The relationship between anomalies of interannual net radiation and 37 GHz polarization difference over North Africa

We show evidence that interannual variations in the net radiation balance at the top-of-atmosphere over North Africa may be linked to variations at the land surface within the Sahelian zone. By utilizing concurrent monthly time series of earth radiation budget and passive microwave measurements obtained from the Nimbus 7 satellite over a five year period from 1979 to 1983, we have examined the spatial distribution of the amplitude of interannual variability of the net radiation at the top of the atmosphere. A strong association is found between the radiation budget anomalies derived from ERB-WFOV measurements and the anomalies of 37 GHz polarized brightness temperature difference derived from SMMR measurements. Previous research has shown that the interannual anomalies in the top-of-atmosphere net radiation balance in the Sahelian zone mostly arise from changes taking place at the surface, not from variations in cloudiness which brings variable rainfall throughout the African Sahel. Since microwave polarization difference is related in part to vegetation cover, we suggest that the main control of the modulation of North Africa's radiation balance may be year to year fluctuations of vegetation cover within the Sahelian zone, likely stemming from fluctuations in the rainfall. This relationship is relevant to the problem of radiation control and feedback on global climate through processes taking place within the biosphere of a semi-arid zone.     

大洋间SST遥联与亚太夏季风异常的关系

用奇异值分解(Singular Value Decomposition,SVD)方法,给出了四季年代际和年际时间尺度上北大西洋和北太平洋海表温度(Sea Surface Temperature,SST)的显著遥相关.用SVD主模态时间系数构造了海温异常指数Ⅰ,分析了它们与同期亚太夏季风和我国东部夏季降水异常的关系.结果表明:两大洋间的SST遥联在年际、年代际时间尺度上都与亚太夏季风相关,其中,年际尺度的两大洋SST遥联与长江流域的降水存在显著相关.     

SST variability in the South Indian Ocean and associated circulation and rainfall patterns over Southern Africa

Summary A general circulation model is used to study the response of the atmosphere to an idealised sea surface temperature (SST) anomaly pattern (warm throughout the southern midlatitudes, cool in the tropics) in the South Indian Ocean region. The anomaly imposed on monthly SST climatology captures the essence of patterns observed in the South Indian Ocean during both ENSO events and multidecadal epochs, and facilitates diagnosis of the model response. A previous study with this anomaly imposed in the model examined differences in the response between that on the seasonal scale (favours enhancement of the original SST anomaly) and that on the decadal scale (favours damping of the anomaly). The current study extends that work firstly by comparing the response on the intraseasonal, seasonal and interannual scales, and secondly, by assessing the changes in the circulation and rainfall over the adjoining African landmass.It is found that the atmospheric response is favourable for enhancement of the original SST anomaly on scales up to, and including, annual. However, as the scale becomes interannual (i.e., 15–21 months after imposition of the anomaly), the model response suggests that damping of the original SST anomaly becomes likely. Compared to the shorter scale response, the perturbation pressure and wind distribution on the interannual scale is shifted poleward, and is more reminiscent of the decadal response. Winds are now stronger over the warm anomaly in the southern midlatitudes suggesting enhanced surface fluxes, upper ocean mixing, and consequently, a damping of the anomaly.Examination of the circulation and rainfall patterns indicates that there are significant anomalies over large parts of southern Africa during the spring, summer and autumn seasons for both short (intraseasonal to interannual) and decadal scales. It appears that rainfall anomalies are associated with changes in the advection of moist tropical air from the Indian Ocean and its related convergence over southern Africa. Over eastern equatorial Africa, the austral autumn season (the main wet season) showed rainfall increases on all time scales, while parts of central to eastern subtropical southern Africa were dry. The signals during summer were more varied. Spring showed generally dry conditions over the eastern half of southern Africa on both short and decadal time scales, with wet areas confined to the west. In all cases, the magnitude of the rainfall anomalies accumulated over a 90 day season were of the order of 90–180 mm, and therefore represent a significant fraction of the annual total of many areas. It appears that relatively modest SST anomalies in the South Indian Ocean can lead to sizeable rainfall anomalies in the model. Although precipitation in general circulation models tends to be less accurately simulated than many other variables, the model results, together with previous observational work, emphasize the need for ongoing monitoring of SST in this region.With 14 Figures     

Warm and cold events in the southeast Atlantic/southwest Indian Ocean region and potential impacts on circulation and rainfall over southern Africa

Summary An important pattern of interannual variability in the southern African region is one where sea surface temperature (SST) in neighbouring waters, particularly in the Agulhas Current, its retroflection region and outflow across the southern midlatitudes of the Indian Ocean, is anomalously warm or cool. Evidence exists of significant rainfall anomalies over large parts of southern Africa during these warm or cool SST events. Here, a general circulation model is used to study the response of the atmosphere in the region to an idealised representation of these SST anomalies. The induced atmospheric circulation and precipitation anomalies over the adjacent southern African landmass on intraseasonal through to interannual time scales are investigated.A nonlinear response to the SST anomalies is found in that the changes to the model atmosphere when warm SST forcing is used are not the reverse (in either pattern or magnitude) to that when cold SST forcing is imposed. For the warm SST anomaly, it is found that the atmospheric response is favourable for enhancement of the original SST anomaly on scales up to, and including, annual. However, as the scale becomes interannual (i.e., 15–21 months after imposition of the anomaly), the model response suggests that damping of the original SST anomaly becomes likely. However, no such coherent timescale dependent response is found when the cold SST anomaly is impose. It is suggested that the relationship of the SST anomaly to the background seasonal climatology may help explain this fundamental difference in the response.Examination of the circulation and rainfall patterns under warm SST forcing indicates that there are significant anomalies over large parts of southern Africa on all scales from intraseasonal through to interannual. On the south coast, rainfall anomalies result from enhanced evaporation of moisture off the SST anomaly. Over the interior, changer in the convergence of moist air streams together with suggestions of a shift in the Walker circulations between southern Africa and the bordering tropical South Atlantic and Indian Oceans appear to be associated with the rainfall anomalies. Similar mechanisms of rainfall perturbation are found when the cold SST anomaly is imposed; however, there is a significant response only on intra-annual to interannual scales. In all cases, the magnitude of the rainfall anomalies accumulated over a 90 day season were of the order of 90–180 mm, and therefore represent a significant fraction of the annual total of many areas. These model results re-inforce previous observational work suggesting that SST anomalies south of Africa, particularly in the retroflection region of the Agulhas Current, are linked with significant rainfall anomalies over the adjacent subcontinent.With 12 Figures     

土壤湿度初始异常对东亚区域气候模拟影响的敏感性试验

利用耦合了CLM3.5陆面模式的区域气候模式RegCM4.0,通过敏感性试验,探讨了人为减小春季初始土壤体积水含量对短期时间尺度东亚夏季气候模拟的可能影响。结果表明:较低的初始土壤湿度场能够明显改变区域的地表能量平衡,引起地表净长波辐射和感热通量的显著增加,进而加强了地表对大气的加热,因而引起东亚大范围地区特别是中国东部、印度北部和中亚地区地表温度、气温的升高。与气温不同,初始土壤湿度场对降水的影响很小而且有较大的不确定性,同时偏暖的下垫面使得对流层中高层出现暖高压异常,但这些影响均不显著。综合来看,土壤湿度初始场的初始异常,对RegCM4.0 模式东亚气候模拟的结果有一定影响,特别是在地表温度、气温和能量平衡方面,应在以后的模拟中加以考虑。     

Numerical simulation of the impact of vegetation index on the interannual variation of summer precipitation in the Yellow River Basin

Two sets of numerical experiments using the coupled National Center for Environmental Prediction General Circulation Model （NCEP/GCM T42L18） and the Simplified Simple Biosphere land surface scheme （SSiB） were carried out to investigate the climate impacts of fractional vegetation cover （FVC） and leaf area index （LAI） on East Asia summer precipitation, especially in the Yellow River Basin （YRB）. One set employed prescribed FVC and LAI which have no interannual variations based on the climatology of vegetation distribution; the other with FVC and LAI derived from satellite observations of the International Satellite Land Surface Climate Project （ISLSCP） for 1987 and 1988. The simulations of the two experiments were compared to study the influence of FVC, LAI on summer precipitation interannual variation in the YRB. Compared with observations and the NCEP reanalysis data, the experiment that included both the effects of satellite-derived vegetation indexes and sea surface temperature （SST） produced better seasonal and interannual precipitation variations than the experiment with SST but no interannual variations in FVC and LAI, indicating that better representations of the vegetation index and its interannual variation may be important for climate prediction. The difference between 1987 and 1988 indicated that with the increase of FVC and LAI, especially around the YRB, surface albedo decreased, net surface radiation increased, and consequently local evaporation and precipitation intensified. Further more, surface sensible heat flux, surface temperature and its diurnal variation decreased around the YRB in response to more vegetation. The decrease of surface-emitting longwave radiation due to the cooler surface outweighed the decrease of surface solar radiation income with more cloud coverage, thus maintaining the positive anomaly of net surface radiation. Further study indicated that moisture flux variations associated with changes in the general circulation also contributed to the precipitation interannual variation.     

Radiative energy partition and cloud radiative forcing at a Po valley site

Values of downward and upward flux densities of solar and terrestrial radiation were continuously recorded between 1 December 2001 and 30 November 2002 using a four-components radiometer at S. Pietro Capofiume (SPC) in northern Italy (44°39′N, 11°37′E, alt. 11 m a.m.s.l.), which is characterized by a weakly-reflective surface. The aim of the study was to investigate the effects of clouds on surface radiation balance (SRB); the cloud fraction (N) has been retrieved through the inverted form of the parameterization proposed by Kasten and Czeplak [Solar Energy 24 (1980) 177] and cloud types estimated following the methodology of Duchon and O'Malley [J. Appl. Meteorol. 38 (1999) 132]. The cloud radiative forcing (CRF) was evaluated through the Bintanja and Van den Broeke [Int. J. Climatol. 16 (1996) 1281] formula and then associated with cloud type. Experimental results showed that during the measuring period the net shortwave (Sw) balance always decreased with increasing N, whereas the net longwave (Lw) balance increased in all seasons. The net radiation available at the surface decreased with increasing N in all seasons except in winter, where no significant dependency was detected.The analysis of the cloud radiative forcing indicates that all seasons were characterized by a net cooling of the surface except winter, where clouds seem to have no effects on the surface warming or cooling. Taking into account the dependence on solar radiation cycle, an intercomparison between the retrieved cloud types seems to indicate that the effect of stratus was a slight cooling whereas that of cumulus clouds was a stronger cooling of the surface. On the contrary, cirrus clouds seem to have slight warming effect on the surface.The annual trends of mean monthly values of shortwave and longwave radiation balances confirmed that the measurement site is characterized by a temperate climate. Moreover, in spite of the cooling effect of clouds, a monthly radiative energy surplus is available all year long for surface–atmosphere energy exchanges. The analysis is also instrumental for the detection of SRB variations.     

Sensitivity of the Earth’s radiation budget to interannual variations in cloud amount

 Using two pairs of coincident long-term satellite derived cloud and earth radiation budget data sets (Nimbus-7 ERB/Nimbus-7 Cloud Climatology and ERBE Scanner/ISCCP-C2), estimates are made of the sensitivity of the top of the atmosphere radiation budget to interannual variations in the total cloud amount. Both sets of analyses indicate that the largest net warming due to interannual cloud cover changes occurs over desert regions, while the largest net cooling occurs in areas of persistent marine stratiform cloud. There is generally a large amount of cancellation between the large shortwave cooling and longwave warming effects in tropical convection regions. However, the Nimbus-7 analysis identifies an area of net warming in the tropical eastern Pacific Ocean which is shown to be associated with the 1982–83 ENSO event. In the zonal mean the Nimbus-7 data sets indicate that interannual cloud cover changes lead to a net warming at low latitudes and net cooling polewards of 25° in both hemispheres. In contrast, the analysis of the ERBE and ISCCP data sets indicates net cooling everywhere except for the Northern Hemisphere equatorwards of 20 °N. For the spatial average between 60 °N and 60 °S the ratio of the shortwave and longwave effects is 0.94 in the Nimbus-7 analysis (i.e. clouds cause a small net warming) and 1.21 in the ERBE-ISCCP analysis (i.e. a net cooling). Given their improved spatial and temporal sampling the analysis using the ERBE and ISCCP data sets should be the more reliable. However, the large differences between the two analyses still raises some issues concerning the confidence with which the sign of the effect of clouds on the radiation budget at these time scales is currently known. Received: 24 October 1995 / Accepted: 8 August 1996     

华北汛期降水量变化中年代际和年际尺度的分离

华北地区汛期(7月和8月)降水量的变化中,不仅具有年际变化的特征,同时还具有明显的年代际变化特征.我们将这两种时间尺度进行分离,即将降水量的变化进行5年滑动平均,以滑动平均值代表年代际变化,并以每年的汛期降水量与滑动平均值的差代表去除了年代际变化后的年际变化.然后,利用NCAR/NCEP再分析资料对与这两种不同时间尺度相关联的大气环流异常进行了分析,发现两者具有不同的表现,说明影响这两种不同时间尺度的物理原因可能是不同的.与年代际变化相对应的大气环流异常主要表现为出现在我国东部地区的南北风异常以及欧亚大陆上空的位势高度异常.若对这两种时间尺度不进行分离,而直接按讨论年际变化的一般方法简单地分析与年际变化相关联的大气环流异常时,所得到的分析结果中既包含与华北降水年代际变化相关联的大气环流异常,又包含与去除年代际变化后的年际变化相关联的大气环流异常.而除去年代际变化后,与华北汛期降水年际变化相关联的大气环流异常主要表现为在东亚上空位势高度和纬向风的异常,意味着急流在涝年位置偏北.     

Impact of clouds on the surface radiation balance of the Arctic Ocean

Summary The relationship between clouds and the surface radiative fluxes over the Arctic Ocean are explored by conducting a series of modelling experiments using a one-dimensional thermodynamic sea ice model. The sensitivity of radiative flux to perturbations in cloud fraction and cloud optical depth are determined. These experiments illustrate the substantial effect that clouds have on the state of the sea ice and on the surface radiative fluxes. The effect of clouds on the net flux of radiation at the surface is very complex over the Arctic Ocean particularly due to the presence of the underlying sea ice. Owing to changes in surface albedo and temperature associated with changing cloud properties, there is a strong non-linearity between cloud properties and surface radiative fluxes. The model results are evaluated in three different contexts: 1) the sensitivity of the arctic surface radiation balance to uncertainties in cloud properties; 2) the impact of interannual variability in cloud characteristics on surface radiation fluxes and sea ice surface characteristics; and 3) the impact of climate change and the resulting changes in cloud properties on the surface radiation fluxes and sea ice characteristics.With 11 Figures     

Climate predictability on interannual to decadal time scales: the initial value problem

Any initial value forecast of climate will be subject to errors originating from poorly known initial conditions, model imperfections, and by "chaos" in the sense that, even if the initial conditions were perfectly known, infinitesimal errors can amplify and spoil the forecast at some lead time. Here the latter source of error is examined using a "perfect model" approach whereby small perturbations are made to a coupled atmosphere-ocean general circulation model and the spread of nearby model trajectories, on time and space scales appropriate to seasonal-decadal climate variability, is measured to assess the lead time at which the error saturates. The study therefore represents an estimate of the upper limit of the predictability of climate (appropriate to the initial value problem) given a perfect model and near perfect knowledge of the initial conditions. It is found that, on average, surface air temperature anomalies are potentially predictable on seasonal to interannual time scales in the tropical regions and are potentially predictable on decadal time scales over the ocean in the North Atlantic. For mid-latitude surface air temperature anomalies over land, model trajectories rapidly diverge and there is little sign of any potential predictability on time scales greater than a season or so. For mean sea level pressure anomalies, there is potential predictability on seasonal time scales in the tropics, and for some global scale annual-decadal anomalies, although not those associated with the North Atlantic Oscillation. For precipitation, the only potential for predictability is for seasonal time anomalies associated with the El-Niño Southern Oscillation. For the majority of the highly populated regions of the world, climate predictability on interannual to decadal time scales based in the initial value approach is likely to be severely limited by chaotic error growth. It is found however that there can be cases in which the potential predictability can be higher than average indicating that there is perhaps some utility in making initial value forecasts of climate in those regions which show low predictability on average.     

我国东部梅雨期降水的年际和年代际变化特征及其与大气环流和海温的关系

基于我国160站59年(1951～2009年)的月降水观测资料、美国气象环境预报中心和国家大气研究中心(NCEP/NCAR)提供的再分析资料和Hadley中心的海表温度(Sea Surface Temperature,简称SST)资料,对我国东部(100°E以东,15°N～40°N)梅雨期(6月和7月)降水的时空变化特...     

华南黑碳气溶胶浓度与南海夏季风关系的年代际突变

利用重建的华南区域黑碳气溶胶(Black Carbon,BC)浓度资料,分析其与南海夏季风在年际尺度上的关系.结果 表明,华南区域BC浓度与南海夏季风的关系在2000年前后有明显的突变,由显著负相关变为显著正相关,即由高BC浓度弱季风变为高BC浓度强季风.通过合成对比分析,发现1988-1999年(第一时间段)的华南B...     

与夏季北太平洋副热带中尺度海洋涡旋相联系的海气关系

采用高分辨率卫星和再分析资料,利用涡旋探测技术、滤波和合成分析等方法,对夏季北太平洋副热带地区中尺度海洋涡旋与大气的耦合关系进行了分析。结果表明:在日时间尺度上,海洋涡旋的海表温度（Sea SurfaceTemperature,简称SST）与海表风速之间不仅存在同位相的正相关关系,还存在反位相的负相关关系,即在涡旋这种中尺度上既存在海洋对大气的强迫,也存在大气对海洋的强迫。海表风速与SST同位相时,对暖（冷）涡来说,向上（下）的净热通量增强,云和降水增多（减少）;其海水温度异常和海流旋度较强,暖（冷）涡较为深厚,一定程度上表明了海洋对大气的强迫。海表风速与SST反位相时,对暖（冷）涡而言,当其处在正（负）位势高度异常、中低层相对湿度较小（大）、气温较高（低）的大气配置下,海表风速较小（大）;同时向下（上）净热通量增强,云和降水减少（增多）;涡旋海水温度异常和海流旋度较弱,这种暖（冷）涡较为浅薄;表明晴空（阴雨）条件下有利于暖（冷）涡的维持,一定程度上反映了大气对海洋的强迫作用。     

Characteristics of the Asian–Pacific Oscillation in Boreal Summer Simulated by BCC_CSM with Different Horizontal Resolutions

The summer Asian–Pacific Oscillation(APO) is a major teleconnection pattern that reflects the zonal thermal contrast between East Asia and the North Pacific in the upper troposphere. The performance of Beijing Climate Center Climate System Models(BCC CSMs) with different horizontal resolutions, i.e., BCC CSM1.1 and BCC CSM1.1(m), in reproducing APO interannual variability, APO-related precipitation anomalies, and associated atmospheric circulation anomalies, is evaluated.The results show that BCC CSM1.1(m) can successfully capture the interannual variability of the summer APO index. It is also more capable in reproducing the APO's spatial pattern, compared to BCC CSM1.1, due to its higher horizontal resolution. Associated with a positive APO index, the northward-shifted and intensified South Asian high, strengthened extratropical westerly jet, and tropical easterly jet in the upper troposphere, as well as the southwesterly monsoonal flow over North Africa and the Indian Ocean in the lower troposphere, are realistically represented by BCC CSM1.1(m), leading to an improvement in reproducing the increased precipitation over tropical North Africa, South Asia, and East Asia, as well as the decreased precipitation over subtropical North Africa, Japan, and North America. In contrast, these features are less consistent with observations when simulated by BCC CSM1.1. Regression analysis further indicates that surface temperature anomalies over the North Pacific and the southern and western flanks of the Tibetan Plateau are reasonably reproduced by BCC CSM1.1(m), which contributes to the substantial improvement in the simulation of the characteristics of summer APO compared to that of BCC CSM1.1.     

The Oscillation between Tropical Indian Ocean and North Pacific:Evidence and Possible Impact on Winter Climate in China

This paper provides evidence that the variation of boreal winter sea level pressure (SLP) over the North Pacific is out-of-phase with SLP fluctuation over the tropical Indian Ocean on both the interdecadal and interannual time scales.Subsequently,a SLP between tropical Indian Ocean and North Pacific (TIO-NP) oscillation index is defined to indicate the variation of such out-of-phase fluctuation.Moreover,the simultaneous surface air temperature and precipitation anomalies in China are closely related to TIO-NP oscillations.Below-normal surface air temperature anomalies in the northern and the eastern part of China,and less rainfall in southern China,correspond to positive TIO-NP oscillation phase with negative SLP anomalies in tropical Indian Ocean and positive anomalies in North Pacific.The TIO-NP oscillation affects China’s winter climate anomalies,possibly through modulating the northeast East Asia winter monsoon.     

Interannual and Interdecadal Variations of the Mid-Atlantic Trough and Associated American-Atlantic-Eurasian Climate Anomalies

The mid-Atlantic trough (MAT) is one of the most prominent circulation systems over the subtropical North Atlantic during the boreal summer, and it can be viewed as a bridge linking the climate in the American-Atlantic-Eurasian region. The upper-tropospheric MAT attains its maximum intensity of 200–150?hPa in June and July. An index measuring the variability of MAT intensity is defined, which reveals significant interannual and interdecadal variations of the trough.

On interannual time scales, the variation of MAT is significantly associated with the North Atlantic Oscillation, a southeastward propagating stationary wave that possibly originates from the northeastern Pacific, and the Atlantic Meridional Mode. A stronger trough is associated with warmer surface temperatures and higher pressure over central-northern North America and the extratropical North Atlantic and with colder surface temperatures and lower pressure over the Arctic, the subtropical North Atlantic, and the northeastern Pacific. In the meantime, significant negative precipitation anomalies occur over the north of the Mediterranean Sea and the Black Sea, as well as the northeastern Atlantic because of the anomalous low-level northeasterly winds over these areas. On an interdecadal time scale, the variation of MAT seems to be related to the Atlantic Multi-decadal Oscillation. Warmer surface temperatures appear over almost the entire North Atlantic, southern Europe, East Asia, and the North Pacific during the weak phase of the trough. A weak trough is also associated with the dipole pattern of anomalous precipitation over the extratropical North Atlantic, Greenland, and northeastern North America, corresponding to a dipole of low-level atmospheric circulation over these regions.     

The initiation of ice sheet growth,Milankovitch solar radiation variations,and the 100 ky ice age cycle

Much work has gone into deciphering the causes of the large scale glacial/interglacial variations in the climate system over the last 900 000 years. While variations on the 41 thousand year (ky) and 23 ky time scales seem to be linearly linked to the variations in the distribution of solar radiation at the top of the atmosphere, Milankovitch solar radiation variations, the causes of the dominant 100 ky cycle in the geologic record are still unknown. One of the aspects of this cycle that is not well understood is how large scale ice sheet growth is initiated. Here we describe the mechanisms by which large scale ice sheet growth may have been initiated by the changes in the seasonal and latitudinal distribution of solar radiation over the past 160 ky. This is done through the use of a coupled energy balance climate-thermodynamic sea ice model that includes a hydrologic cycle which computes precipitation, and a land surface energy balance which determines the net accumulation of snow and ice. Results indicate that the initiation of ice sheet growth is possible during times of extremely low summer solstice solar radiation as a result of a large decrease in ablation during the critical melt season.     

Diversity on the Interannual Variations of Spring Monthly Precipitation in Southern China and the Associated Tropical Sea Surface Temperature Anomalies

There is a continuous and relatively stable rainy period every spring in southern China (SC). This spring precipitation process is a unique weather and climate phenomenon in East Asia. Previously, the variation characteristics and associated mechanisms of this precipitation process have been mostly discussed from the perspective of seasonal mean. Based on the observed and reanalysis datasets from 1982 to 2021, this study investigates the diversity of the interannual variations of monthly precipitation in spring in SC, and focuses on the potential influence of the tropical sea surface temperature (SST) anomalies. The results show that the interannual variations of monthly precipitation in spring in SC have significant differences, and the correlations between each two months are very weak. All the interannual variations of precipitation in three months are related to a similar western North Pacific anomalous anticyclone (WNPAC), and the southwesterlies at the western flank of WNPAC bring abundant water vapor for the precipitation in SC. However, the WNPAC is influenced by tropical SST anomalies in different regions each month. The interannual variation of precipitation in March in SC is mainly influenced by the signal of El Ni?o-Southern Oscillation, and the associated SST anomalies in the equatorial central-eastern Pacific regulate the WNPAC through the Pacific-East Asia (PEA) tele-connection. In contrast, the WNPAC associated with the interannual variation of precipitation in April can be affected by the SST anomalies in the northwestern equatorial Pacific through a thermally induced Rossby wave response. The interannual variation of precipitation in May is regulated by the SST anomalies around the western Maritime Continent, which stimulates the development of low-level anomalous anticyclones over the South China Sea and east of the Philippine Sea by driving anomalous meridional vertical circulation.     

广东大尺度大气水汽汇的年际及年代际变化特征

用1958～2004年实测降雨量和NCEP／NCAR再分析资料,分析了广东地区大尺度水汽汇的年际和年代际变化特征及其与水汽通量变化的关系。结果表明,气候平均而言,广东春夏季大气向地面输送较多水资源,秋季地气间相互交换的水分相当,冬季由地面向大气输送较多的水资源。四季和年水汽汇的年际分量方差贡献均占主导地位,秋、冬季水汽汇的年际分量有约3年的显著周期。除了显著的年际分量外,冬、春季和年水汽汇的年代际分量方差贡献也较显著,占总方差的40％以上,以30多年的长周期变化为主,目前正处于由正位相向负位相转变的过渡期,预示今后广东有偏旱趋势。广东冬春季水汽汇的异常有显著的同相关系。另外,夏、秋季水汽汇的年代际分量有10～15年的显著周期。广东各季大气水汽汇偏强（弱）是由于从热带低纬输送到南海北部至华南地区的水汽增强（减弱）,并伴随着水汽通量的辐合的增强（减弱）造成,但各季水汽通量异常分布型是有差别的。