A new index to describe the tropical Asian summer monsoon

We define a new monsoon index (MV) as the product of relative vorticity and equivalent potential temperature using the long-term NCEP/NCAR reanalysis data. The MV index provides new insights into the intraseasonal and interannual variabilities of the broad-scale tropical Asian summer monsoon (TASM), including the South Asian summer monsoon (SASM) and the South China Sea summer monsoon (SCSSM). On the intraseasonal timescale, the pentad-to-pentad MV index bears a close relationship to the broad-scale rainfall in the TASM regions. Among 29 summers from 1979 to 2007, in 23/27 summers the correlation coefficients are higher than 0.7 in the SASM/SCSSM region. However, in fewer than 9 summers, the correlations between the broad-scale rainfall and the existing circulation indices are higher than 0.7. On the interannual timescale, various existing SASM circulation indices are moderately or well correlated with all-India summer monsoon rainfall, whereas their correlations with broad-scale SASM rainfall are weak. In contrast, the summer mean MV index correlates well with the broad-scale SASM rainfall and all-India summer monsoon rainfall (correlation of 0.73 and 0.65, respectively). In the SCSSM region, the summer mean MV index also bears a close relationship to the SCSSM rainfall, although some discrepancies exist during certain years. The composite strong TASM shows a stronger low-tropospheric low pressure in association with the enhanced westerly winds and moisture transfer, stronger convection, and upper-tropospheric easterly winds, which indicate that the MV index can well capture the features of TASM. Supported by National Basic Research Program of China (Grant No. 2006CB400500), China Postdoctoral Science Foundation (Grant No. 20070410133), Open Foundation of Jiangsu Key Laboratory of Meteorological Disaster (Grant No. KLME0704)     

A hypothesis on onset,advance and withdrawal of the Indian summer monsoon

The paper defines the intertropical convergence zone. (ITCZ) in the Indian monsoon region during the northern summer, identifies it with the northern boundary of the advancing monsoon and suggests that its seasonal movement can serve as an indicator of onset, advance and withdrawal of the monsoon. Evidence suggesting the movement of the ITCZ which is associated with the equatorial trough of low pressure is indirectly furnished by an analysis of the isallobaric or height-tendency field which reveals a distinct gradient towards the north/south during period of advance/withdrawal of the monsoon. A comparative study of the dates of onset of monsoon during two successive years appears to suggest that some of the problems encountered in using rainfall as the sole criterion for determining the onset and advance of the monsoon may be over-come by using the ITCZ concept as proposed in the present paper. Attention is drawn to the effects of synoptic-scale disturbances on the normal dates of onset, advance and withdrawal of the monsoon.     

Australian rainfall trends and their relation to the southern oscillation index

Rainfall is the key climate variable that governs the spatial and temporal availability of water. In this study we identified monthly rainfall trends and their relation to the southern oscillation index (SOI) at ten rainfall stations across Australia covering all state capital cities. The nonparametric Mann–Kendall (MK) test was used for identifying significant trends. The trend free pre‐whitening approach (TFPW) was used to remove the effects of serial correlation in the dataset. The trend beginning year was approximated using the cumulative summation (CUSUM) technique and the influence of the SOI was identified using graphical representations of the wavelet power spectrum (WPS). Decreasing trends of rainfall depth were observed at two stations, namely Perth airport for June and July rainfall starting in the 1970s and Sydney Observatory Hill for July rainfall starting in the 1930s. No significant trends were found in the Melbourne, Alice Springs and Townsville rainfall data. The remaining five stations showed increasing trends of monthly rainfall depth. The SOI was found to explain the increasing trends for the Adelaide (June) and Cairns (April) rainfall data and the decreasing trends for Sydney (July) rainfall. Other possible climatic factors affecting Australian rainfall are also discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Possible role of warming on Indian summer monsoon precipitation over the north-central Indian subcontinent

ABSTRACT

Broad disagreement between modelled and observed trends of Indian summer monsoon (ISM) over the north-central part of the Indian subcontinent (NCI) implies a gap in understanding of the relationship between the forcing factors and monsoonal precipitation. Although the strength of the land–sea thermal gradient (LSG) is believed to dictate monsoon intensity, its state and fate under continuous warming over the Bay of Bengal (BoB) and part of the NCI (23–28°N, 80–95°E) are less explored. Precipitation (1901–2017) and temperature data (1948–2017) at different vertical heights are used to understand the impact of warming in the ISM. In NCI, surface air temperature increased by 0.1–0.2°C decade^?1, comparable to the global warming rate. The ISM precipitation prominently weakened and seasonality reduced after 1950, which is caused by a decrease in the LSG at the depth of the troposphere. Warming-induced increase in local convection over the BoB further reduced ISM precipitation over NCI.     

Precipitable water and dew point temperature relationship during the Indian summer monsoon

Two years of contrasting monsoon rainfall over the Indian subcontinent are studied with reference to (a) total precipitable water and precipitation efficiencies during the respective years and (b) the correlation between the dew point temperature at a particular level and the total precipitable water. It is found that the maximum correlation occurs between the dew point temperature at the 850 mb level and the total precipitable water at an individual station. The precipitation efficiencies are less during the year of bad monsoon. A linear regression equation is attempted between the total precipitable water and dew point temperature.     

Zonal propagation of kinetic energy and convection in the South China Sea and Indian monsoon regions in boreal summer

As early as in the 1980s, Chinese scientists hadfirst proposed that there exits two summer monsoonsystems in Asia, namely the East Asian summer mon-soon (EASM) and the Indian summer monsoon(ISM)[1-4]. The two monsoon systems are quite dif-ferent in characteristics. Since then, such issue andconclusion had been documented and approved by alot of studies in the past two decades, and was appliedin the guideline of the South China Sea summer mon-soon experiment (SCSMEX), which was undertak…     

Dynamical mechanism of the stratospheric quasibiennial oscillation impact on the South China Sea Summer Monsoon

The National Center for the Atmospheric Research (NCAR) middle atmospheric model is used to study the effects of the quasi-biennial oscillation in the stratosphere (QBO) on the tropopause and uppe troposphere, and the relationship between the QBO and South China Sea Summer Monsoon (SCSSM is explored through NCEP (the National Centers for Environmental Prediction)/NCAR, ECMWF (Euro pean Centre for Medium-Range Weather Forecasts) monthly mean wind data and in situ sounding data The simulations show that the QBO-induced residual circulations propagate downwards, and affect the tropopause and upper troposphere during the periods of mid-late QBO phase and phase transition Meanwhile, diagnostic analyses indicate that anomalous circulation similar to SCSSM circulation is generated to strengthen the SCSSM during the easterly phase and anomalous Hadley-like circulation weakens the SCSSM during the westerly. Though the QBO has effects on the SCSSM by meridiona circulation, it is not a sole mechanism on the SCSSM TBO mode.     

Long‐term trend of precipitation in China and its association with the El Niño–southern oscillation

The plausible long‐term trend of precipitation in China and its association with El Niño–southern oscillation (ENSO) are investigated by using non‐parametric techniques. It is concluded that a greater number of decreasing trends are observed than are expected to occur by chance. Geographically, the decreasing trend was concentrated in most parts of China, including the Songliao River, Hai River, Huai River, Yellow River, Zhujiang River, and southern part of the Yangtze River basins, whereas an increasing trend appeared primarily in the western and middle parts of China, mainly including the Inland River basin, and the northern part of the Yangtze River basins. Monthly mean precipitation for the summer and early autumn months generally decreased, with the greatest decrease occurring in August. The precipitation in spring from January to April and later autumn, including September and October, tended to increase. The teleconnection between precipitation and ENSO has been investigated by using the non‐parametric Kendall's τ. The correlation coefficients between the southern oscillation index (SOI) and precipitation show the areas with positive or negative associations. Approximately 20% of the stations exhibit statistically significant correlations between SOI and precipitation, of which 70% show a negative correlation, with most of them appearing in southeast China and several appearing in northwest and northeast China. Similar regional patterns are also observed when the precipitation records are further subdivided into El Niño, La Niña, and neutral periods. Statistical tests for the three kinds of time series were carried out using the non‐parametric Wilcoxon rank‐sum test, and it is noted that the stations with significant differences in precipitation averages are mainly marked in the Yellow River basin and south China. The frequencies of below‐ and above‐average precipitation that occurred during the El Niño, La Niña, and neutral periods are estimated as well. The result shows that greater precipitation may be associated with El Niño episodes in south China, but drought may easily occur during El Niño episodes in the Yellow River basin. Copyright © 2006 John Wiley & Sons, Ltd.     

Tropical drought patterns and their linkages to large-scale climate variability over Peninsular Malaysia

Ocean–atmosphere modes of climate variability in the Pacific and Indian oceans, as well as monsoons, regulate the regional wet and dry episodes in tropical regions. However, how those modes of climate variability, and their interactions, lead to spatial differences in drought patterns over tropical Asia at seasonal to interannual time scales remains unclear. This study aims to analyse the hydroclimate processes for both short- and long-term spatial drought patterns (3-, 6, 12- and 24-months) over Peninsular Malaysia using the Standardized Precipitation Index, Standardized Precipitation Evapotranspiration Index, and Palmer Drought Severity Index. Besides that, a generalized least squares regression is used to explore underlying circulation mechanisms of these spatio-temporal drought patterns. The tested drought indices indicate a tendency towards wetter conditions over Peninsular Malaysia. Based on principal component analysis, distinct spatio-temporal drought patterns are revealed, suggesting North–South and East–West gradients in drought distribution. The Pacific El Nino Southern Oscillation (ENSO), the South Western Indian Ocean (SWIO) variability, and the quasi-biennial oscillation (QBO) are significant contributors to the observed spatio-temporal variability in drought. Both the ENSO and the SWIO modulate the North–South gradient in drought conditions over Peninsular Malaysia, while the QBO contributes more to the East–West gradient. Through modulating regional moisture fluxes, the warm phases of the ENSO and the SWIO, and the western phases of the QBO weaken the southwest and northeast monsoon, leading to precipitation deficits and droughts over Peninsular Malaysia. The East–West or North–South gradients in droughts are related to the middle mountains blocking southwest and northeast moisture fluxes towards Peninsular Malaysia. In addition, the ENSO and QBO variations are significantly leading to short-term droughts (less than a year), while the SWIO is significantly associated with longer-duration droughts (2 years or more). Overall, this work demonstrates how spatio-temporal drought patterns in tropical regions are related to monsoons and moisture transports affected by the oscillations over the Pacific and Indian oceans, which is important for national water risk management.     

Teleconnection between the Indian summer monsoon onset and the Meiyu over the Yangtze River Valley

Based on the Indian and Chinese precipitation data and the NCEP-NCAR reanalysis circulation data, the relationship between the Indian summer monsoon (ISM) onset and the Meiyu over the Yangtze River Valley has been discussed by the methods of correlation analysis and composite analysis. The results show that the date of ISM onset over Kerala in the southwestern coast of the Indian Peninsula is about two weeks earlier than the beginning of the Meiyu over the Yangtze River Valley. After the outbreak of ISM, the teleconnection mode sets up from the western coast of India via the Bay of Bengal (BOB) to the Yangtze River Valley and southern Japan. It is different both in time and space from the telecon- nection mode which is from the northwest of India via the Tibetan Plateau to northern China. The for- mer mode is defined as the "south" teleconnection of the Asian summer monsoon, forming in the pe- riod of ISM onset; while the latter mode is called the "north" teleconnection, mainly occurring in the Asian monsoon culminant period. During the process of the "south" teleconnection’s formation, the Asian monsoon circulation has experienced a series of important changes: ISM onset, the northward movement of the south Asia high (SAH), the onset vortex occurrence, the eastward extension of the stronger tropical westerly belt, and the northeastward jump of the western Pacific subtropical high (WPSH), etc. Consequently, since ISM sets up over Kerala, the whole Asian continent is covered by the upper SAH after about two weeks, while in the mid- and lower troposphere, a strong wind belt forms from the Arabian Sea via the southern India, BOB and the South China Sea (SCS), then along the western flank of WPSH, to the Yangtze River Valley and southern Japan. With the northward moving of the subtropical jet streams, the upper westerly jet stream and the low level jet have been coupled ver- tically over east Asia, while the Yangtze River Valley happens to locate in the ascending motion area between the upper jet stream and the low level jet, i.e. right of the entrance of the upper jet stream and left of the low level jet. Such a structure of the vertical circulation can trigger the Meiyu onset over the Yangtze River Valley.     

南印度洋海温偶极子型振荡及其气候影响

印度洋海表温度（Sea Surface Temperature,简称SST）的方差分析和相关分析表明南印度洋也存在一个海温偶极子型振荡,并定义了一个南印度洋海表温度异常偶极子指数.夏、秋季（南半球冬、春）的南印度洋偶极子指数与后期热带500hPa和100hPa高度场异常有显著而持续的相关,在冬、春达到最大,并可以持续到次年夏、秋.前期夏、秋季节的南印度洋偶极模对次年我国大陆东部夏季降水异常有显著的影响,对应偶极子正位相,次年夏季印度洋、南海（东亚）夏季风偏弱;副高加强且南撤、西伸,南亚高压偏强且位置偏东,易形成我国长江流域降水偏多,华南降水偏少;负位相年反之.后期冬季西太平洋暖池是联系南印度洋偶极子与次年我国夏季降水异常关系的一条重要途径.南印度洋偶极子表现出了明显的独立于ENSO（El Nio-Southern Oscillation,简称ENSO）的特征.     

The relationship between Indian monsoon precipitation along the Qinghai–Tibet Highway and differences in sensible heat flux

Sensible heat flux greatly influences the Indian monsoon. In this study, we calculated sensible heat flux time‐series for 12 sites over the western Tibetan Plateau using Price and Dunne's formula and adjusting the stability function. The time‐series were derived from the field observations from the GEWEX Asian Monsoon Experiment (GAME)/Tibet programme under the Global Energy and Water Cycle Experiment (GEWEX). This paper demonstrates that monthly sensible heat fluxes show strong correlations with corresponding precipitation, and that the correlation coefficients increase with precipitation amount. The preceding winter and spring solid precipitation (snowfall and resulting snowpack) can also influence sensible heat flux in May, but the situation is complex. The correlations between heat flux and snowfall at Amdo, Naqu, and Lhasa are negative, but they are positive at Gaize (also known as Gerze) and Dingri. There is a significant relationship between how the variations from the mean calculated heat fluxes at Amdo differ from those at Rikaze, or Dingri, Cuona and Longzi, and their respective June–September precipitation amounts. This phenomenon may result from changes in circulation. When the sensible heat fluxes are above average north of the influence of the Indian monsoon and below average to the south, the summer monsoon circulation develops early and with greater intensity and precipitation, and vice versa. Copyright © 2006 John Wiley & Sons, Ltd.     

南印度洋副热带偶极模在ENSO事件中的作用

南印度洋副热带偶极模（Subtropical Dipole Pattern,SDP）是印度洋存在的另一种很明显的偶极型海温差异现象,在年际和年代际尺度上均有十分明显的表现.而目前有关印度洋海气相互作用的研究主要集中在赤道印度洋地区,针对南印度洋地区的工作还比较少,特别是有关南印度洋海温与ENSO（El NiDo-Southern Oscillation）事件关系的研究.本文初步探讨了年际尺度上南印度洋副热带偶极型海温变化差异与ENSO事件的关系,发现SDP与ENSO事件有密切的联系,SDP事件就像连接正负ENSO位相转换的一个中间环节,SDP事件前后期ENSO的位相刚好完全相反.进一步,本文通过分析SDP事件前后期海温、高低层风、低层辐合辐散、高空云量和辐射等的变化特征研究了南印度洋偶极型海温异常在ENSO事件中的作用,结果表明：SDP在ENSO事件中的作用不仅涉及海气相互作用的正负反馈过程,还与热带和副热带大气环流之间的相互作用有关,特别是与东南印度洋海温变化所引起的异常纬向风由赤道印度洋向赤道太平洋传播的过程等有十分直接的关系;同时,SDP对ENSO事件的影响在很大程度上还依赖于大尺度平均气流随季节的变换.     

Roles of forced and inertially unstable convection development in the onset process of Indian summer monsoon

The NCEP/NCAR R1 reanalysis data are employed to investigate the impact of forced and inertial instability in the lower troposphere over the Arabian Sea on the onset process of Indian summer monsoon(ISM),and to reveal the important role of zonal advection of zonal geostrophic momentum played in the forced unstable convection.Results show that during the ISM onset the zero absolute vorticity contour(??=0)shifts northward due to the strong cross-equatorial pressure gradient in the lower troposphere over southern Arabian Sea.Thus a region with negative absolute vorticity is generated near the equator in the northern hemisphere,manifesting the evident free inertial instability.When a southerly passes through this region,under the influence of friction a lower convergence that facilitates the convection flourishing at the lower latitudes appears to the north of zero absolute vorticity contour.However,owing to such a traditional inertial instability,the convection is confined near the equator which does not have direct influence on the ISM onset.On the contrary in the region to the north of the zero absolute vorticity contour and to the south of the low pressure center near the surface,although the atmosphere there is inertially stable,the lower westerly jet can develop and bring on the apparent zonal advection of zonal geostrophic momentum.Both theoretical study and diagnosing analysis present that such a zonal advection of geostrophic momentum is closely associated with the zonal asymmetric distribution of meridional land-sea thermal contrast,which induces a convergence center near and further north of the westerly jet in the lower troposphere over the southwestern coast of the Indian Peninsula,providing a favorable lower circulation for the ISM onset.It illustrates that the development of convection over the Arabian Sea in late spring and early summer is not only due to the frictional inertial instability but also strongly affected by the zonal asymmetric distribution of land-sea thermal contrast.Moreover,before the ISM onset due to the eastward development of the South Asian High(SAH)in the upper troposphere,high potential vorticity is transported to the region over the Arabian Sea.Then a local trumpet-shaped stream field is generated to cause the evident upper divergence-pumping effect which favors the ISM onset.When the upper divergence is vertically coupled with the lower convergence resulted from the aforementioned forced unstable convection development near the southwestern coast of Indian Peninsula,the atmospheric baroclinic unstable development is stimulated and the ISM onset is triggered.     

夏季风期间长江流域的水汽输送状态及其年际变化

本文利用NCEP/NCAR再分析资料,分析了长江流域夏季风期间的水汽收支和循环,着重研究了不同月份与水汽收支的年际变化显著相关的大尺度水汽输送和环流异常.流域范围的西南夏季风水汽输送以6、7月最为强烈,经向输送在5～8月造成流域水汽辐合,9月造成辐散;纬向输送在5～7月造成流域水汽辐散,8、9月造成辐合.研究表明,在不同月份,流域的南北边界处的水汽输送在流域水汽收支的年际变化中起着不同的作用.这种变化与大气环流的异常密切相关.在夏季风相对较弱月份（5、8、9月）,流域水汽收支的年际变化极大地受到流域南边界南风水汽输入通道的影响,对应于水汽收入偏丰年,该3个月500 hPa高空在青藏高原东部都存在显著异常低压区,而且,8、9月在中南半岛及其以东洋面存在显著异常反气旋环流,与8月西太副高的向西向南异常伸展,以及9月副高的西伸较弱和南北范围较宽有关,这些异常环流均造成南边界的大量异常水汽输入.而在夏季风十分强盛的6、7月,流域北边界南风水汽输出极大增加,成为流域水汽收入年际变化的关键敏感通道,对应于水汽收入偏丰年,6月500 hPa高空主要受中纬度以黄海和东海为中心的异常低压系统和气旋性异常环流影响,与该区域副高偏南、偏弱有关,而7月则主要受中高纬以外兴安岭为中心的异常高压和反气旋性异常环流影响,应该是由于该区域大陆高压的频繁生成造成的,它们均造成流域北边界水汽输出的异常减少.     

横断山区贡卡湖记录的晚全新世区域水文变化及其与印度夏季风的联系

印度夏季风(ISM)是全球季风系统的重要组成部分,其异常变化会对我国西南地区的生态环境和社会经济带来巨大影响。晚全新世是距离当下最近的地质历史时段,了解晚全新世以来ISM变化及其对区域水文的影响历史与规律,对预测我国西南地区未来的季风降水变化具有重要的参考价值。本文选择位于横断山区且对ISM变化响应敏感的高山湖泊贡卡湖(海拔3529 m)为研究对象,在湖泊中心获取204.5 cm的连续高质量沉积岩芯(GK20B钻孔),利用陆生植物残体AMS¹⁴C测年方法建立了过去3300年来可靠的地层年代序列。通过开展XRF元素扫描、烧失量、总有机碳氮(TOC、TN)及有机碳同位素(δ¹³C_org)等气候与环境代用指标的分析测试,重建了贡卡湖距今3300年以来的水文变化历史。研究结果显示,贡卡湖泥炭与湖相沉积互层的沉积结构与对应的代用指标变化共同揭示了湖泊水位波动,贡卡湖的水位自晚全新世以来整体呈现降低的趋势,响应了在北半球不断减弱的夏季太阳辐射驱动下ISM强度的衰退过程。同时湖泊水位指示的ISM强度存在着百年时间尺度的波动,这可能与地...     

欧亚地区夏季大气环流年际变化的关键区及亚洲夏季风的关联信号

本文利用资料分析和数值模拟方法研究了欧亚地区夏季大气环流的相关性及其与亚洲夏季风的关联信号,以期为欧亚地区的气候变异及可预测性研究提供科学依据.结果表明:欧亚区域同期(JJA)500 hPa高度场年际变化的关键区包括热带区、中纬度的贝加尔湖和巴尔喀什湖之间以及欧洲地中海附近地区;表面气温的关键区主要位于热带海洋;海平面气压的关键区包括热带的海洋性大陆区域、印度洋和非洲大陆赤道附近部分区域、中高纬的贝加尔湖与巴尔喀什湖之间的地区.另外,夏季大气环流年际变化的春季关键区明显西移/南退,特别是表面气温(其西太平洋区不再是关键区).公用气候系统模式CCSM4.0的大气模式在给定海温年际变化的情况下对于上述大气环流相关场及其关键区的模拟基本合理,其中500 hPa高度场的模拟结果较好,海平面气压场的结果逊之;对于同期和前期的结果,模式都有夸大西太平洋海温影响的倾向.对于东亚夏季风指数与大气环流的同期年际变化信号而言,其空间分布基本表现为以30°N为界呈西南东北向的波列状分布;其春季前期信号中,30°N以南的显著区几乎都位于海洋,30°N以北主要位于欧洲、巴尔喀什湖与贝加尔湖之间的地区.南亚夏季风指数的前期显著相关区比同期明显西移/南退.总之,模式的模拟结果和观测结果相当吻合,但其同期模拟结果比前期的更好一些.这些结果说明:模式对于大气环流年际变化的耦合变化信息的刻画是基本合理的,这为利用气候模式进行有关可预测性研究和降尺度预测研究奠定了基础.     

欧亚地区夏季大气环流年际变化的关键区及亚洲夏季风的关联信号

本文利用资料分析和数值模拟方法研究了欧亚地区夏季大气环流的相关性及其与亚洲夏季风的关联信号,以期为欧亚地区的气候变异及可预测性研究提供科学依据.结果表明:欧亚区域同期(JJA)500 hPa高度场年际变化的关键区包括热带区、中纬度的贝加尔湖和巴尔喀什湖之间以及欧洲地中海附近地区;表面气温的关键区主要位于热带海洋;海平面气压的关键区包括热带的海洋性大陆区域、印度洋和非洲大陆赤道附近部分区域、中高纬的贝加尔湖与巴尔喀什湖之间的地区.另外,夏季大气环流年际变化的春季关键区明显西移/南退,特别是表面气温(其西太平洋区不再是关键区).公用气候系统模式CCSM4.0的大气模式在给定海温年际变化的情况下对于上述大气环流相关场及其关键区的模拟基本合理,其中500 hPa高度场的模拟结果较好,海平面气压场的结果逊之;对于同期和前期的结果,模式都有夸大西太平洋海温影响的倾向.对于东亚夏季风指数与大气环流的同期年际变化信号而言,其空间分布基本表现为以30°N为界呈西南东北向的波列状分布;其春季前期信号中,30°N以南的显著区几乎都位于海洋,30°N以北主要位于欧洲、巴尔喀什湖与贝加尔湖之间的地区.南亚夏季风指数的前期显著相关区比同期明显西移/南退.总之,模式的模拟结果和观测结果相当吻合,但其同期模拟结果比前期的更好一些.这些结果说明:模式对于大气环流年际变化的耦合变化信息的刻画是基本合理的,这为利用气候模式进行有关可预测性研究和降尺度预测研究奠定了基础.     

Impacts of El Niño–southern oscillation on global runoff: Characteristic signatures and potential mechanisms

Runoff signatures, including low flow, high flow, mean flow and flow variability, have important implications on the environment and society, predominantly through drought, flooding and water resources. Yet, the response of runoff signatures has not been previously investigated at the global scale, and the influencing mechanisms are largely unclear. Hence, this study makes a global assessment of runoff signature responses to the El Niño and La Niña phases using daily streamflow observations from 8217 gauging stations during 1960–2015. Based on the Granger causality test, we found that ~15% of the hydrological stations of multiple runoff signatures show a significant causal relationship with El Niño–southern oscillation (ENSO). The quantiles of all runoff signatures were larger during the El Niño phase than during the La Niña phase, implying that the entire flow distribution tends to shift upward during El Niño and downward during La Niña. In addition, El Niño has different effects on low and high flows: it tends to increase the low and mean flow signatures but reduces the high flow and flow variability signatures. In contrast, La Niña generally reduces all runoff signatures. We highlight that the impacts of ENSO on streamflow signatures are manifested by its effects on precipitation (P), potential evaporation (PET) and leaf area index (LAI) through ENSO-induced atmospheric circulation changes. Overall, our study provides a comprehensive picture of runoff signature responses to ENSO, with valuable insights for water resources management and flood and drought disaster mitigation.     

Influences of local hydroclimatology and teleconnections on Florida's precipitation and temperature variability

Understanding the influences of local hydroclimatology and two large-scale oceanic-atmospheric oscillations (i.e., Atlantic Multidecadal Oscillation (AMO) and El Niño-Southern Oscillation (ENSO)) on seasonal precipitation (P) and temperature (T) relationships for a tropical region (i.e., Florida) is the focus of this study. The warm and cool phases of AMO and ENSO are initially identified using sea surface temperatures (SSTs). The associations of SSTs and regional minimum, maximum and average surface air temperatures (SATs) with precipitation are then evaluated. The seasonal variations in P-SATs and P-SSTs associations considering AMO and ENSO phases for sites in (1) two soil temperature regimes (i.e., thermic and hyperthermic); (2) urban and non-urban regions; and (3) regions with and without water bodies, are analysed using two monthly datasets. The analyses are carried out using trend tests, two association measures, nonparametric and parametric statistical hypothesis tests and kernel density estimates. Decreasing (increasing) trend in precipitation (SATs) is noted in the recent multi-decadal period (1985–2019) compared to the previous one (1950–1984) indicating a progression towards warmer and drier climatic conditions across Florida. Spatially and temporally non-uniform variations in the associations of precipitation with SATs and SSTs are noted. Strong positive (weak negative) P–T associations are noted during the wet (dry) season for both AMO phases and El Niño, while significant (positive) P–T associations are observed across southern Florida during La Niña in the dry season. The seasonal influences are predominant in governing the P–T relationship over the regions with and without water bodies; however, considerable variations between El Niño and La Niña are noted during the dry season. The climate variability influences on P–T correlations for hyperthermic and thermic soil zones are found to be insignificant (significant) during the wet (dry) season. Nonparametric clustering is performed to identify the spatial clusters exhibiting homogeneous P–T relationships considering seasonal and climate variability influences.