Spatial trends of dry spells over Peninsular Malaysia during monsoon seasons

This study aims to trace changes in the dry spells over Peninsular Malaysia based on the daily rainfall data from 36 selected rainfall stations which include four subregions, namely northwest, west, southwest, and east for the periods of 1975 to 2004. Six dry spell indices comprising of the main characteristics of dry spells, the persistency of dry events, and the frequency of the short and long duration of dry spells will be used to identify whether or not these indices have increased or decreased over Peninsular Malaysia during the monsoon seasons. The findings of this study indicate that the northwestern areas of the Peninsular could be considered as the driest area since almost all the indices of dry spells over these areas are higher than in the other regions during the northeast (NE) monsoon. Based on the individual and the field significant trends, the results of the Mann–Kendall test indicate that as the total number of dry days, the maximum duration, the mean, and the persistency of dry days are decreased, the trend of the frequency of long dry spells of at least 4 days is also found to decrease in almost all the stations over the Peninsula; however, an increasing trend is observed in the frequency of short spells in these stations during the NE monsoon season. On the other hand, during the southwest monsoon, a positive trend is observed in the characteristics of dry spells including the persistency of two dry days in many stations over the Peninsula. The frequency of longer dry periods exhibits a decreasing trend in most stations over the western areas during both monsoon seasons for the periods of 1975 to 2004.     

近50年中国干湿气候界线波动及其成因初探

文中在 10a际尺度上详细分析了中国干湿气候界线波动与气候的干湿变化 ,得出 :过去 5 0a中国干湿气候界线波动显著 ,区域差异大 ,呈现出整体移动和东西、南北相异波动的特征。 2 0世纪 6 0～ 70年代中国干湿气候存在一次突变 ,由较湿润变为干旱 ,但各地干旱程度不同。干湿气候界线波动与气候的干湿变化具有显著的年代际特征。在此基础上分析了气候界线波动的可能原因 ,中国干湿气候界线的波动与气候的干湿变化是西太平洋副热带高压强度位置导致的东南季风、孟加拉湾暖流所导致的西南季风以及高原季风、中纬度西风环流等综合作用的结果。中国各地区干湿位相变化不一致 ,区域差异大 ,是不同环流以及环流的不同强弱组合所致。东南季风、西南季风、高原季风、中纬度西风环流、西太平洋副热带高压的年代际变化是过去 5 0a中国干湿气候界线波动与气候干湿变化年代际变化的根本原因。 2 0世纪 6 0～ 70年代的干湿突变 ,是整个北半球大气环流异常的结果     

Droughts near the northern fringe of the East Asian summer monsoon in China during 1470–2003

Historical annual dry–wet index for 1470–2003 combined with instrumental precipitation since 1951 were used to identify extremely dry years and events near the northern fringe of the East Asian summer monsoon in China—the Great Bend of the Yellow River (GBYR) region. In total, 49 drought years, of which 26 were severe, were identified. Composites of the dry–wet index under the drought years show an opposite wet pattern over the Southeast China. The longest drought event lasted for 6?years (1528–1533), the second longest one 4?years (1637–1640). The most severe 2-year-long drought occurred in 1928–1929, and the two driest single years were 1900 and 1965. These persistent and extreme drought events caused severe famines and huge losses of human lives. Wavelet transform applied to the dry–wet index indicates that the severe drought years are nested in several significant dry–wet variations across multiple timescales, i.e., the 65–85?year timescale during 1600– 1800, 40–55?year timescale before 1640 and 20–35?year timescale mainly from 1550 to 1640. These timescales of dry–wet variations are discussed in relation to those forcing such as cycles of solar radiation, oscillation in the thermohaline circulation and the Pacific Decadal Oscillation (PDO). Comparing 850?hPa winds in Asia in extremely dry and wet years, it was concluded that dry–wet variability in the GBYR region strongly depends upon whether the southerly monsoon flow can reach northern China.     

A Comparative Analysis of Primary and Extreme Characteristics of Dry or Wet Status between Asia and North America

In this study, the Palmer Drought Severity Index (PDSI) was used to analyze the average and extreme dry/wet states of Asia and North America from 1953 to 2003. The results indicate that the two continents underwent drying trends during this period. Compared with North America, Asia showed more severe drought trends. However, more significant and regular seasonal variation for drought was found in North America. The driest regions in Asia were located in the northern region of China, Mongolia, and eastern mid-Siberian plateau. Most regions in central North America were relatively wetter than other regions. The northern and southwestern regions of North America, as well as the Atlantic and Pacific coastal areas, experienced the most drought during this period. A sharp increase of the drought area and the number of extreme drought events took place from 1997 to 2003 in both Asia and North America. Severe drought events were more likely to occur during the summer on both continents. Asia had the most extreme drought events during July, but North America reached its highest drought frequency from June to September. In Asia, a persistent increasing trend of extreme drought emerged throughout the studied period. However, a more complex evolution of drought emerged in North America: a decreasing trend appeared before the mid-1960s and an increasing trend appeared after the late 1970s. A relatively steady dry/wet status was observed between the mid-1960s and the late 1970s. The role of exceptional, extreme drought events with respect to the La Nin?a event was considered during 1997–2003.     

Spatial and temporal variability of precipitation indices during 1961–2010 in Hunan Province,central south China

Based on daily precipitation records at 75 meteorological stations in Hunan Province, central south China, the spatial and temporal variability of precipitation indices is analyzed during 1961–2010. For precipitation extremes, most of precipitation indices suggest that both the amount and the intensity of extreme precipitation are increasing, especially the mean precipitation amount on a wet day, showing a significant positive trend. Meanwhile, both of the monthly rainfall heterogeneity and the contribution of the days with the greatest rainfall show an upward trend. When it comes to rainfall erosivity, most of this province is characterized by high values of annual rainfall erosivity. Although the directions of trends in annual rainfall erosivity at most stations are upward, only 6 of the 75 stations have significant trends. Furthermore, the spatial and temporal variation of dryness/wetness has been assessed by the standardized precipitation index (SPI). The principal component analysis (PCA) was applied to the SPI series computed on 24-month time scales. The results demonstrated a noticeable spatial variability with three subregions characterized by different trends: a remarkable wet tendency prevails in the central and southern areas, while the northern areas are dominated by a remarkable dry tendency.     

西北东部夏季极端干旱事件机理分析

利用国家气候中心提供的1951—2012年逐日降水、温度、综合气象干旱指数、逐月NCEP/NCAR再分析资料等,采用REOF分析、动力诊断、相关分析以及合成分析等方法,从大气环流异常特征、高空急流与季风异常等方面揭示西北东部夏季极端干旱事件的可能机理。研究发现西北东部夏季发生极端干旱时,副热带急流轴"倾斜",且急流与东亚夏季风强度均处于相对偏弱阶段。极端干旱的成因研究表明:急流轴"倾斜"及其强度减弱导致西北东部地区高层大范围的异常辐合;该地区为水汽源区,对流层整层水汽收支显著亏损;此外,该地区低层盛行来自内陆干旱区的异常西南风,东亚夏季风强度偏弱,高低层配置及大尺度环流形势不利于降水产生。     

基于降水数据的云南省近61年旱涝特性研究

选择标准化降水指数(SPI)刻画旱涝特征,基于云南省1954—2014年间32个气象站点逐月降水量资料,采用经验正交函数(EOF)方法、径向基函数(RBF)空间插值法、小波分析法,分析了近61年来云南省SPI序列、旱涝情态的时间特征和空间格局。结果表明:近61年来,云南省整体呈现干旱趋势(SPI变率为-0.009 1),SPI序列在2000年之后变化更加剧烈、速率加快。旱涝等级时间序列中,偏涝至偏旱年份占88.52%,大旱占4.92%,重旱和大涝均占3.28%,且自2003年以后,发生干旱的次数和强度明显增加。旱灾易发区主要分布于2个片区:景洪-思茅-元江站片区,以及沾益站东北地区;洪涝易发区主要分布于3个片区:临沧-大理-华坪沿线的西部片区,昭通站北部区域,及云南省东南部片区。干旱与洪涝事件发生的频率具有较好的对应性,但干旱事件发生的频率要略高于洪涝事件。EOF分析的第一个模态表明云南省整体呈现一致性的变涝或变旱特征,可能受到大尺度气候特征影响,第二模态可能受到地形因素的控制,第三模态可能与季风、大气环流等多种因素的影响有关;相应的时间系数也印证了云南省整体具有干旱趋势。SPI序列存在准2 a、准6 a、准8 a、准18 a、准28 a的周期性特征,且以准28 a为主周期。      

Increasing Flash Floods in a Drying Climate over Southwest China

In a globally warming world,subtropical regions are generally expected to become drier while the tropics and mid–high latitudes become wetter.In line with this,Southwest China,close to 25?N,is expected to become increasingly prone to drought if annual mean precipitation decreases.However,despite this trend,changes in the temporal distribution of moisture supply might actually result in increased extreme rainfall in the region,whose climate is characterized by distinct dry and wet seasons.Using hourly and daily gauge observations,rainfall intensity changes since 1971 are examined for a network of 142 locations in the region.From the analysis,dry season changes are negligible but wet season changes exhibit a significantly strong downward trend [-2.4%(10 yr)~(-1)],particularly during the past 15 years [-17.7%(10 yr)~(-1)].However,the intensity of events during the wettest of 5% hours appears to steadily increase during the whole period [1.4%(10 yr)~(-1)],tying in with government statistical reports of recent droughts and flooding.If the opposing trends are a consequence of a warming climate,it is reasonable to expect the contradictory trend to continue with an enhanced risk of flash flooding in coming decades in the region concerned.     

‘Splendid rains have fallen’: links between El Niño and rainfall variability in the Kalahari, 1840–1900

This study uses a range of published and unpublished historical documentary sources to explore the nature of rainfall variability in the Kalahari Desert and adjacent hardveld regions of central southern Africa during the seventeen Pacific El Niño–Southern Oscillation (ENSO) episodes that occurred between 1840 and 1900. Documentary data are used in two ways. First, maps of relative annual rainfall levels are presented for each of the 12 single-year and five protracted ENSO episodes during the period, in order to identify the associated inter-annual rainfall variations. These suggest that the relationship between ENSO episodes and rainfall variability identified for the twentieth century, whereby warm events are frequently preceded by wetter conditions during the austral summer prior to the event year and succeeded by drought in the following summer, has broadly held for much of the last 160 years. This is despite the long-term fluctuations in precipitation and temperature which are known to have occurred over this period. Droughts are identified following at least thirteen of the 17 single-year and protracted ENSO episodes. Pre-ENSO wetter periods are less common, with only nine of the ENSO episodes preceded by above-normal rainfall. Second, the documentary data are analyzed in detail in order to reveal any evidence for high resolution intra-annual variations in the seasonal distribution of rainfall during ENSO events. Seasonal sequences of rainfall/drought appear to have closely followed contemporary patterns, with heavy rainfall commonly occurring late in the pre-ENSO year or early in the ENSO year(s), and drought at the start of the post-ENSO year. This relationship can be seen to hold most strongly for single-year ENSO warm events and for the first year of protracted events, but rainfall conditions were more variable during the later years of protracted events.     

Impact of drought on wildland fires in Greece: implications of climatic change?

An increasing trend and a statistically significant positive correlation between wildfire occurrence, area burned and drought (as expressed by the Standardized Precipitation Index, SPI) have been observed all over Greece, during the period 1961?C1997. In the more humid and colder regions (Northern and Western Greece) the number of fires and area burned were positively correlated to both summer (SPI6_October) and annual drought (SPI12_September), whereas in the relatively more dry and hot regions (Southern and Central Greece) the number of fires and area burned were correlated only to summer drought. In 1978, Greece entered a period of prolonged drought, possibly as a result of the global climatic change. Data analysis of the period 1978?C1997 revealed a statistically significant increase in the mean annual number of fires, the area burned and the summer and annual drought episodes in the relatively more humid and colder regions (Northern and Western) of Greece (which in the past were characterized by less fires and area burned) compared to the more dry and hot regions (Southern and Eastern Greece), which always presented high fire activity. Additionally, analyzing the two sub-periods (1961?C1977, 1978?C1997) separately, drought was significantly correlated only to fire occurrence during the years 1961?C1977, whereas during 1978?C1997 drought was significantly correlated mainly to area burned. It became obvious that drought episodes, although they are not solely responsible for fire occurrence and area burned, they exert an increasingly significant impact on wildfire activity in Greece.     

基于SPI的1961－2020年昌吉地区作物生长季气象干旱时空特征研究

基于昌吉地区7个气象站1961－2020年降水量资料，计算昌吉地区作物生长季标准化降水指数（SPI-7）。运用趋势分析法、M-K突变检验法和小波分析法探究了昌吉地区作物生长季SPI-7指数的年际和年代变化特征；在此基础上分析了作物生长季干旱的站次比和干旱强度的年际变化,并结合该区实际发生的旱灾对SPI进行了验证。结果表明：1961—2020年昌吉地区作物生长季标准化降水指数以0.08/10 a的速率呈微弱的正趋势（变湿），在年代变化趋势中呈现出变干-变湿-变干的变化波动, 1981年标准化降水指数由低到高突变；干旱强度呈增加趋势，干旱发生的区域面积有轻微减少的趋势；干旱强度在全区范围内主要为轻旱和中旱等级，并表现为全域性干旱和区域性干旱；空间分布上看干旱率最高区域在东部地区，轻旱主要集中在东部，中旱、重旱和特旱集中在西部地区，干旱强度大的区域大致分布在西部地区；在周期性变化方面，SPI指数存在着6年、9年、16年周期震荡；历史旱灾与SPI指数干旱评价结果吻合率较高，SPI指数在昌吉地区作物生长季的干旱监测与分析中具有较好的实用性。     

Boreal summer continental monsoon rainfall and hydroclimate anomalies associated with the Asian-Pacific Oscillation

With the twentieth century analysis data (1901–2002) for atmospheric circulation, precipitation, Palmer drought severity index, and sea surface temperature (SST), we show that the Asian-Pacific Oscillation (APO) during boreal summer is a major mode of the earth climate variation linking to global atmospheric circulation and hydroclimate anomalies, especially the Northern Hemisphere (NH) summer land monsoon. Associated with a positive APO phase are the warm troposphere over the Eurasian land and the relatively cool troposphere over the North Pacific, the North Atlantic, and the Indian Ocean. Such an amplified land–ocean thermal contrast between the Eurasian land and its adjacent oceans signifies a stronger than normal NH summer monsoon, with the strengthened southerly or southwesterly monsoon prevailing over tropical Africa, South Asia, and East Asia. A positive APO implies an enhanced summer monsoon rainfall over all major NH land monsoon regions: West Africa, South Asia, East Asia, and Mexico. Thus, APO is a sensible measure of the NH land monsoon rainfall intensity. Meanwhile, reduced precipitation appears over the arid and semiarid regions of northern Africa, the Middle East, and West Asia, manifesting the monsoon-desert coupling. On the other hand, surrounded by the cool troposphere over the North Pacific and North Atlantic, the extratropical North America has weakened low-level continental low and upper-level ridge, hence a deficient summer rainfall. Corresponding to a high APO index, the African and South Asian monsoon regions are wet and cool, the East Asian monsoon region is wet and hot, and the extratropical North America is dry and hot. Wet and dry climates correspond to wet and dry soil conditions, respectively. The APO is also associated with significant variations of SST in the entire Pacific and the extratropical North Atlantic during boreal summer, which resembles the Interdecadal Pacific Oscillation in SST. Of note is that the Pacific SST anomalies are not present throughout the year, rather, mainly occur in late spring, peak at late summer, and are nearly absent during boreal winter. The season-dependent APO–SST relationship and the origin of the APO remain elusive.     

基于MCI指数的福建省干旱气候特征

采用目前国家干旱监测业务实行的MCI指数,利用Morlet小波、经验正交函数（EOF）等方法,分析了福建省1961—2019年全省66个气象站MCI指数表征的干湿状况和干旱变化特征。结果表明：福建省存在明显的干湿气候特征,具有显著的6—8 a和22 a的周期振荡,内陆山区干湿变化周期比沿海长,在季节尺度上各季均存在多时间尺度和地域差异化的特点;其空间变化具有3种典型模态,反映了季风降水多寡和地形差异。MCI表征的干旱过程时空分布与历史干旱事件相吻合,秋季和冬季是福建省干旱发生频率最高的季节,春季和夏季是干旱强度最强的季节;闽江口以南沿海地区干旱发生率明显高于内陆地区,全省出现同步干旱的机率较小（12%）。     

Drought analysis in the Tons River Basin,India during 1969-2008

The primary focus of this study is the analysis of droughts in the Tons River Basin during the period 1969–2008. Precipitation data observed at four gauging stations are used to identify drought over the study area. The event of drought is derived from the standardized precipitation index (SPI) on a 3-month scale. Our results indicated that severe drought occurred in the Allahabad, Rewa, and Satna stations in the years 1973 and 1979. The droughts in this region had occurred mainly due to erratic behavior in monsoons, especially due to long breaks between monsoons. During the drought years, the deficiency of the annual rainfall in the analysis of annual rainfall departure had varied from ?26% in 1976 to ?60% in 1973 at Allahabad station in the basin. The maximum deficiency of annual and seasonal rainfall recorded in the basin is 60%. The maximum seasonal rainfall departure observed in the basin is in the order of ?60% at Allahabad station in 1973, while maximum annual rainfall departure had been recorded as ?60% during 1979 at the Satna station. Extreme dry events (z score <?2) were detected during July, August, and September. Moreover, severe dry events were observed in August, September, and October. The drought conditions in the Tons River Basin are dominantly driven by total rainfall throughout the period between June and November.     

Trend analysis for rainfall in Delhi and Mumbai, India

Urbanisation has burdened cities with many problems associated with growth and the physical environment. Some of the urban locations in India are becoming increasingly vulnerable to natural hazards related to precipitation and flooding. Thus it becomes increasingly important to study the characteristics of these events and their physical explanation. This work studies rainfall trends in Delhi and Mumbai, the two biggest Metropolitan cities of Republic of India, during the period from 1951 to 2004. Precipitation data was studied on basis of months, seasons and years, and the total period divided in the two different time periods of 1951–1980 and 1981–2004 for detailed analysis. Long-term trends in rainfall were determined by Man-Kendall rank statistics and linear regression. Further this study seeks for an explanation for precipitation trends during monsoon period by different global climate phenomena. Principal component analysis and Singular value decomposition were used to find relation between southwest monsoon precipitation and global climatic phenomena using climatic indices. Most of the rainfall at both the stations was found out to be taking place in Southwest monsoon season. The analysis revealed great degree of variability in precipitation at both stations. There is insignificant decrease in long term southwest monsoon rainfall over Delhi and slight significant decreasing trends for long term southwest monsoon rainfall in Mumbai. Decrease in average maximum rainfall in a day was also indicated by statistical analysis for both stations. Southwest monsoon precipitation in Delhi was found directly related to Scandinavian Pattern and East Atlantic/West Russia and inversely related to Pacific Decadal Oscillation, whereas precipitation in Mumbai was found inversely related to Indian ocean dipole, El Ni?o- Southern Oscillation and East Atlantic Pattern.     

Detecting rainfall trends in twentieth century (1871–2006) over Orissa State, India

Climate change has affected the temperature and rainfall characteristics worldwide. However, the changes are not equal for all regions and have localized intensity and must be quantified locally to manage the natural resources. Orissa is an eastern state in India where agricultural activities mainly depends on the rainfall and thus face problems due to changing patterns of rainfall due to changing climate. In the present study, attempts were made to study temporal variation in monthly, seasonal and annual rainfall over the state during the period from 1871 to 2006. Long term changes in rainfall characteristics were determined by both parametric and non-parametric tests. The analysis revealed a long term insignificant decline trend of annual as well as monsoon rainfall, where as increasing trend in post-monsoon season over the state of Orissa. Rainfall during winter and summer seasons showed an increasing trend. Statistically monsoon rainfall can be considered as very dependable as the coefficient of variation is 14.2%. However, there is decreasing monthly rainfall trend in June, July and September, where as increasing trend in August. This trend is more predominant in last 10?year. Based on departure from mean, rainfall analysis also showed an increased number of dry years compared to wet years after 1950. This changing rainfall trend during monsoon months is major concern for the rain-fed agriculture. More over, this will affect hydro power generation and reservoir operation in the region.     

近44年来我国西北地区干湿特征分析

利用我国西北地区1960—2003年131个测站降水和小型蒸发皿蒸发量资料, 综合考虑降水和蒸发这两个水分平衡最关键的分量构造了降水蒸发均一化干湿指数, 进而研究了西北地区干湿的时空演变特征。结果表明:一致性异常是西北地区近44年干湿特征的最主要空间分布模态; 西北地区干湿异常特征主要分为西风带气候区型, 高原气候区型和季风气候区型; 整个西北地区及其西风带气候区、高原气候区年干湿特征呈较为显著的变湿趋势, 大约在20世纪70年代中期均发生了由干向湿的突变, 而季风气候区表现为变干趋势, 并且在90年代前期发生了由湿向干的突变; 整个西北地区及各分区近44年来主要以年代际周期振荡为主。     

Climatic regime shift and decadal anomalous events in China

Climatic time series from historical documents and instrumental records from China showed temporal and regional patterns in the last two to three centuries, including two multidecadal oscillations at quasi-20-year and quasi-70-year timescales revealed by signal analysis from wavelet transform. Climatic anomalous events on the decadal timescale were identified based on the two oscillations when their positive (or negative) phases coincide with each other to amplify amplitude. The coldest event occurred in the decade of 1965–1975 in eastern China, while the periods of 1920–1930, 1940–1950, and 1988–2000 appeared to be warmer in most parts of China. For the precipitation series in northern China, the dry anomalous event was found in the late 1920s, while the wet anomalous event occurred in the 1950s. A severe drought in 1927–1929 in northern China coincided with the anomalous warm and dry decade, caused large-scale famine in nine provinces over northern China. Climatic anomalous events with a warm-dry or cold-wet association in the physical climate system would potentially cause severe negative impacts on natural ecosystem in the key vulnerable region over northern China. The spatial pattern of summer rainfall anomalies in the eastern China monsoon region showed an opposite variations in phase between the Yellow River Valley (North China) and the mid-low Yangtze River Valley as well as accompanied the shift of the northernmost monsoon boundary. Climatic regime shifts for different time points in the last 200 years were identified. In North China, transitions from dry to wet periods occurred around 1800, 1875, and 1940 while the transitions from wet to dry periods appeared around 1840, 1910, and the late 1970s. The reversal transition in these time points can also be found in the lower Yangtze River. Climatic regime shifts in China were linked to the interaction of mid- and low latitude atmospheric circulations (the westerly flow and the monsoon flow) when they cross the Tibetan Plateau in East Asia.     

贵州夏季降水异常的区域特征

利用贵州省19个测站1951～2000年夏季逐月降水资料,计算了降水量的月平均（区域平均）标准化距平。并进行模糊聚类分析、经验正交函数分解（EOF）和小波分析,研究了贵州夏季降水异常的区域特征。结果表明,贵州夏季降水在近50 a中存在5个明显的气候段：20世纪50年代前期为多雨期;50年代中期到60年代前期为少雨期;60年代中后期为多雨期;70～80年代为少雨期;90年代以后进入多雨期;降水呈增多的趋势。全省一致性是贵州夏季降水的最主要特征,同时还存在区域差异。贵州夏季降水异常有5种空间分布型,即：全省旱（涝）型、东旱（涝）西涝（旱）型、南旱（涝）北涝（旱）型、中东旱（涝）西南涝（旱）型和西南旱（涝）其余涝（旱）型。各型降水具有多时间尺度振荡的特点,存在10～12 a、4～5 a、2～3 a的周期。