Discharge variability in Romania using Palmer indices and a simple atmospheric index of large-scale circulation

ABSTRACT

The aim of this study is to analyse the seasonal characteristics of four Palmer indices calculated on the basis of data from 27 meteorological stations in Romania, and the impact of these indices on river discharges in the period 1931–1998. Our research also tests the influence of large-scale atmospheric circulation on these indices and on discharge. For each season, developments in the empirical orthogonal functions (EOF) and multivariate EOF (MEOF) are achieved. The MEOF representation highlights the overall characteristics of the four Palmer indices. It maximizes specific information for each season compared with individual information of each Palmer index. We then identify geographical areas with homogeneous distribution, taking into account both the discharge distribution and the rotated EOF components of each Palmer index. Finally, we analysed the impact of large-scale atmospheric circulation on hydro-climatic events in Romania by means of the Greenland-Balkan Oscillation Index (GBOI), which is shown to have a greater influence on southeastern Europe than the North Atlantic Oscillation Index (NAOI).     

Identification of extreme events using drought indices and their impact on the Danube lower basin discharge

In this study extreme droughts and extremely wet periods in the Danube upper and middle basin (DUMB) have been highlighted with specific indices. The most widely used indices have been considered to estimate both the dry and wet phenomena severity and the frequency or spatio‐temporal extension. The climatic condition of 15 meteorological stations situated in the Danube basin has been evaluated using four indices: Palmer Drought Severity Index (PDSI), Palmer Hydrological Drought Index (PHDI), Weighted PDSI (WPLM) and Palmer Z‐index (ZIND). The four indices have been analysed separately for each of the four seasons between 1901 and 2000. First the internal structure of the time series of the four indices has been analysed separately. Then the overall temporal characteristic has been analysed by means of the principal component of the Multivariate Empirical Orthogonal Functions decomposition of the four indices (PC1‐MEOF). For the discharge in the Danube lower basin, station Orsova has been chosen, representing an integrator of the discharges from the DUMB. A very close connection has been found between Palmer indices and Danube discharge in all seasons (with correlation coefficients greater then 0.80) excepting the spring season. A classification in five classes of both the four indices separately and the PC1‐MEOF has been achieved in order to highlight extreme events. The impact of phenomena quantified by Palmer indices in DUMB upon discharges in Danube lower basin is evident. It was demonstrated in this study that the Greenland‐Balkan Oscillation (GBO) influences the south‐east European hydro‐climatic regime more than the North Atlantic Oscillation (NAO) Index. Copyright © 2016 John Wiley & Sons, Ltd.     

North Atlantic Oscillation – Concepts And Studies

This paper aims to provide a comprehensive review of previous studies and concepts concerning the North Atlantic Oscillation. The North Atlantic Oscillation (NAO) and its recent homologue, the Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM), are the most prominent modes of variability in the Northern Hemisphere winter climate. The NAO teleconnection is characterised by a meridional displacement of atmospheric mass over the North Atlantic area. Its state is usually expressed by the standardised air pressure difference between the Azores High and the Iceland Low. ThisNAO index is a measure of the strength of the westerly flow (positive with strong westerlies, and vice versa). Together with the El Niño/Southern Oscillation (ENSO) phenomenon, the NAO is a major source of seasonal to interdecadal variability in the global atmosphere. On interannual and shorter time scales, the NAO dynamics can be explained as a purely internal mode of variability of the atmospheric circulation. Interdecadal variability maybe influenced, however, by ocean and sea-ice processes.     

Large-scale climate teleconnections with South Korean streamflow variability

ABSTRACT

Leading patterns of observed seasonal extreme and mean streamflow on the Korean peninsula were estimated using an empirical orthogonal teleconnection (EOT) technique. In addition, statistical correlations on a seasonal basis were calculated using correlation and regression analyses between the leading streamflow patterns and various climate indices based on atmospheric–ocean circulation. The spatio-temporal patterns of the leading EOT modes for extreme and mean streamflow indicate an upstream mode for the Han River, with increasing trends in summer, and a downstream mode for the Nakdong River, with oscillations mainly on inter-decadal time scales in winter. The tropical ENSO (El Niño Southern Oscillation) forcing for both extreme and mean streamflow is coherently associated with summer to winter streamflow patterns. The western North Pacific monsoon has a negative correlation with winter streamflow variability, and tropical cyclone indices also exhibit significant positive correlation with autumn streamflow. Leading patterns of autumn and winter streamflow time series show predictability up to two seasons in advance from the Pacific sea-surface temperatures.     

Teleconnections between oceanic–atmospheric indices and drought over Iran using quantile regressions

ABSTRACT

In this research, the Bayesian quantile regression model is applied to investigate the teleconnections between large oceanic–atmospheric indices and drought standardized precipitation index (SPI) in Iran. The 12-month SPI time series from 138 synoptic stations for 1952–2014 were selected as the drought index. Three oceanic–atmospheric indices, the North Atlantic Oscillation (NAO), the Southern Oscillation Index (SOI) and the Multivariate El Niño/Southern Oscillation Index (MEI), were selected as covariates. The results show that NAO has the weakest impact on drought in different quantiles and different regions in Iran. La Niña conditions amplified droughts through all SPI quantiles in western, Caspian Sea coastal regions and southern regions. The positive phase of MEI significantly modulates low SPI quantiles (i.e. drought conditions) throughout the Zagros region, Caspian Sea coastal regions and southern regions. The study shows that the effect of large oceanic–atmospheric indices have heterogeneous impacts on extreme dry and wet conditions.     

QANAT SYSTEMS IN IRAN

Abstract

A fuzzy rule-based technique is used for modelling the relationship between climatic forcing and droughts in a Central/Eastern European country, Hungary. Two types of climatic forcing'called premises'are considered: atmospheric circulation patterns (CP) and El Niño Southern Oscillation (ENSO). Both the Hess-Brezowsky CP types and ENSO events influence the occurrence of droughts, but the ENSO signal is relatively weak in a statistical sense. The fuzzy rule-based approach is able to learn the high space—time variability of monthly Palmer Drought Severity Index (PDSI) and results in a proper reproduction of the empirical frequency distributions. The “engine” of the approach, the fuzzy rules, are ascertained from a subset called the learning set of the observed time series of premises (monthly CP frequencies and Southern Oscillation Index) and PDSI response. Then an independent subset, the validation set, is used to check how the application of fuzzy rules reproduces the observed PDSI.     

Basin‐scale stream‐flow forecasting using the information of large‐scale atmospheric circulation phenomena

It is well recognized that the time series of hydrologic variables, such as rainfall and streamflow are significantly influenced by various large‐scale atmospheric circulation patterns. The influence of El Niño‐southern oscillation (ENSO) on hydrologic variables, through hydroclimatic teleconnection, is recognized throughout the world. Indian summer monsoon rainfall (ISMR) has been proved to be significantly influenced by ENSO. Recently, it was established that the relationship between ISMR and ENSO is modulated by the influence of atmospheric circulation patterns over the Indian Ocean region. The influences of Indian Ocean dipole (IOD) mode and equatorial Indian Ocean oscillation (EQUINOO) on ISMR have been established in recent research. Thus, for the Indian subcontinent, hydrologic time series are significantly influenced by ENSO along with EQUINOO. Though the influence of these large‐scale atmospheric circulations on large‐scale rainfall patterns was investigated, their influence on basin‐scale stream‐flow is yet to be investigated. In this paper, information of ENSO from the tropical Pacific Ocean and EQUINOO from the tropical Indian Ocean is used in terms of their corresponding indices for stream‐flow forecasting of the Mahanadi River in the state of Orissa, India. To model the complex non‐linear relationship between basin‐scale stream‐flow and such large‐scale atmospheric circulation information, artificial neural network (ANN) methodology has been opted for the present study. Efficient optimization of ANN architecture is obtained by using an evolutionary optimizer based on a genetic algorithm. This study proves that use of such large‐scale atmospheric circulation information potentially improves the performance of monthly basin‐scale stream‐flow prediction which, in turn, helps in better management of water resources. Copyright © 2007 John Wiley & Sons, Ltd.     

A multi‐species dendroclimatic reconstruction of Chilko River streamflow,British Columbia,Canada

Dendroclimatological data were used to reconstruct the discharge history of Chilko River, which drains a glacierized watershed in the Coast Mountains of British Columbia. We correlated ring‐width records from Engelmann spruce (ES) (Picea engelmanni) and mountain hemlock (MH) (Tsuga mertensiana) trees to historical hydroclimate data. Over the period of record, spruce and hemlock radial growth correlates significantly with temperature and snow depth, respectively. We found that a multi‐species approach provided a better model fit and reconstructive power. Using these relationships, we developed generalized linear models for mean June, July, and June‐July discharge. The proxy records provide insights into streamflow variability of a typical Coast Mountains river over the past 240 years and confirm the long‐term influence of the Pacific Decadal Oscillation (PDO) on hydroclimatic regimes in the region. A relationship also exists between the reconstructed June‐July discharge record and the North Pacific (NP) Index, suggesting that winter atmospheric patterns over the North Pacific influence the hydrology of coastal British Columbia. Copyright © 2010 John Wiley & Sons, Ltd.     

Hadley环流与北太平洋涛动的显著关系

利用NCEP/NCAR月平均再分析资料,分析了冬季（11～4月）Hadley环流与北太平洋涛动（NPO）的变化特征以及它们之间的关系.本文选取0°~30°N区域里最大质量流函数来描述北半球Hadley环流强度随时间的演变,利用(20°N, 180°~160°W) 和 (60°N, 180°~160°W) 区域平均的标准化海平面气压差代表NPO强度.结果表明,冬季北半球Hadley环流与NPO的变化形势非常一致,两者都具有显著的年际和年代际变化（70年代前处于负位相,80年代之后处于正位相）,同时还呈现出明显的增强趋势.Hadley环流变化与NPO异常的关系非常密切,在年际和年代际时间尺度上都具有显著的正相关.这种强相关性在大气环流场上可以得到很好的印证.研究还揭示,太平洋地区10°~30°N下沉支和40°~60°N上升支的异常运动可能是这种关系存在的主要内在原因.     

2008年和2012年冬季欧洲气候的差异及成因

2008年冬季（1月和2月）和2012年冬季均发生了较强的拉尼娜事件,但欧洲气候,尤其是西欧在这两年差异较大,2008年异常偏暖,而2012年却出现了极寒事件.诊断表明,大气环流异常是造成气候差异的直接原因.2008年冬季,北大西洋上空大气环流异常呈正位相的北大西洋涛动,有利于欧洲异常偏暖;2012年冬季,北大西洋和欧亚高纬阻塞的长期维持是西欧发生极端严寒的重要原因.通过数值试验,研究了前期海表热状况异常对大气的影响.结果表明：北大西洋海温异常能在一定程度上解释这两年欧洲各自的气候异常;尽管热带海温异常对2012年冬季的北大西洋环流形势和欧洲气候异常起一定的贡献,但不能解释2008年的情形;靠近欧洲的北极海冰异常偏少使得欧洲气候偏冷,对2008年的偏暖气候贡献为负,对2012年则有正贡献.     

The influence of hydroclimatic variability on flood frequency in the Lower Rhine

Climate change is expected to significantly affect flooding regimes of river systems in the future. For Western Europe, flood risk assessments generally assume an increase in extreme events and flood risk, and as a result major investments are planned to reduce their impacts. However, flood risk assessments for the present day and the near future suffer from uncertainty, coming from short measurements series, limited precision of input data, arbitrary choices for particular statistical and modelling approaches, and climatic non‐stationarities. This study demonstrates how historical and sedimentary information can extend data records, adds important information on extremes, and generally improves flood risk assessments. The collection of specific data on the occurrence and magnitude of extremes and the natural variability of the floods is shown to be of paramount importance to reduce uncertainty in our understanding of flooding regime changes in a changing climate. For the Lower Rhine (the Netherlands and Germany) estimated recurrence times and peak discharges associated with the current protection levels correlate poorly with historical and sedimentary information and seem biased towards the recent multi‐decadal period of increased flood activity. Multi‐decadal and centennial variability in flood activity is recorded in extended series of discharge data, historical information and sedimentary records. Over the last six centuries that variability correlates with components of the Atlantic climate system such as the North Atlantic Oscillation (NAO) and Atlantic Multi‐decadal Oscillation (AMO). These climatic non‐stationarities importantly influence flood activity and the outcomes of flood risk assessments based on relatively short measurement series. Copyright © 2016 John Wiley & Sons, Ltd.     

Inter-annual variability in snow cover depletion patterns and atmospheric circulation indices in the Upper Irtysh basin,Central Asia

The Irtysh River is the main water resource of Eastern Kazakhstan and its upper basin is severely affected by spring floods each year, primarily as a result of snowmelt. Knowledge of the large-scale processes that influence the timing of these snow-induced floods is currently lacking, but critical for the management of water resources in the area. In this study, we evaluated the variability in winter–spring snow cover in five major sub-basins of the Upper Irtysh basin between 2000 and 2017 as a possible explanatory factor of spring flood events, assessing the time of peak snow cover depletion rate and snow cover disappearance from the moderate-resolution imaging spectroradiometer (MODIS) MOD10A2 data set. We found that on average, peak snow cover retreat occurs between 22 March and 14 April depending on the basin, with large interannual variations but no clear trend over the MODIS period, while our comparative analysis of longer-term snow cover extent from the National Oceanic and Atmospheric Administration Climate Data Record data set suggests a shift to earlier snow cover disappearance since the 1970s. In contrast, the annual peak snow cover depletion rate displays a weak increasing trend over the study period and exceeded 5,900 km²/day in 2017. The timing of snow disappearance in spring shows significant correlations of up to 0.82 for the largest basin with winter indices of the Arctic Oscillation (AO) over the region. The primary driver is the impact of the large-scale pressure anomalies upon the mean spring (MAM) air temperatures and resultant timing of snow cover disappearance, particularly at elevations 500–2,000 m above sea level. This suggests a lagged effect of this atmospheric circulation pattern in spring snow cover retreat. The winter AO index could therefore be incorporated into long-term runoff forecasts for the Irtysh. Our approach is easily transferable to other similar catchments and could support flood management strategies in Kazakhstan and other countries.     

Large‐scale climatic influences on precipitation and discharge for a British river basin

This article aims to identify the large‐scale climate variables that yield significant statistical relationships with precipitation and discharge for a British river basin (Dyfi). Ranked correlation analysis was performed between gridded ERA‐40 atmospheric data and Dyfi precipitation and discharge for individual months. Precipitation and discharge demonstrate significant negative correlation with mean sea level pressure (MSLP). Strongest MSLP correlation areas move from north of Britain in winter to central Britain in summer; this shift is associated with a displacement of geopotential (Z) and zonal wind (U). Movement of significant correlation regions (not captured by the North Atlantic Oscillation Index) highlights the dynamic nature of precipitation and river flow generating weather systems throughout the year. Existence of strong significant correlation shows potential for exploiting large‐scale climate variables in forecasting precipitation and river flow in Britain. Copyright © 2010 John Wiley & Sons, Ltd.     

ENSO and the natural variability in the flow of tropical rivers

This paper examines the relationship between the annual discharges of the Amazon, Congo, Paran á, and Nile rivers and the sea surface temperature (SST) anomalies of the eastern and central equatorial Pacific Ocean, an index of El Niño-Southern Oscillation (ENSO). Since river systems are comprehensive integrators of rainfall over large areas, accurate characterization of the flow regimes in major rivers will increase our understanding of large-scale global atmospheric dynamics. Results of this study reveal that the annual discharges of two large equatorial tropical rivers, the Amazon and the Congo, are weakly and negatively correlated with the equatorial Pacific SST anomalies with 10% of the variance in annual discharge explained by ENSO. Two smaller subtropical rivers, the Nile and the Paraná, show a correlation that is stronger by about a factor of 2. The Nile discharge is negatively correlated with the SST anomaly, whereas the Paraná river discharge shows a positive relation. The tendency for reduced rainfall/discharge over large tropical convection zones in the ENSO warm phase is attributed to global scale subsidence associated with major upwelling in the eastern Pacific Ocean.     

Identifying non-stationary and long-term river–aquifer interactions as a response to large climatic patterns and anthropogenic pressures using wavelet analysis (Mancha Oriental Aquifer,Spain)

The objective of this study was to analyse periodicities and the long-term variability of monthly Júcar River–Mancha Oriental Aquifer interactions (RAI) and regionally measured precipitation (PP) with special focus on the correlations between these local hydrological variables and the large climatic patterns governing the Iberian Peninsula, represented by their teleconnection indices – the North Atlantic Oscillation index (NAOi) and the Western Mediterranean Oscillation index (WeMOi). To that end, wavelet analysis has been applied since it not only provides insight into the time-series dynamics but also permits statistical interpretation and correlation analysis. As a result, several periodicities have been detected: intermittent semi-annual periodicity in PP and the NAOi and annual periodicity in the RAI, NAOi and WeMOi time series. Long cycles (approximately 14 years) are also observed in the PP and WeMOi time series. The cross-wavelet spectra show a correlation between the RAI and the rest of the variables on the semi-annual and the annual scales, while wavelet coherence detects common behaviour with longer cycles – 5–6 years between the NAOi and the RAI and cycles of both 1–5 years and 7–10 years between PP and the RAI. Furthermore, results show that the periodicities in the teleconnection indices and precipitation propagate into the RAI with certain lead times: 3 months between the RAI and PP and 6 months between the RAI and the NAOi. The results indicate that the detected periodicities and the coherence between the studied variables could have applications in strategic planning on a river basin scale, taking into account the propagation times and the frequency scale. This methodological approach can be applied into strategic water resource planning independently of the geographical location of the hydrogeological system, the basin size and the climate region.     

Influence of the North Atlantic Oscillation (NAO) on climatic factors and estuarine water temperature on the Basque coast (Bay of Biscay): Comparative analysis of three seasonal NAO indices

The effect of the North Atlantic Oscillation (NAO) indices on climatic conditions and their subsequent influence on water temperature of two Basque estuaries (estuary of Bilbao and estuary of Urdaibai) were assessed by transfer function (TF) models for the period 1997–2006. Results showed that air temperature had an immediate (lag=0) and significant negative response to the NAO, whereas rainfall was not correlated with this climate index. The negative correlation between NAO and air temperature was found to be stronger with the seasonal indices derived from the differences in surface pressure between Iceland and Azores than with that derived from the principal component time-series of the leading eigenvector of the sea-level pressure in an Atlantic sector. The correlations between rainfall and river discharge, and between air temperature and water temperature were positive and highly significant in both estuaries. The response of water temperature to air temperature was immediate in both estuaries, whereas one-quarter lagged responses were also observed in the estuary of Bilbao, which is deeper and more stratified than the estuary of Urdaibai. Our study provides evidence that on the Basque coast the NAO plays an important role in climate variations, which in turn affect estuarine water temperature.     

Observed trends and relationships between ENSO and standardized hydrometeorological drought indices in central Chile

Droughts are natural phenomena that severely affect socio economic and ecological systems. In Chile, population and economic activities are highly concentrated in its central region (i.e. between latitudes 29°S and 40°S), which periodically suffers from severe droughts affecting agriculture, hydropower, and mining. Understanding the dynamics of droughts and large-scale atmospheric processes that influence the occurrence of dry spells is essential for forecasting and efficient early detection of drought events. Central Chile's climate is marked by a significant El Niño Southern Oscillation (ENSO) influence that might help to better characterize droughts as well as to identify the effects of ENSO on the spatial and temporal characteristics of meteorological and hydrological droughts in the region. We analysed the behaviour of the Standardized Precipitation Evapotranspiration Index (SPEI) and Standardized Streamflow Index (SSI) time series for 6-month accumulation periods over the austral winter and summer seasons. Multiple linear regression (MLR) and Generalized Linear Models (GLM) showed a significant ENSO influence on dry events for SPEI-6 and SSI-6 during winter and summer. We found that the magnitude of correlation between ENSO and SPEI-6 has changed over the last decades becoming weaker in winter periods and increasing in spring summer periods. Increasing trends in meteorological and hydrological drought events were also identified, along all latitudes, with significant trends during winter in the southern latitudes, and during summer in the semi-arid and Mediterranean zones. These results indicate that drought mitigation actions and policies are necessary to overcome their adverse effects. Given the monthly persistence of ENSO and its relationship to drought indices, there are opportunities for drought monitoring and seasonal forecasting that could become part of national drought management systems.     

Statistical and geostatistical investigations into the effects of the Gabcikovo hydropower plant on the groundwater resources of northwest Hungary / Analyses statistiques et géostatistiques des effets de la centrale hydroélectrique de Gabcikovo sur les ressources en eaux souterraines du Nord-Ouest de la Hongrie

Abstract

Abstract The construction of the Gabcikovo hydropower plant and the diversion of the Danube River over 25 km into an artificial channel in 1992 influenced the groundwater regime of the region considerably. Statistical and geostatistical methods are used to quantify changes of different groundwater characteristics on the Hungarian side of the river based on observations in the time period 1960–2000. External drift kriging was used to interpolate groundwater levels and the other related variables. While mean groundwater levels did not change appreciably, there are significant changes in the variability. Standard deviations of the groundwater levels and the amplitude of the annual cycle decreased near the old river bed of the Danube. The water-level fluctuations of the Danube have a decreased influence on the groundwater dynamics. Interrelationships between water levels in wells have also changed.     

Impact of the North Atlantic Oscillation on streamflow in Western Iran

The Middle East region, where arid and semi‐arid regions occupy most of the land, is extremely vulnerable to any natural or anthropogenic reductions in available water resources. Much of the observed interannual‐decadal variability in Middle Eastern streamflow is physically linked to a large‐scale atmospheric circulation patterns such as the North Atlantic Oscillation (NAO). In this work, the relationship between the NAO index and the seasonal and annual streamflows in the west of Iran was statistically examined during the last four decades. The correlations were constructed for two scenarios (with and without time lag). The associations between the annual and seasonal streamflows and the simultaneous NAO index were found to be poor and insignificant. The possibility of streamflow forecasting was also explored, and the results of lag correlations revealed that streamflow responses at the NAO signal with two and three seasons delays. The highest Spearman correlation coefficient of 0.379 was found between the spring NAO index and the autumn streamflow series at Taghsimab station, indicating that roughly 14% of the variance in the streamflow series is associated with NAO forcing. Copyright © 2013 John Wiley & Sons, Ltd.     

Hydroclimatic influence of large‐scale circulation on the variability of reservoir inflow

In this study, the nature of basin‐scale hydroclimatic association for Indian subcontinent is investigated. It is found that, the large‐scale circulation information from Indian Ocean is also equally important in addition to the El Niño‐Southern Oscillation (ENSO), owing to the geographical location of Indian subcontinent. The hydroclimatic association of the variation of monsoon inflow into the Hirakud reservoir in India is investigated using ENSO and EQUatorial INdian Ocean Oscillation (EQUINOO, the atmospheric part of Indian Ocean Dipole mode) as the large‐scale circulation information from tropical Pacific Ocean and Indian Ocean regions respectively. Individual associations of ENSO & EQUINOO indices with inflow into Hirakud reservoir are also assessed and found to be weak. However, the association of inflows into Hirakud reservoir with the composite index (CI) of ENSO and EQUINOO is quite strong. Thus, the large‐scale circulation information from Indian Ocean is also important apart form the ENSO. The potential of the combined information of ENSO and EQUINOO for predicting the inflows during monsoon is also investigated with promising results. The results of this study will be helpful to water resources managers due to fact that the nature of monsoon inflow is becoming available as an early prediction. Copyright © 2009 John Wiley & Sons, Ltd.