Annual and interannual variations of large-scale circulation over the tropics as revealed from OLR data

In this study, the outgoing longwave radiation (OLR) data of eight years have been used to investigate the annual and interannual variations of large-scale circulation over the tropics. The results of EOF analysis indicate that the first three eigenvectors reflect the essential features in OLR annual variation. However, they contribute little to the interannual change. In comparison, the fourth to the sixth components have very large year-to-year variations. The pronounced anomalies are located over the tropics with east-west dipole pattern. It implies that ENSO events are related closely to the variation of the Walker circulation.     

CHANGE OF CLIMATE AND ITS INFLUENCE ON THE CROPPING SYSTEM IN CHINA

Tne global change of climate and its influence on the cropping system in China have been investigatedin this paper.It is found that the temperature was increased during the last decade and the precipitationdecreased in northern China and increased in southern China during the last 30 years.The sea level hasbeen rising by about 21—26 cm in the coastal areas south of 30°N in China during the last 100 years.The most of results as simulated by the general circulation models(GCMs)show that the temperature increasewould amount to about 2°—4°C in the most parts of China and precipitation and soil moisture might bedecreased in northern China and increased in sourthern China due to doubling of carbon dioxide(CO_2).The effects of doubled CO_2 on growth period and climatic yield capability in China have been estimatedroughly.It is shown that the regions of the growth period in China would be moved northward about fivedegrees latitude and the climatic yield capability might be increased by about 10％ in the most parts of China.     

NUMERICAL EXPERIMENTS ON THE EFFECT OF QINGHAIXIZANG PLATEAU SNOW COVER ON SUMMER MONSOON FORMATION

The calculating schemes of underlying surface processes in the model described by Li et al.(1989)are modified with inclusion of simple land surface processes and oceanic mixed layer processes,then a simulation on the zonal wind along 90°E from the Northern to the Southern Hemisphere with moun-tains is performed.Comparisons of the results and the observations show that the modified model not onlyhas an excellent stability in calculation but also can better display the seasonal change of the wind field,theability of the present model is improved as compared with that of the previous one.Based on the simulations,the authors investigate the effects of Qinghai-Xizang Plateau snow cover on theformation of South Asian monsoon by thickcning the snow depth and by increasing the snow albedo.Themain results arc as follows:The summer meridional circulation over the south of the Plateau and its vicinityis weakeued,and the precipitation reduced.However,over the northern tropics,the circulation is enhanced,and the ecipitation is increased,and the land and the air above it become warmer,the tropical easterly jetis weakened.     

VARIATIONS OF 30—60 DAY OSCILLATION IN ATMOSPHERE BEFORE AND DURING 1982 EL NINO*

By using the upper-wind data from July 1980 to June 1983,the variations of the low-frequency oscillation(LFO) in the atmosphere before and during 1982 El Nino have been investigated.Before the El Nino,the LFO propagates from west to east over the equator of the Eastern Hemisphere and from east to west over 20°N.The eastward propagating LFO over the equator consists of zonal wavenumber 1 propagating eastward and zonal wavenumber 2 with a character of stationary wave.The oscillation of zonal wavenumber 2 can modulate the oscillation strength.After the onset of the El Nino,the propagating directions of the LFO over the equator and 20°N of the Eastern Hemisphere change to be westward and eastward,respectively.The LFO over the western Pacific weakens rapidly and one coming from middle and high latitudes propagates to the equator.From the phase compositions of streamline fields for the zonal wavenumber 1 of equatorial westward propagatirg LFO,it is found that the atmospheric heat source in the equator of the eastern Pacific(EEP)excites a series of the equatorial cyclones and anticyclones which move northward and westward and form the westward propagating LFO over the equator.With the wavelength of 20000km,this kind of equatorial wave is similar to the mixing Rossby-gravity wave.In its westward and northward movement,the circulation in East Asia is modified.This may be the mechanism of the influence of El Nino on the climate of China.     

The atmospheric heat budget in summer over Asia monsoon area

For better understanding the mechanism of monsoon formation and designing the numerical simulation of the general atmospheric circulation, a new approach of calculating atmospheric radiation is proposed to investigate the distribution of the atmospheric heat source, and the budget of heat component is recalculated. The results show that there is a tremendous atmospheric heat source region over central India, northeast of the Bay of Bengal, east of the South China Sea and about 10 °N at the west Pacific, among which the heating center with a maximum heating rate of 8 ℃/day is located over the Bay of Bengal and the average rate in the Plateau is about 1 ℃/day.     

THE ASIAN SUMMER MONSOON AND ITS RELATIONS TO THE RAINFALL IN CHINA

In this paper, four problems are discussed: (1) the monsoon circulation over southern Asia; (2) the seasonal variation of the general circulation of the atmosphere; (3) the influence of the monsoon on the rainfall in southwestern China; and (4) the source region of water vapor for the rainfall in the Changjiang valley.     

The medium-range variations of the summer monsoon circulation system over east asia

The Asian monsoon circulation system can be divided into two subsystems, i.e., the East Asian monsoon system(EA MS) and the Indian monsoon system (IMS). In this paper the main elements including the Indian monsoon trough, the South Asian high. the upper easterly jet etc. and the interactions between EAMS and IMS arc dealt with. The basic emphasis is put on the medium-range variations of the EAMS. Some significant results arc obtained.     

亚洲上空夏季平均环流的结构及其热源分析

本文运用Boogaard的资料(对国内部分经过补充订正)和用连续方程计算了亚洲地区各层的垂直运动场。另外设计了一个间接计算热源的方法并计算了该地区的大气热源。在此基础上对亚洲地区夏季(7月)平均环流结构进行了研究。其主要结果如下:(1)在30°N以南地区西太平洋副热带高压的下沉主要来自东侧高空气流,也来自南侧的Hadley环流的上升支和高原及大陆上升气流,后两者均在300mb以下。而30°N以北,高原及大陆上升气流除了部分下沉于西太平洋地区外,主要在太平洋中部槽槽后下沉。(2)亚洲夏季(7月)大气热源中心在孟加拉湾北岸,而不在西藏高原的上空。     

CHARACTERISTICS OF SEASONAL VARIATION OF RAINFALL OVER THE TIBETAN PLATEAU DURING SUMMER 1998 AND ITS IMPACT ON EAST ASIAN WEATHER*

The seasonal variation of rainy season over the Tibetan Plateau in summer 1998 is analyzed by using daily observational rainfall data for Lhasa from 1955 to 1996,and rainfall data at 70 stations from January to August of 1998 over the Tibetan Plateau (TP) and adjacent regions,as well as TBB data from May to August of 1998.The onset date of rainy season for Lhasa is climatologically 6 June.Among the analyzed years,the earliest onset date is 6 May,while the latest may delay to 2 July.The obvious inter-decadal variation can be found in the series of onset date.The onset date of summer 1998 over middle TP (onset date of Lhasa) is 24 June,which is relatively later than the normal case.The onset for rainy season of 1998 started over southeast and northeast parts of TP and then propagated westward and northward.The convection over east and west parts of TP shows that there is a quasi 12-15 day oscillation.In June,the convection over middle and lower reaches of Yangtze River is formed by the westward propagation of convection over subtropical western Pacific.while in July.it is formed by the eastward propagation of convection over TP.Besides,it is also found that there exists good negative and obvious advance and lag correlation between the convection over the middle and western TP and that over the subtropical western Pacific and southern China.Therefore it can be inferred that a feedback zonal circulation with a quasi two-three week oscillation exists between the ascending region of TP and descending region of subtropical western Pacific,i.e.the convection over TP may affect the subtropical high over western Pacific and vice versa.     

夏季青藏高原东部降水变化与副热带高压带活动的研究

用青藏高原上常规的台站资料和NCEP/NCAR的再分析资料,分析和研究了1993/1994年夏季5—8月青藏高原东部降水变化与西太平洋副热带高压南北移动的关系。结果表明:1993年夏季的副热带高压异常偏南,5—8月副高脊线北移过程中,还伴有准双周南北移动过程;而1994年夏季的副热带高压异常偏北,5—8月副高脊线北移过程中,则主要伴有30—60d的低频变化的南北移动过程。同时,在分析青藏高原上的天气变化特征时,发现这两年高原东部的降水变化特征也有明显的不同,夏季高原上降水的活跃和中断与副高脊线南北移动变化有类似的特征。因此,夏季西太平洋副热带高压南北移动可能与高原东部降水的中断/活跃有一定的关系。