CHARACTERISTICS OF FREQUENCY SPECTRUM VARIATION OF INTRASEASONAL OSCILLATION OF CONVECTION DURING SOUTH CHINA SEA SUMMER MONSOON

1 INTRODUCTIONSituated in the region of East Asian Monsoon(EAM), weather and climate in China are deeplyinfluenced by the activities of EAM. Especially inflood seasons, distribution of precipitation, movementof rain-bands and occurrence of droughts and fl…     

CHARACTERISTICS OF FREQUENCY SPECTRUM VARIATION OF INTRASEASONAL OSCILLATION OF CONVECTION DURING SOUTH CHINA SEA SUMMER MONSOON

Datasets of equivalent temperature of black body (TBB) and sea surface temperature (SST) ranging from 1980 to 1997 are used to diagnose and analyze the characteristics of frequency spectrum and strength of intraseasonal variation of convection. The relationship between the strength of intraseasonal oscillation of convection, strength of convection itself and SST in the South China Sea (SCS) is studied. It is shown that, there are distinguishable annual, interannual and interdecadal variations in both strength and frequency spectrum of intraseasonal variation of convection in SCS. There are connections between strength of convection, strength of ISO1 in the summer half (s.h.) year and SST in ensuing winter half (w.h.) year in SCS. The strong (weak) convection and strong (weak) ISO1 are associated with negative (positive) bias of SST in ensuing w.h. year in SCS.     

南海夏季风对流季节内振荡的频谱变化特征

利用1980～1997年TBB资料和海表温度（SST）资料,诊断分析了南海对流季节内变化频谱及强度的变化特征,并探讨对流季节内振荡强度与对流本身强度以及南海SST之间的关系。结果表明,南海对流季节内变化强度及频谱存在明显的年内变化、年际变化和年代际变化特征;南海地区夏半年对流及其ISO1强度与后期冬季南海SST有一定的关系,当夏半年对流较强（弱）,ISO1较强（弱）时,则后期冬季南海SST偏低（高）。     

2004年南海夏季风活动概述

利用NCEP逐日再分析资料、向外长波辐射（OLR）和卫星降水资料,分析了2004年影响南海夏季风活动的一些特征。结果表明,2004年由于西太平洋副热带高压主体位置比正常偏北、偏西,导致南海夏季风爆发日期比正常偏晚（5月19日）、强度偏大,并主要在南海地区活动。南海季风爆发期间存在明显的季节内振荡,有两个显著振荡周期：一个峰值为20-30天,另一个峰值为40～50天。南海夏季风期间,主要的水汽通量输送集中在南海一西太平洋地区,向北到达华南的水汽输送减少,致使华南地区干旱。     

GENERAL SITUATION ABOUT ACTIVITY OF SUMMER MONSOON OVER SOUTH CHINA SEA IN 2004 ASIAN SUMMER MONSOON INDEX

Based on daily NCEP reanalysis data, OLR and satellite rainfall data, the characteristic of the activities of South China Sea summer monsoon(SCSSM) in 2004 were analyzed. The results showed that the establishment of SCSSM was little later than normal and the intensity was stronger than normal. Influenced by the location of the northwest Pacific subtropical high, which was much northward and westward than normal, SCSSM was active mainly in the South China Sea areas. There existed obvious intraseasonal oscillation and two significant periods of SCSSM, one was about 20-30 days and the other about 40-50 days. The transportation of moisture was concentrated on the South China Sea and the northwest Pacific regions, reducing the northward transportation and resulting in drought in southern China     

南海-西北太平洋地区大气准双周振荡对TC生成的调节作用

通过对南海-西北太平洋地区大气10～20 d准双周振荡(QBWO)不同位相的划分(A～D),研究了QBWO对南海-西北太平洋海域热带气旋(TC)生成的调节作用。将TC分为强热带风暴及以下级别(TS)和台风及以上级别(TY),并将QBWO分为干湿位相,发现南海海域生成的TS(TY)在干湿位相的比与西北太平洋海域生成的TS(TY)在干湿位相的比相等,这表明QBWO对TS(TY)生成的调节作用在南海和西太平洋地区可能相同。从A位相到C位相,南海和西北太平洋地区TC的生成频数均逐渐增多,D位相时期,TC生成最少,多数TC发生在QBWO的对流活动湿位相,少数TC发生在干位相。南海-西北太平洋海域TC的生成受到QBWO的明显调制。从位相A到位相C,低频对流和低频风场逐渐向西北方向移动,低频对流强度持续加强,低频风场逐渐由异常西风-东风-西风转为异常东风-西风-东风配置,西北太平洋地区季风槽加强,使得TC生成频数逐渐增多。此外,在QBWO活跃位相,非绝热加热增强和纬向风垂直切变减弱也有利于TC的生成。     

南海-西北太平洋季风槽强度的变化特征及其与热带气旋活动的关系

利用1979—2005年NCEP/DOE AMIP-Ⅱ再分析逐日平均资料,根据西北太平洋季风槽的特点,研制了能较好表征季风槽活动的强度指数和位置指数,并分三段描述不同区域季风槽的活动特征及其对生成于南海-西北太平洋季风槽的热带气旋（MTTC）活动的影响,结果表明：（1） 季风槽强度指数和位置指数呈正相关关系,季风槽强度越强,其位置越偏北。（2） 季风槽强度存在明显的年际变化,1994年前以4～5年的变化周期较为显著,1994年后2～3年的周期较明显。（3） 不同区域季风槽强度的影响因子不同。前期海温场的异常将导致大气环流异常,致使不同区域的季风槽强度异常。（4） 季风槽强度与MTTC频数异常密切相关,3个不同区域的季风槽以南海季风槽强度与MTTC频数异常的关系最为密切。MTTC异常偏少年大多出现在季风槽总体偏弱,各区域季风槽也偏弱的年份;MTTC异常偏多年可能出现在季风槽偏强的年份,也可能出现在季风槽总体偏弱但南海季风槽偏强的年份,且后者出现的概率更大一些。（5） 季风槽强度的季节内变化能为TC的生成和发展提供有利条件,季风槽同时处于30～60天振荡和准双周10～20天振荡的活跃期时,有利于MTTC的生成。     

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.     

IMPACT OF TROPICAL INTRASEASONAL OSCILLATION ON THE TRACKS OF TROPICAL CYCLONES IN THE WESTERN NORTH PACIFIC

In this work,an index of tropical 20-90 d oscillation(intra-seasonal oscillation;ISO)in the western North Pacific(WNP)was determined via the combined empirical orthogonal function(EOF)method using daily outgoing longwave radiation(OLR)field data from the National Oceanic and Atmospheric Administration(NOAA),daily wind field data(at 850 hPa)from the European Centre for Medium-Range Weather Forecasts(ECMWF)and referencing the Madden-Julian oscillation(MJO)index proposed by Wheeler and Hendon.An in-depth investigation was conducted to examine the impact of the ISO on changes in tropical cyclone(TC)tracks in the WNP during different ISO phases.The research results indicate that during the easterly phase of the ISO,under the impact of the northeastern airflow of anti-cyclonic ISO circulation,the easterly airflow south of the western Pacific subtropical high is relatively weak,and TCs generated in the subtropical high tend to change their tracks east of 140°E;during the westerly phase,there is a relatively high probability that TCs change their tracks west of 140°E.This work also analyzed the ISO flow field situation in cases of typhoons and determined that the track of a tropical cyclone will experience a sudden right turn when the center of the ISO cyclonic(anti-cyclonic)circulation coincides with that of the cyclone.     

西北太平洋热带气旋强度变化的统计特征

用中国气象局整编的1949-2003年共55年的《台风年鉴》和《热带气旋年鉴》资料,依据平均值与标准差的数学涵义,给出了TC突然增强、缓慢增强、强度稳定、缓慢减弱和突然减弱的标准,分析了西北太平洋热带气旋(TC)强度变化的年代际、年际、月际、日变化和区域分布的基本特征。结果表明:(1)1960年代以前,T℃的年平均增强或减弱幅度较小。(2)在TC出现较为频繁的夏秋季节,8月份TC强度变幅较小。TC在14时(北京时,下同)最易发展,20时最易减弱;08时TC增强速度最快,02时最慢;02时TC减弱速度最快, 20时最慢。(3)TC频数和增强TC频数的高值区位于海南岛以东的南海北部中国近海区域和菲律宾以东洋面,减弱类TC频数极值区在吕宋岛及其东部海域、海南岛以西的北部湾、广东沿岸。(4)TC突然增强不出现在30°N以北的中高纬地区和0—5°N的低纬地区。TC突然减弱多出现在125°E以西的中国近海大范围海域,在0～5°N的低纬地区基本不出现。     

RESEARCH ON THE INTERANNUAL AND INTERDECADAL VARIABILITIES OF THE MONSOON TROUGH AND THEIR IMPACTS ON TROPICAL CYCLONE GENESIS OVER THE WESTERN NORTH PACIFIC OCEAN

In this paper, we mainly summarize and review the progresses in recent climatological studies (by CMSR, IAP/CAS and some associated domestic and international institutions) on the interannual and interdecadal variabilities of monsoon troughs and their impacts on tropical cyclones and typhoons (TCs) geneses over the western North Pacific Ocean. The climatological characteristics of monsoon troughs and four types of circulation patterns favorable to TCs genesis over the western North Pacific Ocean in summer and autumn are given in this paper. It is also shown in this paper that the monsoon trough over the western North Pacific Ocean has obvious interannual and interdecadal variabilities. Especially, it is revealed in this paper that the interannual and interdecadal variabilities of the monsoon trough over the western North Pacific Ocean influence the TCs genesis not only through the impact on distributions of the vorticity in the lower troposphere and the divergence in the upper troposphere, the water vapor in the mid- and lower troposphere and the vertical shear of wind fields between the upper and lower troposphere over the western North Pacific Ocean, but also through the dynamical effects of the transition between convectively coupled tropical waves and providing disturbance energy. Besides, some climatological problems associated with TCs activity over the western North Pacific Ocean that need to be studied further are also pointed out in this paper.     

夏季西北太平洋热带气旋生成频次与热带海温关系的年代际变化

探讨了夏季(6—8月)西北太平洋(Western North Pacific,WNP)热带气旋生成频次(Tropical Cyclone Genesis Frequency,TCGF)与热带海温关系的年代际变化,发现影响WNP TCGF的热带海温型在1991/1992年发生了年代际变化。在1990年代初之前,TCGF正异常对应的热带海温异常(Sea Surface Temperature Anomaly,SSTA)呈现东部型La Ni?a衰减位相,前冬至春季WNP局地暖SSTA在其西北侧激发气旋异常,夏季时由热带印度洋冷SSTA继续维持。在1990年代初之后,TCGF正异常对应的热带SSTA呈现东部型La Ni?a向中部型El Ni?o快速转换的位相,夏季中太平洋暖SSTA在其西北侧激发气旋异常,同时热带东印度洋至海洋性大陆以及热带大西洋的冷SSTA通过垂直环流圈加强中太平洋的辐合上升运动,进一步维持其西北侧气旋异常。由于激发气旋异常的暖SSTA在第二个年代相较第一个年代明显偏南偏东,气旋异常和TCGF正异常在第二个年代也整体偏南且向东扩展至更远的区域。WNP TCGF与热带海温关系的年代际变化与1990年代初之后厄尔尼诺-南方涛动演变速率加快有关。      

南海夏季风爆发的EOF分析

本文利用1983～1992年的NCEP资料．对南海夏季风爆发做经验正交函数分解，分析了主要模态的时空变换特征。结果表明：太阳辐射北移，是南海夏季风爆发的最重要的因素。南海夏季风爆发前后，在典型季风区850hPa上东西风有一次重大调整。南半球中高纬西风带槽脊振幅的增强和北半球副热带系统经向环流的加大是南海夏季风爆发的重要原因。西太平洋副高的迅速减弱东撤，导致南海夏季风的爆发。     

2007 年南海夏季风季节内振荡的北传及影响因子

利用2007年全球降水气候计划GPCP(the Global Precipitation Climatology Project)卫星红外窗口导出的全球降水指数GPI(the Global Precipitation Index)的日降水资料及频率-波数分析方法,分析2007年南海夏季风季节内振荡(Intraseasonal Oscillation,ISO)的传播特征,并使用美国国家环境预报中心(NCEP)/美国大气研究中心(NCAR)再分析的逐日资料,探讨影响其传播的主要因子.结果表明,南海夏季风ISO有明显的北传趋势,并且明显比南传分量占优.影响南海夏季风ISO北传的主要因子是平均纬向风垂直切变和平均经向风对异常水汽的输送.之所以异常经向风对平均水汽的输送及海-气相可作用的影响在南海地区不重要,而在印度季风区有一定的贡献,是因为平均水汽和纬向风分布在两个地区的差异.     

COMPOSITE ANALYSIS OF SUMMER MONSOON ONSET PROCESS OVER SOUTH CHINA SEA

Based on the method of composite analysis, the onset process and preceding signs of summer monsoon over the South China Sea (SCS) is investigated. The result indicates that convection activities appear first over the Indo-China Peninsula prior to the onset of the monsoon, then around the Philippines just at the point of onset, implying that the convection activities around the Philippines serve as one of the reasons leading to the SCS monsoon onset. Before the SCS monsoon onset, the equatorial westerly over the Indian Ocean (75°E 95°E ) experiences noticeable enhancement and plays an important role on the SCS monsoon onset. It propagates eastward rapidly and causes the establishment and strengthening of equatorial westerly in the southern SCS, on the one hand, it results in the migration southward of the westerly on south side of the south-China stationary front by means of shift northeastward of the westerly and convection over the Bay of Bengal, on the other. Further study also shows that the intensification of equatorial westerly in the Indian Ocean (75°E 95°E) and the southern SCS is closely related to the reinforcement of the Southern-Hemisphere Mascarene high and Australian high, and cross-equatorial flow northward around Somali, at 85°E and 105°E, respectively.     

SURFACE OBSERVATIONS IN THE TROPICAL CYCLONE ENVIRONMENT OVER THE SOUTH CHINA SEA

In this paper,the observational data from Marine and Meteorological Observation Platform(MMOP)at Bohe,Maoming and buoys located in Shanwei and Maoming are used to study the characteristics of air-sea temperature and specific humidity difference and the relationship between wind and wave with the tropical cyclones over the South China Sea(SCS).The heat and momentum fluxes from eddy covariance measurement(EC)are compared with these fluxes calculated by the COARE 3.0 algorithm for Typhoon Koppu.The results show that at the developing and weakening stages of Koppu,both these differences between the sea surface and the near-surface atmosphere from the MMOP are negative,and data from the buoys also indicate that the differences are negative between the sea surface and near-surface atmosphere on the right rear portion of tropical cyclones(TCs)Molave and Chanthu.However,the differences are positive on the left front portion of Molave and Chanthu.These positive differences suggest that the heat flux is transferred from the ocean to the atmosphere,thus intensifying and maintaining the two TCs.The negative differences indicate that the ocean removes heat fluxes from the atmosphere,thus weakening the TCs.The wind-wave curves of TCs Molave and Chanthu show that significant wave height increases linearly with 2-min wind speed at 10-m height when the wind speed is less than 25 m/s,but when the wind speed is greater than 25 m/s,the significant wave height increases slightly with the wind speed.By comparing the observed sensible heat,latent heat,and friction velocity from EC with these variables from COARE 3.0 algorithm,a great bias between the observed and calculated sensible heat and latent heat fluxes is revealed,and the observed friction velocity is found to be almost the same as the calculated friction velocity.     

1998年南海西南季风活动的初步分析

利用NCEP再分析资料和OLR、SST观测数据,分析了1998年南海西南季风的建立日期、强度的多时间尺度变化特征、与海面温度的相互作用以及对广东降水的影响.得出南海西南季风建立的日期为5月17日(5月4候).1998年为弱季风年,OLR具有1个月左右的振荡周期,西南风具有半个月左右的振荡周期.孟加拉湾地区季风和105°E越赤道气流是南海季风低频变化的重要策源地.1998年南海季风弱,主要是由于初春赤道东太平洋海温正距平,并导致南海-阿拉伯海海温正距平的结果.     

THE CHARACTERISTICS OF LOW-FREQUENCY OSCILLATIONS OVER THE SOUTH CHINA SEA

Using 1975-1993 (with 1978 missing) data of the outgoing longwave radiation (OLR), characteristics of seasonal variation of low-frequency oscillations in the South China Sea and its relation to the establishment and activity of the summer monsoon there are studied. As is shown in the result, the low-frequency oscillation in the South China Sea is much stronger in the period of summer monsoon than in that of winter monsoon and the summer monsoon there usually begins to set up in a negative phase of the first significant low-frequency oscillation for the early summer. The study also reveals that the circulation for the low-frequency oscillation during the summer monsoon in the Sea is embodied as north-south fluctuations of the ITCZ and east-west shifts of western ridge point of the West Pacific subtropical high, suggesting close correlation between the low-frequency oscillation and the active and break (decay) of the South China Sea monsoon. In the meantime. the work illustrates how the low-frequency oscillation in the South China Sea are superimposed with the seasonal variation of the general circulation. so that the summer inonsoon covers the establishment of the I^st, intensification of the 2^nd and 3^rd the low-frequency oscillations and decay of the 4^th oscillation.     

西北太平洋夏季风的气候学研究

西北太平洋季风区(5°~25°N,120°~160°E)是亚洲-太平洋季风区与亚洲-澳大利亚季风区的一个重要部分,也是一个独立的季风区。文中对西北太平洋夏季风进行了气候学研究,发现西北太平洋夏季风的爆发发生在31候(6月上旬)前后。爆发后,夏季风经历了3次活跃-中断循环,每个循环都相应于独立的干期和湿期。第1个循环是从6月初到7月中旬,降水与低层西风的中心主要位于5°~10°N。第2个循环是7月下旬至9月下旬,降水与低层西风明显向东北方向移动,位于10°~20°N。这个循环是西北太平洋夏季风最强盛的时期。第3个循环是从9月末到10月末,降水与低层西风又向南退回到5°~10°N,达到了西北太平洋夏季风最弱的阶段。这个循环的结束也就预示着西北太平洋夏季风的结束。西北太平洋季风区有明显的季节内振荡(ISO),这种气候的季节内振荡(CISO)主要由30~60 d与10~20 d两种周期组成,但是主要以30~60 d的低频振荡为主。根据西北太平洋的对流和低层西风在不同位相分布的分析,可以看出西北太平洋的低频对流和西风是向西向北传播的。西北太平洋的季风降水、对流与西风的活跃-中断循环在很大程度上受30~60和10~20 d低频振荡的调制。     

CONNECTION OF THE SOUTH CHINA SEA SUMMER MONSOON TO MARITIME CONTINENT CONVECTION AND ENSO

The relationship between the intensity of the South China Sea summer monsoon (SCSSM) and the Nino3.4 index and anomalous atmospheric circulation patterns associated with a strong and weak SCSSM are investigated using the NCEP/NCAR reanalysis data, Extended Reconstructed Sea Surface Temperature (ERSST) data and Climate Prediction Center Merged Analysis of Precipitation (CMAP) data. The SCSSM is significantly positively correlated with the Nino3.4 index in the succeeding northern autumn and winter. In the strong minus weak SCSSM composite, a positive East Asia-Pacific teleconnection (EAP) pattern and a negative Europe-Asian-Pacific teleconnection (EUP) pattern appear in the 500 hPa height difference field; low-level cross-equatorial flows are strengthened over the Maritime Continent (MC) region; positive (negative) precipitation anomalies occur in the South China Sea and western north Pacific (MC). A possible mechanism through which SCSSM affects ENSO is proposed. A strong (weak) SCSSM strengthens (weakens) cross-equatorial flows over the MC. The anomalous cross-equatorial flows cool (warm) the SST around the MC through enhanced (reduced) surface latent heat fluxes. The cooling (warming) further leads to suppressed (enhanced) convection over the MC, and causes the anomalous westerly (easterly) in the equatorial western Pacific, which favors the onset of El Ni?o (La Ni?a) through modulating the positive air-sea feedback process.