南海大气季节内振荡特征及其与ENSO循环的关系

采用1979—2009年NCEP/NCAR再分析资料、NOAA向外长波辐射(outgoing long-wave radiation,简称OLR)及扩展重建海表面温度资料,对南海大气季节内振荡(Intra-seasonal Oscillation,简称ISO)特征及其与ENSO循环的关系进行了诊断分析。结果表明:1)南海大气ISO的30~60 d周期在5—10月均显著。一般年南海大气ISO的对流传播在纬向上存在东传和西传,在经向上具有南北半球季节性摆动的特征。以低频动能表征的南海大气ISO强度年代际变化特征明显,近31 a来趋势增强,年变化呈单峰结构,峰值在7—8月。2)南海大气ISO的对流与ENSO循环显著相关,其强度在El Nio(La Nia)年减弱(增强)。与一般年对比,南海大气ISO对流在El Nio和La Nia年均表现为西传减弱、北传显著。北传特征表现为强对流活跃带于春季(4—5月)北跳至北半球(在La Nia年最北可至35°N),但在北半球的传播方向与一般年相比存在显著差异。3)南海大气ISO强度与ENSO循环关系密切,在El Nio(La Nia)年减弱(增强),两者表现为约半年(6~8个月)的滞后相关。Nio3区海表面温度异常序列与南海大气ISO强度的相关在中西太平洋地区和El Nio成熟前的春、秋季最显著,同时相关中心伴随低频动能高值区东移。     

THE NORTH-SOUTH ANTI-PHASE DISTRIBUTION OF RAINFALL IN MEIYU PERIODS AND ITS RELATIONSHIP WITH QUASI-BIWEEKLY OSCILLATION IN THE ATMOSPHERE

Based on the daily rainfall datasets from 740 stations in China from 1954 to 2005 and the NCEP/NCAR reanalysis data, the relationship between the north-south anti-phase distribution(APD) of rainfall during Meiyu periods and the Quasi-Biweekly Oscillation(QBWO) in the atmosphere was analyzed. Diagnostic results are as follows:(1) there was significant north-south oscillation of Meiyu rainfall during the 16 years from 1954 to 2005. Since the 1990 s, the APD enhanced significantly and showed 2- and 4-6-year period. In the region with more rainfall, the QBWO was always more active.(2) The APD of Meiyu and north-south movements of precipitation in eastern China belong to the same phase.(3) The 10-25 day filtered water vapor flux could spread to the area north of 30°N in 1991. The divergence of the water vapor flux which propagated from middle- and higher- latitudes to the of Yangtze-Huaihe River Basins(YHRB) was significant in 1991, but the latitudes that the water vapor flux could reach were further southward and there was no southward propagation of divergence in 1993.(4) The locations of Western Pacific Subtropical High(WPSH) and 10-25 day anti-cyclone, which modulated WPSH's advancement in and out of the South China Sea, were relatively northward in 1991. Furthermore, the vertical circulation showed north-south deviation between 1991 and 1993, just as other elements of the circulation did.     

RELATIONSHIP BETWEEN TROPICAL ISO AND TROPICAL CYCLONE ACTIVITY OVER THE SOUTH CHINA SEA

The relationship between the tropical intra-seasonal oscillation (ISO) and tropical cyclones (TCs) activities over the South China Sea (SCS) is investigated by utilizing the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) global reanalysis data and tropical cyclone best-track data from 1949 to 2009. The main conclusions are: (1) A new ISO index is designed to describe the tropical ISO activity over the SCS, which can simply express ISO for SCS. After examining the applicability of the index constructed by the Climate Prediction Center (CPC), we find that the convection spatial scale reflected by this index is too large to characterize the small-scale SCS and fails to divide the TCs activities over the SCS into active and inactive categories. Consequently, the CPC index can’t replace the function of the new ISO index; (2) The eastward spread process of tropical ISO is divided into eight phases using the new ISO index, the phase variation of which corresponds well with the TCs activities over the SCS. TCs generation and landing are significantly reduced during inactive period (phase 4-6) relative to that during active period (phase 7-3); (3) The composite analyses indicate distinct TCs activities over the SCS, which is consistent with the concomitant propagation of the ISO convective activity. During ISO active period, the weather situations are favorable for TCs development over the SCS, e.g., strong convection, cyclonic shear and weak subtropical high, and vice versa; (4) The condensation heating centers, strong convection and water vapor flux divergence are well collocated with each other during ISO active period. In addition, the vertical profile of condensation heat indicates strong ascending motion and middle-level heating over the SCS during active period, and vice versa. Thus, the eastward propagation of tropical ISO is capable to modulate TCs activities by affecting the heating configuration over the SCS.     

热带季节内振荡与南海热带气旋活动的关系

利用1949—2009年台风年鉴和NCEP/NCAR再分析资料分析了热带季节内振荡(ISO)与南海热带气旋活动的关系。主要结论包括：(1) 针对南海地区定义了纬向风指数,经验证该指数能较好描述南海大气ISO的具体特征;(2) CPC120指数反映的对流空间尺度相对于南海小尺度海域太大,且不能很好判明南海地区气旋活动归属于活跃与否两类,这从反面证明了纬向风指数对南海地区热带ISO活动的适用性;(3) 利用纬向风指数把热带ISO活动划分了8个位相,发现其位相变化与南海热带气旋活动有较好的对应关系：不活跃期(第4～6位相)时,气旋在南海生成和登陆均明显减少,而在活跃期(第7～3位相)时情况相反;(4) 合成分析表明,热带ISO东传时,伴随对流中心位置的东移,南海气旋活动表现也不同,其活跃期以第2位相特征最明显,此时南海地区对应强对流和气旋式切变,同时副高强度较弱位置偏东,这些均有利于南海气旋的生成和发展;不活跃期则情况相反,以第6位相特征最明显;(5) 进一步从能源供应角度来探讨发现,南海地区的水汽凝结加热中心、强对流中心及水汽通量散度均配合一致。此外,活跃位相时整层热源垂直剖面均反映南海上空为强上升运动和中层加热;不活跃位相则情况相反。因此,热带ISO东传也会影响南海上空加热配置,从而影响热带气旋活动。     

热带MJO和ENSO对西北太平洋热带气旋影响研究综述

对西北太平洋热带气旋的活动规律作了综合评述,简单回顾了热带季节内振荡（MJO）特征和厄尔尼诺/南方涛动（ENSO）特征以及两者间关系,较系统地总结了近年来国内外学者关于MJO和ENSO对西北太平洋海域热带气旋活动影响和机理等方面的研究成果。讨论的热带气旋活动特征包括源地、频数、路径、强度、生命期和登陆等几个方面,并简单讨论了目前该领域存在的科学问题和未来可能的研究方向。     

Modulation of Tropical Cyclone Activity Over the Northwestern Pacific Through the Quasi-Biweekly Oscillation

The quasi-biweekly oscillation (QBWO) is the second most dominant intraseasonal mode for circulation over the Northwestern Pacific (WNP) during boreal summer. In this study, we investigated how the QBWO modulates tropical cyclone (TC) activities over the WNP from dynamic and thermodynamic perspectives. The propagation of the QBWO can be divided into four phases through empirical orthogonal function analysis of the vorticity at 850 hPa, which was proven to be effective in extracting the QBWO signal. TC generation and landings are significantly enhanced during the active period (phases 1 and 2) relative to the inactive period (phases 3 and 4). Composite analyses show the QBWO could significantly modulate TC activity as it propagates northwestward by changing the atmospheric circulation at both high and low levels. Cumulus convection provides an important link between TCs and the QBWO. The major component of the atmosphere heat source is found to be the latent heat release of convection. The condensation latent heat centers, vertical circulation, and water vapor flux divergence cooperate well during different phases of the QBWO. The vertical profile of the condensation latent heat indicates upper-level heating (cooling) during the active (inactive) phases of the QBWO. Thus, the northwestward propagation of the QBWO can modulate TC activity by affecting the configuration of atmospheric heating over the WNP.     

AN EMPIRICAL ORTHOGONAL FUNCTION ANALYSIS FOR THE WAVE TRAINS IN EAST ASIAN SUMMER

This study examines the wave trains at 500 hPa occurring in East Asian summer by using the Empirical Orthogonal Function (EOF) analysis as a diagnostic tool. The results are summarized as follows: (1) A wave train pattern (OKJ pattern) originating from the upstream areas of the Sea of Okhotsk to the subtropical regions could display its strong signal in early and middle summer. The OKJ pattern is clearly recognized in the first EOF component in Eurasia. (2) The other wave train pattern originating from the Philippines via Japan to North America (the P-J pattern) shows quite strong signals in the whole summer. Although the P-J pattern is described as the second EOF component around the area from East Asia to Northeast Pacific Ocean, the variance contribution is the same as that of OKJ pattern in the first EOF component. (3) The composite analyses indicate that the OKJ and P-J wave trains could coexist to some extents.