong over the Bay of Bengal (BOB) and warm temperature anomalies appeared at the mid-upper troposphere over the northern BOB. In the meantime, warm horizontal thermal advection occurred over the northern Indo-China peninsula and South China. This warm advection seemed to play an important role in the winter-to-summer transition of the patterns of mean meridional temperature gradient in the 500–200-mb layer over South Asia.The PSU/NCAR Mesoscale Model Version 5 (MM5) is used to understand the physical link between the latent heating associated with the monsoon depression over BOB and the establishment of SCS monsoon. Full-physics simulations, for a 6-day period coinciding with the onset of the observed monsoon, reproduce realistically the evolution of the monsoon depression and monsoon onset process. It is found that the condensational heating over BOB is important for the formation of large-scale circulation pattern that favors the establishment of SCS monsoon. In an experiment without latent heating, the winter-to-summer reversal of meridional temperature gradient over South Asia was delayed and the onset of SCS summer monsoon did not occur during the simulation period.     

Springtime Convective Quasi-Biweekly Oscillation and Interannual Variation of Its Intensity over the South China Sea and Western North Pacific

This study investigates characteristics of the convective quasi-biweekly oscillation(QBWO) over the South China Sea(SCS) and western North Pacific(WNP) in spring, and the interannual variation of its intensity. Convective QBWO over the WNP and SCS shows both similarities and differences. Convective QBWO over the WNP originates mainly from southeast of the Philippine Sea and propagates northwestward. In contrast, convective QBWO over the SCS can be traced mainly to east of the Philippines and features a westward propagation. Such a westward or northwestward propagation is probably related to n = 1 equatorial Rossby waves. During the evolution of convective QBWO over the WNP and SCS, the vertical motion and specific humidity exhibit a barotropic structure and the vertical relative vorticity shows a baroclinic structure in the troposphere. The dominant mode of interannual variation of convective QBWO intensity over the SCS–WNP region in spring is homogeneous. Its positive phase indicates enhanced convective QBWO intensity accompanied by local enhanced QBWO intensity of vertical motion throughout the troposphere as well as local enhanced(weakened) QBWO intensity of kinetic energy, vertical relative vorticity,and wind in the lower(upper) troposphere. The positive phase usually results from local increases of the background moisture and anomalous vertical shear of easterlies. The latter contributes to the relationship between the dominant mode and QBWO intensities of kinetic energy, vertical relative vorticity, and wind. Finally, a connection between the dominant mode and the sea surface temperature anomalies in the tropical Pacific Ocean is demonstrated.     

Intraseasonal variability of latent-heat flux in the South China Sea

Intraseasonal variability (ISV) of latent-heat flux in the South China Sea (SCS) is examined using 9 years of weekly data from January 1998 to December 2006. Using harmonic and composite analysis, some fundamental features of the latent-heat flux ISVs are revealed. Intraseasonal latent-heat flux has two spectral peaks around 28–35 and 49–56 days, comparable with the timescales of the atmospheric ISV in the region. Active monsoon is clearly correlated with positive and negative phases of the ISV of latent-heat flux in the SCS. The characteristics of the intraseasonal latent-heat flux variations in summer are remarkably different from those in winter. The amplitudes of significant intraseasonal oscillations are about 35 and 80 W?m^?2 during summer and winter monsoons, respectively. In summer, the intraseasonal latent-heat flux perturbations are characterized by slow eastward (about 1° latitude/day) and slower northward (about 0.75° longitude/day) propagations, probably in a response to eastward and northward propagating Madden-Julian oscillations (MJOs) from the equatorial Indian Ocean. In contrast, the perturbations appear to remain in the northern SCS region like a quasi-stationary wave in winter. In summer, the intraseasonal latent-heat flux fluctuations are highly correlated with wind speed. In winter, however, they are primarily associated with winds and near-surface air humidity. In addition, the intraseasonal SST variation is estimated to significantly reduce the amplitude of the intraseasonal latent-heat flux by 20% during winter.     

Intraseasonal and interannual zonal circulations over the Equatorial Indian Ocean

El Ni?o Southern Oscillation (ENSO) and given phases of the Madden?CJulian Oscillation (MJO) show similar regional signatures over the Equatorial Indian Ocean, consisting in an enhancement or reversing of the convective and dynamic zonal gradients between East Africa and the Maritime Continent of Indonesia. This study analyses how these two modes of variability add or cancel their effects at their respective timescales, through an investigation of the equatorial cellular circulations over the central Indian Ocean. Results show that (1) the wind shear between the lower and upper troposphere is related to marked regional rainfall anomalies and is embedded in larger-scale atmospheric configurations, involving the Southern Oscillation; (2) the intraseasonal (30?C60?days) and interannual (4?C5?years) timescales are the most energetic frequencies that modulate these circulations, confirming the implication of the MJO and ENSO; (3) extreme values of the Indian Ocean wind shear result from the combination of El Ni?o and the MJO phase enhancing atmospheric convection over Africa, or La Ni?a and the MJO phase associated with convective activity over the Maritime Continent. Consequences for regional rainfall anomalies over East Africa and Indonesia are then discussed.     

Origins of intraseasonal rainfall variations over the southern South China Sea in boreal winter

本文主要探讨冬季南海南部季节内降水变化的来源,数据主要来自于TRMM和NCEP-DOE资料,方法主要有谱分析和回归分析。研究发现南海南部的降水变化在10–20天和30–60天时间尺度上有不同的来源。10–20天时间尺度上,东亚冬季风超前于南海南部降水异常2天。而在30–60天时间尺度上,两者几乎同时出现。此外,热带东南印度洋上的降水异常超前于南海南部一周左右,这表明热带印度洋上的对流引起的环流异常可能对南海南部30–60天时间尺度上降水异常有重要的作用,但对10–20天时间尺度上降水异常的作用并不明显。     

华南6月持续性致洪暴雨与孟加拉湾对流异常活跃的关系

利用NCEP/NCAR再分析逐日及逐月资料、NOAA卫星观测的向外长波辐射（Outgoing Long-wave Radiation,OLR）资料、我国160站月降水资料及华南45站逐日降水资料,分析了2005年6月华南持续性致洪暴雨的发生与孟加拉湾对流异常活跃及西太平洋副热带高压演变的关系,并对华南6月有无持续性大范围暴雨年进行合成分析,结果表明华南持续性大范围暴雨发生的年份,孟加拉湾对流相对活跃,西太平洋副高强度偏强、位置偏西;反之亦然。华南6月持续性大范围暴雨发生前,孟加拉湾对流经历了一次活跃过程,同时西太平洋副高也明显经历了一次增强西伸过程。孟加拉湾对流活跃可能通过东西向垂直环流诱使西太平洋副高增强西伸,从而对华南暴雨的形成产生影响。     

Intraseasonal Oscillation of the South China Sea Summer Monsoon and Its Influence on Regionally Persistent Heavy Rain over Southern China

The intraseasonal oscillation（ISO） in the South China Sea summer monsoon（SCSSM） and its influence on regionally persistent heavy rain（RPHR） over southern China are examined by using satelhte outgoing long wave radiation,NCEP/NCAR reanalysis,and gridded rainfall station data in China from 1981 to 2010.The most important feature of the ISO in SCSSM,contributing to the modulation of RPHR,is found to be the fluctuation in the western Pacific subtropical high（WPSH）,along with a close link to the Madden-Julian oscillation（MJO）.Southern China is divided into three regions by using rotated empirical orthogonal functions（REOFs）for intraseasonal rainfall,where the incidence rate of RPHR is closely linked to the intraseasonal variation in rainfall.It is found that SCSSM ISOs are the key systems controlling the intraseasonal variability in rainfall and can be described by the leading pair of empirical orthogonal functions（EOFs） for the 850-hPa zonal wind over the SCS and southern China.Composite analyses based on the principal components（PCs） of the EOFs indicate that the ISO process in SCSSM exhibits as the east-west oscillation of the WPSH,which is coupled with the northward-propagating MJO,creating alternating dry and wet phases over southern China with a period of 40 days.The wet phases provide stable and lasting circulation conditions that promote RPHR.However,differences in the ISO structures can be found when RPHR occurs in regions where the WPSH assumes different meridional positions.Further examination of the meridional-phase structure suggests an important role of northward-propagating ISO and regional air-sea interaction in the ISO process in SCSSM.     

上游赤道西风在南海夏季风暴发中的重要作用

利用17年（1979～1995）的NMC全球格点分析资料及从NOAA极轨卫星观测的地球向外长波辐射（OLR）资料,研究了南海夏季风暴发的年际变化特征。用本文的定义标准,南海季风从1979年到1995年间暴发的平均日期是5月第4候。南海夏季风暴发前后,南海周围地区对夏季风暴发的直接影响来自南海上游70°～90°E的赤道西风的加强和北抬东伸,同时索马里越赤道气流的加强和阿拉伯海高压在海上消失通过影响赤道西风也间接影响到南海。其中阿拉伯海反气旋的消失可能是南海夏季风暴发的一个重要信号。南海夏季风暴发的年际变化较     

近20 a孟加拉湾海表温度变化对南海夏季风爆发早晚的影响

基于美国NOAA现代极高分辨率辐射仪(Advanced Very High Resolution Radiometer)提供的1993—2012年逐日海表温度(SST)资料,利用季节经验正交函数(S-EOF)和相关分析等统计方法,研究了孟加拉湾海表温度变化对南海夏季风爆发的影响。结果表明,孟加拉湾的(6~14°N,85~95°E)海区海表温度变化对南海夏季风爆发早晚具有重要指示意义,该海区海表温度异常(SSTA)与南海夏季风的爆发日期存在密切的正相关,通过了0.05信度的显著性检验,即当孟加拉湾海表温度正(负)异常时,南海夏季风晚(早)爆发。应用德国马普气象研究所的ECHAM5全球大气环流模式在孟加拉湾关键海区进行了敏感性数值试验,发现在关键海区降低其5月海表温度02℃的情况下,南海夏季风爆发日期相应提前5 d左右,而在升高02℃情况下,南海夏季风推后10 d左右爆发。在孟加拉湾5月海表温度降低的情况下,促使80~100°E的越赤道气流增强,南海区域西风分量增强,进而促使南海夏季风提前爆发。     

华南南部前汛期气温与阿拉伯海和孟加拉湾北部海温的关系

利用国家气候中心整编的1951—2008年中国160站的月平均气温资料,选出华南地区10个代表站,分析了华南南部前汛期气温异常的季节、年际和年代际变化的时空特征。结果表明,58年的温度变化总趋势是增加的,1950—1990年代初为相对低温期,1990年代中期开始温度明显升高。华南南部前汛期气温异常存在2,4和5年的年际周期和12年和19年的年代际周期。在研究华南南部前汛期气温异常的基础上,还分析了华南南部前汛期气温与阿拉伯海和孟加拉湾北部海温的关系,发现前一年10~11月阿拉伯海和孟加拉湾北部海域存在一个与次年华南南部前汛期气温有较好正相关的关键区(85°~97°E,5°~16°N)(简称关键区),当前一年10~11月关键区海温异常偏高(偏低),次年华南南部前汛期气温偏高(偏低),关键区海温异常和华南南部前汛期气温异常都具有准2年、准5年和准12年的振荡周期,冷、暖水年次年的华南南部前汛期环流形势存在很大的差异。     

阿拉伯海和孟加拉湾夏季风气候特征的差异

利用 1 958— 1 997年 3— 1 1月 NCEP/NCAR850和 2 0 0 h Pa再分析风场资料 ,研究了南亚夏季风活动的气候特征。发现南亚不同区域的经、纬向风季节变化及纬向风的峰型结构差异明显。夏季在阿拉伯海和孟加拉湾分别有两个最明显西南风速中心 ,这两个地区的西南风活动明显不同 ,孟加拉湾西南风出现及向北推进到2 0°N时间比阿拉伯海明显偏早 ,结束时间偏晚 ,是南亚季风区西南夏季风维持时间最长的地区。对比分析强、弱夏季风年 6— 8月高、低层经、纬向风距平垂直切变矢量风场发现 ,热带低纬度越赤道气流及南亚季风区的流场结构明显不同。     

南海夏季风季节内振荡的年际变化研究

利用NCEP/NCAR风场、 垂直速度再分析资料和NOAA的逐日向外长波辐射 (简称OLR) 资料, 运用Butterworth带通滤波方法, 对南海夏季风季节内振荡 (简称ISO) 的特征和年际变化情况进行了初步研究。气候平均状况下ISO在亚洲季风区存在5个活动中心, 其中包括南海南部地区 (5°N～15°N, 110°E～120°E)。气候平均状况下南海ISO在5～9月有三次比较明显的活跃过程。以850 hPa风在西南方向上的投影 (Vsw) 经30～60天带通滤波后的绝对值在南海区域的夏季平均定义了当年南海夏季风ISO活动指数, 以此确定了南海夏季风ISO活跃年和不活跃年, 并由此提取了两者在逐日序列和水平分布场特征上的差异。通过对南海夏季风ISO活跃年与不活跃年ISO信号差别的研究发现: 在南海夏季风ISO活动较活跃年份的一个完整周期的8个位相合成中, 低频对流中心从热带印度洋西部东传到达南海后, 受Walker环流和西太平洋副热带高压底层东风气流阻挡等其它动力和热力因素的影响转向东北传。春季南海地区较强的Walker环流和对流活动有可能作为南海夏季风ISO活跃的前期信号。     

孟加拉湾西南季风与南海热带季风季节内振荡特征的比较

采用美国国家环境预报中心的向外长波辐射和风场资料及日本气象厅的降水资料,用30-60d滤波后的夏季风指数在孟加拉湾和南海的区域平均值分别代表孟加拉湾西南季风和南海热带季风季节内振荡,对两支季风的季节内振荡特征进行比较分析,发现孟加拉湾西南季风的季节内振荡和南海热带季风的季节内振荡在夏季风期间(5-10月)都有约3次半的波动.夏季风期间,在阿拉伯海-西太平洋纬带上,夏季风的季节内振荡有4次从阿拉伯海的东传和3次从西太平洋的西传,其中7月后东传可直达西太平洋.孟加拉湾和南海在夏季风期间都有4次季节内振荡的经向传播,但孟加拉湾在约15°N以南为季节内振荡从热带东印度洋的北传,在约15°N以北则为副热带季风季节内振荡的南传;而在南海则是4次季节内振荡从热带的北传.在以孟加拉湾西南季风季节内振荡和南海热带季风季节内振荡分别划分的6个位相中,都存在1-3位相和4-6位相中低频对流、环流形势相反的特征,这是由热带东印度洋季节内振荡的东传和北传所致.热带印度洋季节内振荡沿西南-东北向经过约14d传到孟加拉湾,激发了孟加拉湾西南季风季节内振荡的东传,经过约6d到达南海,激发了南海热带季风季节内振荡的北传,经过约25d到达华南,形成热带印度洋季节内振荡向华南的经纬向接力传播(45d).孟加拉湾西南季风季节内振荡所影响的降水主要是在20°N以南的热带雨带随低频对流的东移而东移;而南海热带季风季节内振荡所影响的降水除了这种热带雨带随低频对流的东移外,还有在20°N以北的东亚副热带地区存在雨带随南海低频对流的北移而北移.     

Relative Roles of Intraseasonal and Above-seasonal Components in the South China Sea Summer Monsoon Onset

Multi-scale contributions are involved in the South China Sea (SCS) summer monsoon (SCSSM) onset process. The relative roles of intraseasonal oscillation and above-seasonal component in the year-to-year variation of the SCSSM onset are evaluated in this study. The 30-90-day and above-90-day components are major contributors to the year-to-year variation of the SCSSM onset, and the former contributes greater portion, while the 8-30-day component has little contribution to the onset. In the early onset cases, the 30-90-day westerly winds move and extend eastward from the tropical Indian Ocean (TIO) to the SCS monsoon region relatively earlier, and replace the easterly winds over the SCS with the cooperation of the 30-90-day cyclone moving southward from northern East Asia. The westerly anomalies of the above-90-day component in spring jointly contribute to the early SCSSM onset. In the late onset cases, the late eastward expansion of 30-90-day westerly wind over the TIO, accompanied by the late occurrence and weakening of the 30-90-day anticyclone over the SCS, and its late withdraw from the SCS, as well as the persistent easterly anomalies of above-90-day component, suppress the SCSSM onset. However, the SCSSM outbreaks in the obvious weakening stage of 30-90-day easterly anomalies. The easterlies-to-westerlies transition of the 30-90-day 850- hPa zonal wind over the SCS in spring is closely associated with sea surface temperature in the tropical western Pacific in preceding winter and spring, while the interannual variation of the above-90-day zonal wind in April-May is closely related to the decaying stage of the El Ni?o-Southern Oscillation events.     

中国东部夏季风雨带季节内变异模态的研究

根据中国气象局国家气候中心提供的1951～2005年中国160个站月降水量资料,用扩展的EOF（EEOF）分析、小波变换、相关分析和合成分析方法,研究了中国东部夏季风雨带季节性南北变动的主导模态和时空变化的特征,分析了各模态的气候特点及其与西太平洋副热带高压季节内变异的关系。 研究表明,我国夏季雨带季节性进退活动不仅是多态的而且是多时间尺度的现象。主导模态主要有六种,为了对各模态雨带推进过程的不同特点有个概要的认识, 分别称之为:梅涝型、伏旱型、梅旱型、南涝北旱型、常态型和南旱北涝型。这六种模态基本上反映了我国东部夏季雨带季节性移动异常的各种情况。在时间尺度上,各模态普遍存在准两年、3～5年和准8年的年际变化,以及准10年、准15年、准24年和准30年的年代际变化。但各模态的优势周期存在明显差别,尤其是年代际变化。分析还表明,西太平洋副热带高压的季节性变异是影响我国东部夏季雨带季节内时空变化最直接和最主要的原因。基本上, 不同型式的副高变异对应有不同的雨带季节内变化模态和天气气候特征。这些研究结果可为我们进一步探讨中国夏季雨带季节内变异各模态形成的物理过程和成因提供基础。     

Intraseasonal oscillations of the monsoon circulation over South Asia

The space–time structure of the three-dimensional circulation over the South Asian monsoon region has been studied using the ERA-40 reanalysis of the European Centre for Medium-Range Weather Forecasts. Applying multi-channel singular spectrum analysis on combined daily values of horizontal winds and pressure vertical velocity at ten vertical levels for the period 1958–2001, two leading intraseasonal nonlinear oscillations were extracted. The first oscillation has an average period of 50?days and propagates northeastward from the Indian Ocean to the Indian subcontinent. The second oscillation has a period of 30?days and propagates northwestward from the West Pacific to the Indian region. Both the oscillations exhibit the oscillatory and propagation features at all vertical levels from 1,000 to 100?hPa. The two oscillations correspond well with similar oscillations found in outgoing longwave radiation and precipitation in earlier studies. The wind oscillations also account for the active and break phases of the Indian monsoon. The vertical structures and propagation of specific humidity and temperature are found to be consistent with those of the winds in each oscillation. The structure and movement of regional Hadley and Walker circulations have also been described. The analyses provide further strong evidence for the existence of two distinct monsoon intraseasonal oscillations.     

Variation of the total ozone column during tropical cyclones over the Bay of Bengal and the Arabian Sea

The variation of the total ozone column (TOC) before, during, and after tropical cyclones occurring in the pre-monsoon and post-monsoon periods over the Bay of Bengal and the Arabian Sea for the period 1997–2009 is presented. From the analysis, it is concluded that TOC decreases steadily before and during the formation of a cyclone, followed by a more or less increasing trend after dissipation of the cyclone. It is also observed that, when the cyclone reaches its peak intensity indicated by its maximum wind speed, there is a sudden fall in TOC over those regions where the cyclone has intensified. The observed variation of TOC is consistent with existing chemical and dynamical theories.     

The 30–60-day Intraseasonal Variability of Sea Surface Temperature in the South China Sea during May–September

This study investigates the structure and propagation of intraseasonal sea surface temperature(SST) variability in the South China Sea(SCS) on the 30–60-day timescale during boreal summer(May–September). TRMM-based SST, GODAS oceanic reanalysis and ERA-Interim atmospheric reanalysis datasets from 1998 to 2013 are used to examine quantitatively the atmospheric thermodynamic and oceanic dynamic mechanisms responsible for its formation. Power spectra show that the 30–60-day SST variability is predominant, accounting for 60% of the variance of the 10–90-day variability over most of the SCS. Composite analyses demonstrate that the 30–60-day SST variability is characterized by the alternate occurrence of basin-wide positive and negative SST anomalies in the SCS, with positive(negative) SST anomalies accompanied by anomalous northeasterlies(southwesterlies). The transition and expansion of SST anomalies are driven by the monsoonal trough–ridge seesaw pattern that migrates northward from the equator to the northern SCS. Quantitative diagnosis of the composite mixed-layer heat budgets shows that, within a strong 30–60-day cycle, the atmospheric thermal forcing is indeed a dominant factor, with the mixed-layer net heat flux(MNHF) contributing around 60% of the total SST tendency, while vertical entrainment contributes more than 30%. However, the entrainment-induced SST tendency is sometimes as large as the MNHF-induced component, implying that ocean processes are sometimes as important as surface fluxes in generating the30–60-day SST variability in the SCS.