Projected future changes in synoptic systems influencing southwest Western Australia

Rainfall in the southwest of Western Australia (SWWA) is sensitive to shifts in the hemispheric scale circulation due to its location at the northward extent of the influence of mid-latitude fronts. A step-drop in the 1970s to a new winter rainfall regime has caused great concern for water users in the region. The synoptic systems at the height of winter in the latter half of the 20th century over this region have been described in Hope et al. (Clim Dyn, 2006) using a self-organising map, and in this study the projected future shifts in those systems has been examined. Bounds are placed on the possible responses by examining a number of different models and, into the future, two scenarios at the upper (SRES A2) and lower (SRES B1) limits of plausible human induced emissions. Rainfall taken directly from the models captures the rainfall decline in the 1970s, and, although it is not as large as observed in any one model, all the models express a decline, which is a very strong result. Into the future the rainfall decline is dramatic. The scenario at the upper bound of emissions, where atmospheric concentrations of greenhouse gases continue to rise strongly, shows a rainfall decline right through to the end of the century. The shift in synoptic systems for most models is to far fewer troughs and more high pressure systems across the region. One model exhibits a different signature, with a shift to more systems with a zonal structure. The fact that there is a rainfall decline shown by all models, yet the synoptic changes are different, highlights how sensitive SWWA rainfall is to the different responses of climate models to increasing greenhouse gases. In the B1 scenario, the concentrations rise only slowly in the second half of the century and the shift is still to drier conditions, but it is not as striking. These results show that increasing concentrations of greenhouse gases lead to increasingly dry conditions in SWWA, and as the atmospheric concentrations rise, the synoptic response intensifies.     

Synoptic aspects of wet and dry conditions in central Australia: observations and GCM simulations for 1 × CO2 and 2 × CO2 conditions

Synoptic climatological patterns that produce anomalous wet conditions in central Australia during the period from September to April have been studied. The analysis was done by using observed daily rainfall data at a number of stations, wind and mean sea level pressure from the European Centre for Medium Range Weather Forecasts (ECMWF), Tropical Ocean and Global Atmosphere (TOGA) data from 1985 to 1991, and the CSIRO 9-level (CSIR09) global climate model (GCM) simulated data for 1 × CO₂ and 2 × CO₂ experiments. On the basis of rainfall values above 99.5 percentile in observed and simulated data, wet days have been selected to study the synoptic-scale weather systems that produce anomalous wet events in central Australia. As the vast majority of days in central Australia are dry, the same number of days with no rainfall for both observed and simulated conditions have been selected randomly. The observed synoptic climatological patterns have been compared with the results of the control simulation of CSIRO9. A comparison between CSIRO9 simulated synoptic patterns and observed synoptic patterns reveals that the model fairly well captures the synoptic climatological characteristics which produce anomalous wet and contrasting dry weather conditions during the period from September to April. Under enhanced greenhouse experiments, the main features of the synoptic patterns are intensified both for wet and dry conditions, which result in an increase in extreme weather conditions, an increase in rainfall intensity, a spatial expansion of the heavy rainfall region during wet days, and an expansion of the dry area during dry days. During anomalous wet conditions, the low pressure area is intensified, monsoonal winds and southeasterlies are strengthened and strong wind shear over tropical Australia is simulated. During this condition, the monsoon shear line moves poleward particularly over the Northern Territory. In contrast, during dry conditions, the anticyclonic circulation over the continent is strengthened.     

与西澳州西南部冬季降水相联系的大气环流特征分析

西澳大利亚州西南部(SWWA)是西澳大利亚州首府Perth的所在地,也是西澳州政治、经济、文化、教育和旅游的中心.自20世纪中期以来,SWWA地区雨季降水持续减少.本文利用近60年的观测及再分析数据,分析了已知的影响澳大利亚降水的热带海洋模态:厄尔尼诺—南方涛动(ENSO)、印度洋偶极子(IOD)和ENSOModoki...     

Impacts of convection schemes on simulating tropical-temperate troughs over southern Africa

This study examines southern African summer rainfall and tropical temperate troughs (TTTs) simulated with three versions of an atmospheric general circulation model differing only in the convection scheme. All three versions provide realistic simulations of key aspects of the summer (November–February) rainfall, such as the spatial distribution of total rainfall and the percentage of rainfall associated with TTTs. However, one version has a large bias in the onset of the rainy season. Results from self-organizing map (SOM) analysis on simulated daily precipitation data reveals that this is because the occurrence of TTTs is underestimated in November. This model bias is not related to westerly wind shear that provides favorable conditions for the development of TTTs. Rather, it is related to excessive upper level convergence and associated subsidence over southern Africa. Furthermore, the model versions are shown to be successful in capturing the observed drier (wetter) conditions over the southern African region during El Niño (La Niña) years. The SOM analysis reveals that nodes associated with TTTs in the southern (northern) part of the domain are observed less (more) often during El Niño years, while nodes associated with TTTs occur more frequently during La Niña years. Also, nodes associated with dry conditions over southern Africa are more (less) frequently observed during El Niño (La Niña) years. The models tend to perform better for La Niña events, because they are more successful in representing the observed frequency of different synoptic patterns.     

20世纪90年代以来东北暴雨过程特征分析

使用1990～2005年全国730站日降水资料和NCEP格点分析资料对1990～2005年东北地区大暴雨过程进行了分类研究,探讨21世纪前后夏季东北暴雨的主要特征.按照东北地区日降雨量大于50 mm的站点数不少于5个的标准,统计出1990～2005年东北地区的69个暴雨个例(共90天).在统计的基础上,进一步对造成大范围暴雨过程的天气形势进行分类研究.考虑阻塞高压、热带、副热带系统和西风带之间的相互关系,将暴雨过程的主要影响系统大致分为6类:(1)台风与西风带系统(西风槽、东北低涡)的远距离相互作用(20个,28.9%);(2)登陆台风(或南来低涡)北上与西风带系统(西风槽、东北低涡)相互作用(16个,23.2%);(3)台风直接暴雨(1个,1.5%);(4)低槽冷锋暴雨(16个,23.2%);(5)低空切变型暴雨(2个,2.9%);(6)东北低涡暴雨(14个,20.3%).在所有个例中与台风有关的共有37个,超过一半,占总数53.6%.台风的远距离水汽输送或登陆台风北上与西风带系统相互作用是东北地区产生大暴雨或持续性大暴雨的重要环流条件.此外,东北低涡和西风槽前系统造成暴雨个例也比较多,也是东北地区大范围暴雨的重要影响系统,低槽冷锋暴雨和东北低涡暴雨也各分为4小类.低空切变暴雨的切变线一般在低层较为明显.上述分析表明,夏季东北地区暴雨过程种类繁多,情况较为复杂,且进入新世纪以来该区降雨过程较为活跃,值得深入研究.     

珠海市暴雨天气的气候特征及影响形势分析

利用1962—2010年珠海市降雨资料和NCEP/NCAR再分析资料,应用小波分析、相关分析等统计方法,对珠海市暴雨天气气候特征及其影响系统进行分析。结果表明,珠海市白天发生暴雨的次数比夜间多,但夜间暴雨的雨量比白天大;大部分暴雨都出现在汛期,汛期暴雨具有次数多、雨量大的特点;暴雨占全年总降雨的比率越来越大,近阶段容易出现旱涝急转的气象灾害。普查分析得出,影响珠海市暴雨的天气形势可分为热带气旋型、锋面低槽型、低空急流型、高空槽和切变线型、辐合带北抬型、副高边缘和东风波型等。     

IPCC AMIP模式对西南澳类季风环流的模拟

以西南澳类季风环流为出发点,考察了IPCC第四次评估报告AMIP提供的12个大气环流模式对于澳大利亚西南部(SWWA)地区降水的季节演化特征,西南澳类季风环流(SWAC)的季节特征、季节演化、对应的异常环流型及其年际变率的模拟性能进行了评估。结果表明,除了NCAR_CAM3模式以外,其余模式均能较好的再现SWWA地区近地层盛行风向季节性反转及副热带高压脊线的季节性跳跃特征。对副热带高压脊线的季节演化特征,虽然大部分模式可以模拟出其季节移动特征,但是对于副热带高压脊线的北跳、南撤时间、到达位置和年内振幅均不能很好模拟。其次,除了NCAR_CAM3,其余模式基本能刻画出与SWAC相联系的异常环流型结构;而对于SWAC的年际变率,基本所有模式均不能较好模拟。整体权衡,GISS_MODEL_E_R在模拟SWAC环流的年际变率方面表现出较其它模式稍好的性能,大致可以模拟出与观测SWAC相似的特征,对SWWA地区的冬季降水显示出了与观测相似的显著影响。     

A comparative synoptic climatology of cool-season rainfall in major grain-growing regions of southern Australia

Two distinct synoptic weather systems, cut-off lows and fronts, deliver most of the cool-season rainfall to the cropping regions of southern Australia. A comparative synoptic climatology of daily rainfall events over approximately five decades reveals both spatial and temporal variations of the dominant synoptic types. The rainfall characteristics and associated large-scale drivers differ between the two synoptic types. Understanding regional rainfall depends on understanding these differences. Cut-off lows contribute one half of growing season rainfall in southeast Australia, while frontal systems associated with Southern Ocean depressions contribute about a third. The proportions are reversed in the Central Wheat Belt (CWB) of Western Australia where Southern Ocean fronts are the dominant source of growing season rainfall. In the southern island state of Tasmania, topography strongly influences the outcome with cut-off lows contributing about half the rainfall near the east coast and fronts dominating a short distance to the west. Cut-off lows generally contribute their highest proportion of rainfall in the austral autumn and spring while frontal rainfall is at its maximum in late winter. Cut-off low rainfall contributes more strongly in percentage terms to the recent decline in rainfall. The distribution of synoptic types is explained by the dominant long-wave structure in the winter half of the year. The major trough near Western Australia favours frontogenesis to the southwest of the CWB but fronts moving out of the region encounter a persistent meridional ridge in the Tasman Sea where there is a high frequency of blocking events.     

江淮梅雨期强降水和大气不同时间尺度波动的关系

中国的江淮梅雨具有多时间尺度特征,利用1979-2017年欧洲中期预报中心逐日再分析资料(ERA-interim)和台站逐日降水观测数据,采用滤波和合成分析等统计方法,分析了江淮梅雨期间不同时间尺度强降雨过程的特征,对比研究了不同时间尺度强降水对应的大气环流系统波动的演变特征.研究表明江淮梅雨降水集中期开始前1～9天和...     

Climatic change over the Lowveld of South Africa

There has been a 38% decrease in expected annual rainfall totals over the Lowveld, in the eastern part of South Africa, during the last two decades. The downward trend in mean annual rainfall is not replicated in the rest of the summer rainfall region above the escarpment. Rainfall variability over the Lowveld has been increasing since about the 1950s, although the increase in variability appears to have been slowing down in more recent years. Changes in the frequency and intensity of El Niño/Southern Oscillation extreme events are only partly responsible for the observed desiccation and increase in rainfall variability. The CSIRO 9-level general circulation model simulates, for 2 × CO₂ conditions, an insignificant decrease of 10% in the annual mean and a slight increase in the inter-annual variability of rainfall over the Lowveld. Other general circulation models likewise simulate only small changes in annual mean rainfall over the region. However, the simulated increase in rainfall variability by the CSIRO 9-level model is likely to be conservative since the model, being linked to a slab ocean, is unable to represent important features of ocean-atmosphere coupling in the region. Significant changes in the frequencies of extreme drought events and of heavy rains in the Lowveld are likely to occur even with only small changes in the rainfall climatology of the region.     

华南初夏干旱及多雨年份的季风环流特征

本文根据1961—1980年我国华南5月降水量资料,划分5月的旱或涝。并挑选1963、1966(干旱)1973、1975(涝)四年作为典型年,从天气气候角度分析了初夏季风环流结构与华南5月旱涝关系。华南地区旱5月和涝5月在夏季风来临早迟、持续期长短、强度强弱等方面都有明显差异。华南5月出现旱或涝,不仅在同期全球环流,而且在前期环流,特别是北半球中高纬度几个活动中心也有明显的差异。1月以阿留申低压及冰岛低压,4月以太平洋高压及大西洋高压差别比较明显。华南5月降水量的多少与前期3月低纬气压指标值有较好关系,涝      

Future projections of synoptic weather types over the Arabian Peninsula during the twenty-first century using an ensemble of CMIP5 models

An assessment of future change in synoptic conditions over the Arabian Peninsula throughout the twenty-first century was performed using 20 climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) database. We employed the mean sea level pressure (SLP) data from model output together with NCEP/NCAR reanalysis data and compared the relevant circulation types produced by the Lamb classification scheme for the base period 1975–2000. Overall, model results illustrated good agreement with the reanalysis, albeit with a tendency to underestimate cyclonic (C) and southeasterly (SE) patterns and to overestimate anticyclones and directional flows. We also investigated future projections for each circulation-type during the rainy season (December–May) using three Representative Concentration Pathways (RCPs), comprising RCP2.6, RCP4.5, and RCP8.5. Overall, two scenarios (RCP4.5 and RCP 8.5) revealed a statistically significant increase in weather types favoring above normal rainfall in the region (e.g., C and E-types). In contrast, weather types associated with lower amounts of rainfall (e.g., anticyclones) are projected to decrease in winter but increase in spring. For all scenarios, there was consistent agreement on the sign of change (i.e., positive/negative) for the most frequent patterns (e.g., C, SE, E and A-types), whereas the sign was uncertain for less recurrent types (e.g., N, NW, SE, and W). The projected changes in weather type frequencies in the region can be viewed not only as indicators of change in rainfall response but may also be used to inform impact studies pertinent to water resource planning and management, extreme weather analysis, and agricultural production.     

Evidence of climatic change in Nigeria based on annual series of rainfall of different daily amounts, 1919–1985

Annual series of light rainfall, moderate rainfall and heavy rainfall are computed for 4 zones arranged from south to north in Nigeria: Coastal, Guinea-Savanna, Midland and Sahelian zones. Daily rainfall data for the period 1919–85 are utilized. Each series is examined for evidence of change in structure in terms of pattern of decrease and increase in dry and wet years, the overall trend, and the occurrence of runs of dry and wet years. The northern Nigeria (Midland and Sahel) heavy rainfall series and the Sahel moderate rainfall series are found to depict evidence of climatic change as defined by Landsberg (1975) that climatic conditions must change to a new equilibrium position with the values of climatic elements changed significantly. On the other hand Landsberg's definition of climatic fluctuations as involving temporary deflection which can revert to earlier conditions is found to fit the 4 regional light rainfall series and the Midland area moderate rainfall series. The southern Nigeria moderate and heavy rainfall series are found to depict only evidence of high frequency oscillations about a stable long-term mean. The recent drought in Nigeria north of about 9° N is shown to be associated with a large decline in moderate and heavy rainfalls over this part of the country.     

Occurence Frequency of Storm Wind Waves in the Baltic,Black, and Caspian Seas under Changing Climate Conditions

The article proposes the method of climatic forecast of the occurrence frequency of synoptic conditions causing severe hydrometeorological events as well as severe events that are genetically related to them, in particular, storm wind waves. The choice of sea level pressure field as an indicator of atmospheric conditions of storm waves is substantiated. The algorithm for the method implementation is developed. It includes the processing of observational/reanalysis data; wind wave simulation; the systematization of synoptic conditions that accompany storm waves under the modern climate; the assessment of the ability of climate models of atmospheric and oceanic general circulation to simulate correctly the frequency of the revealed types of synoptic conditions for the modern climate; and the forecast of the frequency of these types for the possible scenarios of the future climate.     

2012年7月21日北京特大暴雨的多尺度特征

本文采用观测和NCEP分析场资料对2012年7月21日发生在北京地区的特大暴雨过程的天气形势、水汽来源和中尺度对流系统的特征进行了研究。结果如下:“7.21”北京暴雨过程是高低空与中低纬系统共同配合的结果,暴雨发生在“东高西低”的环流形势下,低涡、切变线、低槽冷锋和低空急流为此次过程的主要影响天气系统;孟加拉湾至西太平洋地区热带辐合带(ITCZ)活跃,其中热带气旋的活动有利于水汽向东亚大陆输送,此次暴雨过程中华北地区的水汽源地包括孟加拉湾和我国东部的渤海、黄海等,低层的水汽主要来自东部,中层的水汽主要来自孟加拉湾;北京的强降雨有两段,第1段降雨虽然发生在冷锋前,但有明显冷空气的侵入,并与地形和东风的作用有关,第2段降雨对流的组织和增强与冷锋强迫相关。在有利环境下,中尺度对流系统频繁发生发展,持续时间长,且稳定少动是此次特大暴雨形成的重要原因。     

The exceptionally wet year of 2014 over Greece: a statistical and synoptical-atmospheric analysis over the region of Thessaloniki

The highest rainfall totals (912.2 mm) and the largest number of raindays (133 days), since 1958, were recorded in Thessaloniki during the year of 2014. Extreme precipitation heights were also observed on a seasonal, monthly and daily basis. The examined year presented the highest daily rainfall intensity, the maximum daily precipitation and the largest number of heavy precipitation days (greater than 10 mm), and it also exceeded the previous amounts of precipitation of very wet (95th percentile) and extremely wet (99th percentile) days. According to the automatic circulation type classification scheme that was used, it was found that during this exceptionally wet year, the frequency of occurrence of cyclonic types at the near surface geopotential level increases, while the same types decreased at a higher atmospheric level (500 hPa). The prevailing type was type C which is located at the centre of the study area (Greece), but several other cyclonic types changed during this year not only their frequency but also their percentage of rainfall as well as their daily precipitation intensity. It should be highlighted that these findings differentiated on the seasonal-scale analysis. Moreover, out of the three teleconnection patterns that were examined (Scandinavian Pattern, Eastern Mediterranean Teleconnection Pattern and North Sea-Caspian Pattern), the Scandinavian one (SCAND) was detected during the most of the months of 2014 meaning that it was highly associated with intense precipitation over Greece.     

Synoptic typing: interdisciplinary application methods with three practical hydroclimatological examples

Synoptic classification is a methodology that represents diverse atmospheric variables and allows researchers to relate large-scale atmospheric circulation patterns to regional- and small-scale terrestrial processes. Synoptic classification has often been applied to questions concerning the surface environment. However, full applicability has been under-utilized to date, especially in disciplines such as hydroclimatology, which are intimately linked to atmospheric inputs. This paper aims to (1) outline the development of a daily synoptic calendar for the Mid-Atlantic (USA), (2) define seasonal synoptic patterns occurring in the region, and (3) provide hydroclimatological examples whereby the cascading response of precipitation characteristics, soil moisture, and streamflow are explained by synoptic classification. Together, achievement of these objectives serves as a guide for development and use of a synoptic calendar for hydroclimatological studies. In total 22 unique synoptic types were identified, derived from a combination of 12 types occurring in the winter (DJF), 13 in spring (MAM), 9 in summer (JJA), and 11 in autumn (SON). This includes six low pressure systems, four high pressure systems, one cold front, three north/northwest flow regimes, three south/southwest flow regimes, and five weakly defined regimes. Pairwise comparisons indicated that 84.3 % had significantly different rainfall magnitudes, 86.4 % had different rainfall durations, and 84.7 % had different rainfall intensities. The largest precipitation-producing classifications were not restricted to low pressure systems, but rather to patterns with access to moisture sources from the Atlantic Ocean and easterly (on-shore) winds, which transport moisture inland. These same classifications resulted in comparable rates of soil moisture recharge and streamflow discharge, illustrating the applicability of synoptic classification for a range of hydroclimatological research objectives.     

West African palaeosynoptic patterns at the last glacial maximum

Summary The composite method and an atmospheric moisture budget is applied to study the present synoptic summer situation in the West African Sahel. This allows the information of the synoptic scale flow systems at the last glacial maximum to be obtained. Relying on the results of general circulation models and of local geological findings the palaeosynoptic situation was found to consist of the same constituents as today's given pattern. In both time slices the strength of the midtropospheric African Easterly Jet determines the intensity and frequency of the Sahelian synoptic disturbances: the Easterly Waves and the Squall Lines. The glacial rainfall intensity of the Squall Lines, the most effective rain-bearing system of the Sahel, is calculated to be 2/3 of the modern one. Additionally, only half the number of Squall Lines propagated across the West African continent per summer month. Thus the glacial rainfall amount of the Sahel was roughly 30% of the value observed today.With 6 Figures     

Stable isotopic variations in precipitation in Southwest China

This study analyzes the relationships of stable isotopes in precipitation with temperature, air pressure and humidity at different altitudes, and the potential influencing mechanisms of control factors on the stable isotopes in precipitation in Southwest China. There appear marked negative correlations of the δ18O in precipitation with precipitation amount, vapor pressure and atmospheric precipitable water (PW)at the Mengzi, Simao and Tengchong stations on the synoptic timescale; the marked negative correlations between the δ18O in precipitation and the diurnal mean temperature at 400 hPa, 500 hPa, 700 hPa and 850hPa are different from the temperature effect in middle-high-latitude inland areas. In addition, the notable positive correlation between the δ18O in precipitation and the dew-point deficit △Td at different altitudes is found at the three stations. Precipitation is not the only factor generating an amount effect. Probably,the amount effect is related to the variations of atmospheric circulation and vapor origins. On the annual timescale, the annual precipitation amount weighted-mean δ18O displays negative correlations not only with annual precipitation but also with annual mean temperature at 500 hPa. It can be deduced that, in the years with an abnormally strong summer monsoon, more warm and wet air from low-latitude oceans is transported northward along the vapor channel located in Southwest China and generates abnormally strong rainfall on the way. Meanwhile, the abnormally strong condensation process will release more condensed latent heat in the atmosphere, and this will lead to a rise of atmospheric temperature during rainfall but a decline of δ18O in the precipitation. On the other hand, in the years with an abnormally weak summer monsoon, the precipitation and the atmospheric temperature during rainfalls decrease abnormally but the δ18O in precipitation increases.     

Subseasonal to multidecadal variability of northeast monsoon daily rainfall over Peninsular Malaysia using a hidden Markov model

This study involves the modelling of a homogeneous hidden Markov model (HMM) on the northeast rainfall monsoon using 40 rainfall stations in Peninsular Malaysia for the period of 1975 to 2008. A six hidden states HMM was selected based on Bayesian information criterion (BIC), and every hidden state has distinct rainfall characteristics. Three of the states were found to correspond by wet conditions; while the remaining three states were found to correspond to dry conditions. The six hidden states were found to correspond with the associated atmospheric composites. The relationships between El Niño–Southern Oscillation (ENSO) and the sea surface temperatures (SST) in the Pacific Ocean are found regarding interannual variability. The wet (dry) states were found to be well correlated with a Niño 3.4 index which was used to characterize the intensity of an ENSO event. This model is able to assess the behaviour of the rainfall characteristics with the large scale atmospheric circulation; the monsoon rainfall is well correlated with the El Niño–Southern Oscillation in Peninsular Malaysia.