WAVELET ANALYSIS OF INTERANNUAL VARIATION OF TROPICAL CYCLONES IN GUANGDONG

Climatological laws are studied for the annual frequency of tropical cyclone occurrence and the date of the yearly first landfall, which take place in the Guangdong province or pose serious threats on it from 1951 to 1999, using the data in the Yearly Book on Typhoons. A new method that has developed over recent years for the study of temporal sequences, the wavelet analysis, is used, in addition to more common statistical approaches. By analyzing two wavelet functions, MHAT and MORLET, we have compared the results of transformation of the wavelets provided that other conditions remain unchanged. It is discovered that the variance of MORLET wavelet has better indication of primary periods; period-time sequence charts can reflect major affecting periods for individual sections of time; when compared with the original sequence, the chart shows a little shift. On the other hand, such shift is absent in the MHAT wavelet, but its higher frequency part of variance covers up the primary periods to make its variance less predominant as compared to the MORLET wavelet. Besides, the work compares two different assumptions of an amplifying factor a. It is found that primary periods can be shown more clearly in the variance when a takes the exponential of 2 than it takes values continuously. Studying the annual frequency of tropical cyclones and the date of first appearance for periodic patterns, we have found that the primary periods extracted by this approach are similar to those obtained by wavelet transformation.     

广东热带气旋年际变化的小波分析

利用《台风年鉴》资料，分析了1951－1999年登陆或严重影响广东的热带气旋的年频数和初旋登陆日期的气候规律。除了利用一般的统计方法以外，还利用近年来分析时间序列比较新的方法，即小波分析。通过对MHAT和MORLET两个小波函数进行分析，分析比较了两个不同小波在其他条件相同情况下的变换结果，分析指出MORLET小波的方差能够比较好的显示出主要的周期，周期－时间序列图可以体现出在各个时段的主要影响周期，MORLET的周期－时间序列图与原序列相比，有较小的飘移现象：MHAT小波虽然没有这种飘移现象，但是其方差的高频部分掩盖了主要周期，使得主要周期与MORLET小波相比方差没有那么明显。同时对放大因子a的两种不同取法做了比较，发现利用a取为2的指数的方法选取a比一般连续的选取a在方差上更能体现主要的周期。年频数的初旋日期的周期分析发现周期分析提取的主要周期与小波变换后得到的主要周期类似。     

1991、1998和2016年三个大水年长江中下游夏季降水季节内特征的对比

针对长江中下游三个大水年1991、1998和2016年,利用NCEP/NCAR大气环流再分析资料和CMAP降水资料,对比了夏季降水的季节内特征,分析了引起降水季节内变化的大气环流季节内振荡ISO演变及源地。小波分析表明,三年季节内降水周期差异明显,分别为20~30 d、20~40 d和10~20 d。随之,以东亚季风区季节内振荡指数及热带外Rossby波活动通量,诊断了引起三年季节内活动异常的热带和中纬度ISO变率特点。结果显示影响三年季节内降水的ISO差异较大。1991年受到来自印度洋10~30 d和中纬度高层Rossby波10~30 d的ISO共同影响,造成周期为20~30 d的低频降水;1998年ISO来源路径单一,受中北太平洋30~60 d和10~30 d的ISO西传叠加作用,降水表现为20~40 d的振荡;引起2016年季节内降水异常的ISO源地较多,既有来自印度洋向东北传播30~60 d的ISO,又有来自太平洋向西北传播10~30 d的ISO,还有来自热带外10~30 d的ISO,三者在长江中下游汇合,引起降水10~20 d的振荡。研究结果对认识长江中下游夏季集中降水的形成有重要意义。      

NATURE OF PRECIPITATION AND ACTIVITY OF CUMULUS CONVECTION DURING THE 1991 MEIYU SEASON OF CHANGJIANG-HUAIHE RIVER BASIN

Seasonal variability regarding the nature of precipitation and the activity of cumulus convectionduring the 1991 Meiyu season of Changjiang-Huaihe River Basin(Jianghuai)has been investigatedby calculating apparent heat source/apparent moisture sink and analyzing TBB(cloud-topblackbody radiation temperature)data.It is found that three periods of strong ascending motionduring the Meiyu season lead to three episodes of heavy rain,and the latent heat due to theprecipitation is of the sole heat source of the atmosphere.The nature of precipitation showsdistinct seasonal variability,from frontal precipitation of the first episode to the extremely strongconvective precipitation of the third episode.TBB field of East Asia may well reflect not only theintensity of convection and rainfall,but also the movement of rain belt and convection belt.In thewhole Meiyu season.convection belt mainly stays in Jianghuai.but may shift within the domain ofEast Asia.Its locating in Jianghuai or not determines the maintenance or break of Meiyu.In thethird episode,the narrow convection belt over Jianghuai is mainly caused by southwest monsoonwhich takes moist and convective atmosphere from tropical ocean.     

NATURE OF PRECIPITATION AND ACTIVITY OF CUMULUS CONVECTION DURING THE 1991 MEIYU SEASON OF CHANGJIANG-HUAIHE RIVER BASIN*

Seasonal variability regarding the nature of precipitation and the activity of cumulus convection during the 1991 Meiyu season of Changjiang-Huaihe River Basin(Jianghuai)has been investigated by calculating apparent heat source/apparent moisture sink and analyzing TBB(cloud-top blackody radiation temperature)data.It is found that three periods of strong ascending motion during the Meiyu season lead to three episodes of heavy rain,and the latent heat due to the precipitation is of the sole heat source of the atmosphere.The nature of precipitation shows distinct seasonal variability,from frontal precipitation of the first episode to the extremely strong convective precipitation of the third episode.TBB field of East Asia may well reflect not only the intensity of convection and rainfall,but also the movement of rain belt and convection belt.In the whole Meiyu season.convection belt mainly stays in Jianghuai.but may shift within the domain of East Asia.Its locating in Jianghuai or not determines the maintenance or break of Meiyu.In the third episode,the narrow convection belt over Jianghuai is mainly caused by southwest monsoon which takes moist and convective atmosphere from tropical ocean.     

南海登陆热带气旋对长江三角洲地区降水的影响

利用热带气旋年鉴资料统计了1949~2000年南海热带气旋影响期间我国长三角地区的最大降水量，计算有南海热带气旋影响时最大降水量大于50 mm的条件概率为60%。并把历年引起长三角地区50 mm以上降水的南海气旋频数序列作小波变换，发现其具有周期性振荡的特点，并且振荡周期在频率的分布上也有一定的规律。把南海热带气旋频数序列与夏季3个月副高面积指数之和的序列做交叉谱分析，发现两者具有准2年和准5年的耦合周期，气旋频数序列在位相上超前约半年。最后用1951~1999年的500 hPa月平均高度资料统计频数异常年的500 hPa高度场的距平和t统计量，发现在西太平洋、大西洋和极地都有显著的异常。     

热带太平洋SSWA特征及其在ENSO循环中的演变

应用1970年1月至1989年12月热带太平洋SSTA（海面温度距乎）和SSWA（海面风距平）资料,分析发现：热带太平洋SSWA的主要空间特征和海气耦合主要空间型均表现为ElNino（LaNina）盛期的SSWA分布。热带太平洋SS－WA的主要传播特征与ENSO循环过程中SSWA的演变是一致的,表现为赤道中东太平洋海面阻平东、西风的转换和赤道东太平洋SSWA辐散、辐合的转换。赤道中东太平洋距平东风（西风）向赤道距平西风（东风）的转换伴随着中西太平洋副热带SSWA偶极子气旋（反气旋）的生消,偶极子气旋（及气旋）表现为明显的北半球强于南半球。     

TBB资料揭示的热带大气低频振荡周期的年变化特征

１９８０－１９９２年ＧＭＳＴＢＢ格点资料，分析了１３年平均的ＴＢＢ功率谱随时间的变化，讨论了低频振荡周期的年变化特征及其可能的成因。结果表明，热带大气低频振荡周期有很明显的年变化。北半球热速睦区比赤道附近和南半球热带地区的周期变化幅度更大。总体上，冬季比夏，秋季低频振荡周期更长。     

Inter-decadal shift of the prevailing tropical cyclone tracks over the western North Pacific and its mechanism study

On the basis of observations and results from the combination of a statistical formation model and a trajectory model, the inter-decadal shift of prevailing TC tracks in the western North Pacific (WNP) are examined. The contributions of the changes in large-scale steering flows and tropical cyclone (TC) formation locations to the observed inter-decadal shift are investigated and their relative importance is determined. This study focuses on two periods, 1965–1986 (ID1) and 1987–2010 (ID2), which are determined based on the abrupt change of the annual category 4 and 5 TC frequency derived from the Bayesian change-point detection analysis. It is found that the models can well simulate the primary features of prevailing TC tracks on the inter-decadal timescale. From ID1 to ID2, a significant decrease in the frequency of TC occurrences is observed over the central South China Sea and well simulated by the models. Areas with a remarkable increase in the TC frequency, which extends from the Philippine Sea to the eastern coast of China and in the west of the WNP basin, are also reasonably simulated. Above changes in the prevailing TC tracks are attributed to (1) intensified cyclonic circulation centered over the western part of China and (2) more westward-southward expansion and intensification of the subtropical high over the WNP. Further analysis reveals that the inter-decadal shift in prevailing TC tracks is mainly resulted from the combined effects of changes in large-scale steering flows and TC formation locations. Although both contribute to the inter-decadal shift in the prevailing TC tracks, changes in large-scale steering flows play a more important role compared to changes in TC formation locations.     

Amplification of the solar signal in the summer monsoon rainband in China by synergistic actions of different dynamical responses

A rainband meridional shift index (RMSI) is defined and used to statistically prove that the East Asian summer monsoon rainband is usually significantly more northward in the early summer of solar maximum years than that of solar minimum years. By applying continuous wavelet transform, cross wavelet transform, and wavelet coherence, it is found that throughout most of the 20th century, the significant decadal oscillations of sunspot number (SSN) and the RMSI are phase-locked and since the 1960s, the SSN has led the RMSI slightly by approximately 1.4 yr. Wind and Eliassen–Palm (EP) flux analysis shows that the decadal meridional oscillation of the June rainband likely results from both a stronger or earlier onset of the tropical monsoon and poleward shift of the subtropical westerly jet in high-solar months of May and June. The dynamical responses of the lower tropical monsoon and the upper subtropical westerly jet to the 11-yr solar cycle transmit bottom-up and top-down solar signals, respectively, and the synergistic actions between the monsoon and the jet likely amplify the solar signal at the northern boundary of the monsoon to some extent.     

NUMERICAL STUDY OF THE EFFECTS OF PERSISTENT WARMER SEASURFACE TEMPERATURE IN TROPICAL INDIAN OCEAN ONATMOSPHERIC CIRCULATION IN THE EARLY SUMMERIN EAST ASIA IN 1991

By employing the CCM1（R15L12）long-range spectral model, study is undertaken of the effects of sea surface temperature anomaly(SSTA) for tropical Indian ocean on circulation transformation in the early summer in East Asia in 1991. The results indicate that warmer SSTA contributes to the increasing of the temperature over the Plateau in early summer, resulting in the intensification of tropical easterly jet on 100 hPa and northward shift of Northern Hemisphere subtropical westerly jet in May. It is obviously favorable for the subtropical high enhancement over western Pacific Ocean in May and subtropical westerly jet maintaining at 35～40 °N in June, making the Mei-Yu come earlier and stay over the Changjiang basin in 1991. Furthermore, warmer SSTA is also advantageous to averaged temperature rise in East Asia land region and Nanhai monsoon development. These roles are helpful in accelerating the seasonal transition for East Asia in early summer.     

郑州近54年降水变化的多时间尺度分析

利用Morlet小波分析方法,对1951~2004年郑州市年降水距平时间序列的小波变化特征进行分析,揭示降水变化的多时间尺度结构,分析其中存在的主要周期振荡和突变点,并与功率谱分析结果进行比较。结果表明:郑州市年降水存在多时间尺度的周期变化特征,郑州市年平均降水存在3 a、准5 a、准9 a和准21 a的主周期。郑州市降水变化存在着明显的多时间尺度,即年代际尺度和年际尺度的周期性变化。郑州市降水变化还显示出自己的突变点分布及其位相结构。在降水量总趋势下降的前提下,2004年后的一段时间还将处在一个偏多期。     

Decadal change in relationship between western North Pacific tropical cyclone frequency and the tropical Pacific SST

In this study, we examine the relationship between the number of tropical cyclones (TCs) in the western North Pacific and the tropical Pacific sea surface temperature (SST) during the main TC season (July–November) for the period of 1965–2006. Results show that there are periods when TC frequency and the tropical Pacific SST are well correlated and periods when the relationship breaks down. Therefore, decadal variation is readily apparent in the relationship between the TC frequency and the SST variations in the tropical Pacific. We further examine the oceanic and atmospheric states in the two periods (i.e., 1979–1989 vs. 1990–2000) when the marked contrast in the correlation between the TC frequency and the tropical Pacific SST is observed. Before 1990, the analysis indicates that oceanic conditions largely influenced anomalous TC frequency, whereas atmospheric conditions had little impact. After 1990, there the reverse appears to be the case, i.e., atmospheric conditions drive anomalous TC frequency and oceanic conditions are relatively unimportant. A role of atmosphere and ocean in relation to the TC development in the western North Pacific changes, which is consistent with the change of the correlations between the TC frequency and the tropical Pacific SST.     

WAVELET CHARACTERISTICS OF RAINFALL VARIATION IN THE THREE GORGE REGION OF CHANGJIANG RIVER*

Making the climatic diagnosis of Changjiang Three Gorge Region by the maximum entropy spectrum analysis and wavelet analysis,we discovered that the main periods of the annual and wintertime precipitation variation in the east subregion differ from that in the west subregion.The former is a period of about 2 years,and the latter is about 5-6 years.Their wavelet patterns are also apparently different between two subregions:the alternate positive and negative wavelet patterns for the west,but not so regular for the east.These provide a climatic background for the implementation of Three Gorge Project.We also found that the historical reason of the main periods by combing synthetically spectrum analysis and wavelet analysis could be realized.In addition we suggest a gist of the climate forecast by analogy analysis based on the 2-dimensional wavelet figures.     

福建前汛期持续性强降水的大气低频特征分析

本文采用福建1979—2010年66个气象站逐日降水资料和NCEP再分析资料和OLR资料,揭示了福建前汛期降水的低频特征,分析了福建前汛期持续性强降水同期的大气低频特征以及前期低频信号的演变特征,结果表明:(1)福建前汛期降水存在显著的低频周期,出现频率较高的前三个低频周期分别为:10~20、30~60和20~30 d;约63%的年份出现两种以上显著的低频周期;10~90 d低频变化占前汛期降水总方差的20%~30%;(2)前汛期总雨量与降水低频信号的强度呈显著正相关关系,持续性强降水的强度和持续时间与降水的低频特征关系密切,以BWO(ISO)为主的年份,持续性强降水的持续时间较短(长)。(3)福建前汛期持续性强降水期间日本以东、索马里以东和南海三个关键区皆为低频反气旋,日本至渤海湾南下的冷空气与索马里越赤道气流、南海南部越赤道气流在福建上空持续相遇形成低频气旋,导致福建上空低层低频辐合、高层低频辐散,对流活跃。(4)福建前汛期持续性强降水与热带及副热带大气低频变化密切相关,热带MJO的东传以及东亚西北太平洋地区低频信号的北传对福建持续性强降水过程的延伸期预报具有一定的指示意义。     

江淮下游6～7月雨量趋势的预测

对１９５１～１９９４年６～７月江淮下游（６站平均）雨量距平进行小波变换,结果表明：该区域６～７月雨量存在着两个明显的周期振荡,一个是２８～２９年的长周期,另一个是８～９年的中周期．文中分析了两个周期的不同位相所对应的雨量特征,提出６～７月雨量趋势的预测思路,预测结果与实况对比表明,对雨量偏少年及连续两个干旱年有较好的预测能力     

Interannual and interdecadal variability of ocean temperature along the equatorial Pacific in conjunction with ENSO

In this paper, the leading modes of ocean temperature anomalies (OTA) along the equatorial Pacific Ocean are analyzed and their connection with El Niño-Southern Oscillation (ENSO) and interdecadal variation is investigated. The first two leading modes of OTA are connected with the different phases of the canonical ENSO and display asymmetric features of ENSO evolution. The third leading mode depicts a tripole pattern with opposite variation of OTA above the thermocline in the central Pacific to that along the thermocline in the eastern and western Pacific. This mode is found to be associated with so-called ENSO-Modoki. Insignificant correlations of this mode with the first two leading modes suggest that ENSO-Modoki may be a mode that is independent to the canonical ENSO and also has longer time scales compared with the canonical ENSO. The fourth mode reflects a warming (cooling) tendency above (below) the thermocline since 2000. Both the first and second modes have a large contribution to the interdecadal change in thermocline during 1979–2012. Also, the analysis also documents that both ENSO and OTA shifted into higher frequency since 2000 compared with that during 1979–1999. Interestingly, the ENSO-Modoki related OTA mode does not have any trend or significant interdecadal shift during 1979–2012. In addition, it is shown that first four EOF modes seem robust before and after 1999/2000, suggesting that the interdecadal shift of the climate system in the tropical Pacific is mainly a frequency shift and the changes in spatial pattern are relatively small, although the mean states over two periods experienced some significant changes.     

IMPACT OF LARGE-SCALE CIRCULATION ON THE INTERDECADAL VARIATIONS OF THE WESTERN NORTH PACIFIC TROPICAL CYCLONE

Based on the annual frequency data of tropical cyclones from 1960 to 2005 and by the polynomial fit and statistical analysis, this work has discovered that TC activity in the 46a exhibits significant decadal- scale variability. It has two high frequency periods (HFP) and two low frequency periods (LFP). Significant differences in the number of TCs between HFP and LFP are found in active TC seasons from July to October. Differences of large-scale circulation during HFP and LFP have been investigated with NCEP/NOAA data for the season. In HFP, the condition includes not only higher sea surface temperature,lower sea level pressure, larger divergence of upper air, larger relative vorticity at low levels and smaller vertical shear, but also 500-hPa wind vector being more available for TC activity and moving to western North Pacific, the position of the subtropical anticyclone over the western Pacific shifting more northward,and South Asian Anticyclone at 100-hPa being much smaller than that in LFP. The precipitation of western North Pacific has no clear influence on TC activity.     

东亚夏季风和ENSO关系的不稳定性

通过本项研究，发现了东亚夏季风和ENSO的相互关系在长期变化中是不稳定的。不稳定指的是在一段时期两者关系比较紧密而在另一段时期两者关系比较微弱。文章揭示：在东亚季风和ENSO关系紧密时期（HCP）和关系微弱时期（LCP）夏季大气环流的年际变率有显著差别。在关系紧密时期，南热带东太平洋区的信风、热带东太平洋区的低层大气温度、两个半球的副热带高压系统等的年际变率均显著高于关系微弱时期。并且，HCP和LCP时期中国夏季降水和ENSO的关系也有明显差异。     

登陆中国不同强度热带气旋的变化特征

根据《热带气旋等级》国家标准(2006),将热带气旋(TC)划分为热带低压(TD)、热带风暴(TS)、强热带风暴(STS)、台风(TY)、强台风(STY)、超强台风(SSTY)6个等级,利用中国气象局整编的1949—2006年共58年的《台风年鉴》和《热带气旋年鉴》资料,分析了登陆中国大陆、海南和台湾不同强度TC变化特征。结果表明：(1) 不同强度登陆TC频数存在年际和年代际变化,在长期趋势上,TD、TS登陆频数呈现显著的线性递减趋势,STY登陆频数呈现显著增加趋势。(2) 登陆TD、TS、STS存在6—8年的周期变化,TY具有准16年的周期变化。(3) 登陆TD、TS主要生成于南海东北部海面,登陆TY、STY、SSTY多生成于巴士海峡东南部海面和菲律宾以东洋面。(4) 在年代际变化上,南海生成的登陆TD、TS频数有减少趋势,TY、STY有增多趋势。