Rotated Empirical Orthogonal Function Research Articles

Regionalization study of maximum daily temperature based on grid data by an objective hybrid clustering approach

Regionalization plays an important role in climatology and hydrology research, as it can be used to understand the changing pattern of a hydroclimate variable better and explore its underlying drivers effectively by identifying homogeneous regions which are determined by topography and climatic conditions. However, as climatic drivers change over time, homogeneous regions should vary when the intra-annual patterns are of interest. In this paper we investigate the regionalization and changing patterns of maximum daily temperature in China based on the maximum air temperature (Tmax) derived from the CN05.1 daily surface air temperature gridded data from 1981 to 2010. We also propose an objective variable-based regionalization approach by combining the rotated empirical orthogonal function (REOF) method and K-means clustering analysis, to determine homogeneous regions effectively for such a large volume of daily grid data. The REOF approach is not able to identify discrete regional boundaries easily, but it is efficient at determining the number of regions and their rough spatial location. Furthermore, while clustering analysis suffers from convergence when the number of observations is large, it is able to provide clean boundaries between regions. The hybrid method tends to overcome their individual disadvantages while maintaining their individual advantages. The results show that the patterns of homogeneous regions have distinct features in both summer and winter seasons. The reasonableness of regionalization is then examined by the first principal component within regions, which shows the homogeneity within regions, and the scatterplots of Tmax time series between regions, which show the heterogeneity between regions. Furthermore, the boxplots of normalized Tmax within regions for both summer and winter seasons also clearly show the different characteristics of inter-annual variability in each region. In order to reveal detailed process of the change in the number and pattern of regions, we further conducted the regionalization for individual months and explored the potential linkages between regionalization and climatic conditions over time. This research is noteworthy in promoting research on regionalization with consideration of seasonal and intra-annual variation as well as extreme temperature events. The proposed hybrid regionalization method is broadly applicable to other regionalization studies.

Interdecadal variability in pan-Pacific and global SST, revisited

Interest in the “Interdecadal Pacific Oscillation (IPO)” in the global SST has surged recently on suggestions that the Pacific may be the source of prominent interdecadal variations observed in the global-mean surface temperature possibly through the mechanism of low-frequency modulation of the interannual El Nino-Southern Oscillation (ENSO) phenomenon. IPO was defined by performing empirical orthogonal function (EOF) analysis of low-pass filtered SST. The low-pass filtering creates its unique set of mathematical problems—in particular, mode mixing—and has led to some questions, many unanswered. To understand what these EOFs are, we express them first in terms of the recently developed pairwise rotated EOFs of the unfiltered SST, which can largely separate the high and low frequency bands without resorting to filtering. As reported elsewhere, the leading rotated dynamical modes (after the global warming trend) of the unfiltered global SST are: ENSO, Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). IPO is not among them. The leading principal component (PC) of the low-pass filtered global SST is usually defined as IPO and it is seen to comprise of ENSO, PDO and AMO in various proportions depending on the filter threshold. With decadal filtering, the contribution of the interannual ENSO is understandably negligible. The leading dynamical mode of the filtered global SST is mostly AMO, and therefore should not have been called the Interdecadal “Pacific” Oscillation. The leading dynamical mode of the filtered pan-Pacific SST is mostly PDO. This and other low-frequency variability that have the action center in the Pacific, from either the pan-Pacific or global SST, have near zero global mean.

Comprehensive assessment of drought risk in the arid region of Northwest China based on the global palmer drought severity index gridded data

Droughts are extremely widespread natural disasters, which cause the most severe losses among natural disasters. The comprehensive drought risk in Northwest China (NWC) was evaluated based on the self-calibrating (SC) Palmer Drought Severity Index (PDSI) and copula method. The major conclusions are the following: (1) based on the rotated empirical orthogonal function (REOF), a significant consistency in the spatial distribution of the monthly averaged SC-PDSI was observed in NWC, especially in the subregions Inner Mongolia Plateau (IM), Hexi Corridor (HX), and Qiangtang Plateau (QT); (2) the largest frequency was obtained for slight drought and slight wet conditions, while extreme drought and extreme wet showed the lowest values; (3) with respect to the PDSI-th, the Clayton, Arch12, Arch12, Arch12, Arch12, and Frank played the major roles in the copula weight in the subregions IM, HX, Qinghai River Basin (QH), QT, North Xinjiang (NXJ), and South Xinjiang (SXJ), respectively. In terms of PDSI-pm, Arch12, Clayton, Gaussian, Arch12, Clayton, and Clayton dominated the weights of multi-copula functions in the regions IM, HX, QH, QT, NXJ, and SXJ, respectively; and (4) the frequency and probability of droughts in each area differed. The least drought events occurred in the QT and the most emerged in the HX for SC-PDSI.

Spatial and Temporal Characteristics of Drought and Flood in Quanzhou Based on Standardized Precipitation Index (SPI) in Recent 55 Years

To analyze the characteristics of drought and flood variations in Quanzhou during recent 55 years, the standardized precipitation index (SPI) and Empirical Orthogonal Function (EOF) and Rotated Empirical Orthogonal Function (REOF) were calculated by using the monthly precipitation data from 6 meteorological bureaus across Quanzhou for 1960-2014. Results showed that: 1) During 1960-2014, the drought and flood showed Periodic variation characteristics in Quanzhou, the specific period of frequent drought was 1963-1972, 1977-1986 and 2009-2011, and the specific period of frequent flood was 1961-1962, 1972-1975, 1990-1992 and 1997-2007; the drought and flood did not have significant tendency of variation in Spring and Summer, and the drought and flood showed a non-significant downward trend in Autumn and Winter. 2) The drought and flood variation had relatively consistent trend in Quanzhou, but there was a big difference on the northwest mountainous area, the middle semi-mountainous area and the southeast coast area in some periods. 3) The precipitation cell and distribution in every season were Nan’an and Anxi, but there was a big difference in rainfall less area: it was Yongchun and Dehua in Spring, Chongwu and Jinjiang in Summer, Chongwu in Autumn, Dehua and Chongwu in Winter.

Spatio‐temporal pattern of meteorological droughts and its possible linkage with climate variability

ABSTRACTSpatial pattern of meteorological droughts is a critical element for investigating drought condition. Empirical orthogonal function (EOF) and rotated EOF (REOF) methods are widely used to discriminate dominant patterns. Previous works focused on case applications but left the associated physical meanings unexplored. This study employs the EOF, wavelet analysis, and stepwise regression methods to distinguish spatio‐temporal patterns of meteorological droughts with pattern interpretation. For this purpose, monthly precipitation data were used from 73 meteorological stations in the Poyang Lake basin of China for the period of 1960–2007. Standardized Precipitation Index (SPI) was applied to quantify meteorological droughts. Our results identified four leading EOF/REOF patterns at the basin scale. Seasonal SPI series showed more dominant periods than monthly SPI series, indicating periodic complexity of regional droughts. The dominant periods coincided with those of the EOF/REOF time coefficients. The four leading EOF/REOFs were significantly related to the dominant time scales of 2, 3, 6, 11, 16, and 17 years, except for the fifth EOF/REOF. Stepwise regression analysis further indicated that the four leading EOF/REOFs be likely the superposition of SPI variation from inter‐annual and inter‐decadal scales. The SPI series at different time scales (1–36 months) showed significant correlations with climate teleconnection modes, confirming the existence of identified spatio‐temporal pattern. The Niño3.4 and Southern Oscillation were the primary teleconnection modes that influenced the variability of meteorological droughts over the basin. The Pacific Decadal Oscillation and the North Atlantic Oscillation also contributed to the regional dry/wet alternation. The findings provide important implications for understanding spatial and temporal variation of regional droughts.

Regionalization based on spatial and seasonal variation in ground-level ozone concentrations across China

Owing to the vast territory of China and strong regional characteristic of ozone pollution, it's desirable for policy makers to have a targeted and prioritized regulation and ozone pollution control strategy in China based on scientific evidences. It's important to assess its current pollution status as well as spatial and temporal variation patterns across China. Recent advances of national monitoring networks provide an opportunity to insight the actions of ozone pollution. Here, we present rotated empirical orthogonal function (REOF) analysis that was used on studying the spatiotemporal characteristics of daily ozone concentrations. Based on results of REOF analysis in pollution seasons for 3years' observations, twelve regions with clear patterns were identified in China. The patterns of temporal variation of ozone in each region were separated well and different from each other, reflecting local meteorological, photochemical or pollution features. A rising trend in annual averaged Eight-hour Average Ozone Concentrations (O3-8hr) from 2014 to 2016 was observed for all regions, except for the Tibetan Plateau. The mean values of annual and 90 percentile concentrations for all 338 cities were 82.6±14.6 and 133.9±25.8μg/m3, respectively, in 2015. The regionalization results of ozone were found to be influenced greatly by terrain features, indicating significant terrain and landform effects on ozone spatial correlations. Among 12 regions, North China Plain, Huanghuai Plain, Central Yangtze River Plain, Pearl River Delta and Sichuan Basin were realized as priority regions for mitigation strategies, due to their higher ozone concentrations and dense population.

Analysis on dry-wet characteristics in the Zhujiang River Basin in recent 55 years

Based on Palmer Drought Severity Index (PDSI), this paper analysed the spatial distribution of dry-wet condition in the Zhujiang River Basin in 1961-2015, and studied the dry-wet climatic regionalization by Rotated Empirical Orthogonal Function (REOF). The results show most areas in the Zhujiang River Basin was in normal state of dry-wet condition and the whole basin can be divided into 7 dry-wet climate zones. The above conclusions reflect the dry-wet characteristics in the basin reasonably, which can provide a reference on prediction, monitoring and evaluation of some kinds of extreme climatic disasters such as floods and droughts inextricably linked with dry-wet condition.

Variation of Precipitation and Streamflow in the Upper and Middle Huaihe River Basin, China, from 1959–2009

ABSTRACT An, G. and Hao, Z., 2017. Variation of precipitation and streamflow in the Upper and Middle Huaihe River Basin, China, from 1959–2009. In: Zhi, Y. and Guido Aldana, P.A. (eds.), Sustainability of Water Resources and the Development of Coastal Environments: Select Proceedings from the 2016 International Conference on Water Resource and Environment (WRE2016). Journal of Coastal Research, Special Issue No. 80, pp. 69–79. Coconut Creek (Florida), ISSN 0749-0208. Variation of precipitation and streamflow in the Upper and Middle Huaihe River Basin (UMHRB) during 1959–2009 is characterized in this study with the Mann-Kendall (MK) test and Sen's slope estimator. The rotated empirical orthogonal function (REOF) was selected to obtain the sub-basins over UMHRB. The cumulative sum of departures of modulus coefficient (CSDMC) was used in this study to detect the temporal variability of streamflow and precipitation. The results are drawn as follows: (1) Four sub-basins were obtained by REOF: Region I: norther...

Increasing drought has been observed by SPEI_pm in Southwest China during 1962–2012

Drought is regarded as an environmental disaster that has significantly impacted on economic and social development in recent decades, particularly in Southwest China (SWC). Here, we presented an analysis of spatiotemporal variations of meteorological drought in SWC. We computed the standardized precipitation evapotranspiration index using the Penman–Monteith equation (SPEI_pm) and explored spatiotemporal variational evolution of drought in SWC during 1962–2012 utilizing the Mann–Kendall test, continuous wavelet transform (CWT), and rotated empirical orthogonal function (REOF). We also analyzed the relationship between the rotated principal component time series (RPCs) and the 74 circulation indices. The results showed that (1) SWC displayed a dry trend at annual and seasonal scale with a 2–4-year significant period. The SWC can be divided into six sub-climate regions with different aridity and wetness features. (2) Among the six regions, the Eastern Yunnan Plateau showed an obvious dry trend while Hanzhong Basin and the Eastern Mountain Areas showed a wet trend that was not significant. There were 2–8-year periods of oscillation in general within the variation of the six regions. (3) The indices of the strength of the polar vortex in Asia (APVA) may affect the drought evolution of the whole study area. The change of drought in Eastern Yunnan Plateau and Sichuan Basin may be associated with Tibetan Plateau High.

Variability of precipitation extremes and dryness/wetness over the southeast coastal region of China, 1960–2014

ABSTRACTThe spatio‐temporal variability of precipitation extremes was investigated over the southeast coastal region of China, based on daily precipitation records from 73 meteorological stations for the period of 1960–2014. The standardized precipitation index (SPI), rotated empirical orthogonal function (REOF), and wavelet analysis methods were used to assess the characteristics of dryness/wetness patterns, as well as their correlations with the El Niño/Southern Oscillation (ENSO). Most of the extreme indices exhibited increasing trends at the regional scale over the past 55 years and generally had larger magnitudes than in other regions, with heavy precipitation that was more concentrated in time. The annual total wet day precipitation (PRCPTOT), total precipitation on very wet days (R95p), and total precipitation on extremely wet days (R99p) displayed sharp increases, with magnitudes of 22.60 (P = 0.33), 21.50 (P = 0.02) and 7.80 (P = 0.06) mm decade−1, respectively. In contrast, the regional averaged maximum consecutive wet days (CWD) decreased by −0.10 days decade−1 (P = 0.48). The stations with sharp increases in extreme heavy precipitation were mainly located in the north and coastal areas. In addition, three dominant dryness/wetness patterns (REOFs) corresponding to different climatic partitions were identified. With regard to the time coefficients (PCs) of each REOF, no significant trend in PC1 was detected, while an increasing trend (P < 0.001) in PC2 and decreasing trend (P < 0.001) in PC3 were found. The evolution of dryness/wetness occurred over a 4‐ to 12‐year period in the first three REOFs, and their PCs all had significant positive correlations with ENSO at a 12‐month lag time. This study suggests that the risks of heavy precipitation and flooding are likely to increase in southeast China, particularly in the northern and coastal areas, and extreme events may be strengthened in the southern subtropical zone during certain seasons.

Variability of Cloudiness over Mountain Terrain in the Western United States

Abstract This study investigates the spatial and temporal variability of cloudiness across mountain zones in the western United States. Daily average cloud albedo is derived from a 19-yr series (1996–2014) of half-hourly Geostationary Operational Environmental Satellite (GOES) images. During springtime when incident radiation is active in driving snowmelt–runoff processes, the magnitude of daily cloud variations can exceed 50% of long-term averages. Even when aggregated over 3-month periods, cloud albedo varies by ±10% of long-term averages in many locations. Rotated empirical orthogonal functions (REOFs) of daily cloud albedo anomalies over high-elevation regions of the western conterminous United States identify distinct regional patterns, wherein the first five REOFs account for ~67% of the total variance. REOF1 is centered over Northern California and Oregon and is pronounced between November and March. REOF2 is centered over the interior northwest and is accentuated between March and July. Each of the REOF/rotated principal components (RPC) modes associates with anomalous large-scale atmospheric circulation patterns and one or more large-scale teleconnection indices (Arctic Oscillation, Niño-3.4, and Pacific–North American), which helps to explain why anomalous cloudiness patterns take on regional spatial scales and contain substantial variability over seasonal time scales.

Characterizing present and future drought changes over eastern China

ABSTRACTThis study aimed to characterize present and future drought changes over eastern China using observations and Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. We used the standardized precipitation evapotranspiration index (SPEI) to characterize droughts at the timescales of 3‐ and 12‐month. We distinguished the spatial patterns of drought regimes (DRs) using the rotated empirical orthogonal function (EOF) on the SPEI. Our results identified three DRs centered in northern China (DR1), northeastern China (DR2), and southern China (DR3). Droughts in these regimes have increased in recent decades. Most CMIP5 models reproduce at least two of the DRs. The rotated EOF results indicate that the three DRs can jointly explain 46–68% of the SPEI variance (compared with approximately 60% in the observations). Among the 33 CMIP5 models analysed in this study, 13 simulated all three DRs well and showed a strong correlation pattern (>0.5). Further analysis indicated that both the external natural and greenhouse gas forcing experiments in CMIP5 reproduced the DRs, implying that natural variability and anthropogenic activity play important roles in the formation of these DRs. With continued climate warming, the three DRs over eastern China will persist. Despite the uncertainties in drought changes that mainly depend on the potential evapotranspiration methods, climate model results suggest that droughts would be aggravated under warming scenarios. The probability of severe droughts increase by the end of the century: 33% in DR1, 25% in DR2, 34% in DR3 in RCP4.5 and almost double in RCP8.5 scenario in Thornthwaite method based SPEI estimates.

Climate response and spatial-temporal model on the inter-annual change of winter temperature-salinity in the East China Sea

Spatial distributions and time variation characteristics were analyzed using Rotated Empirical Orthogonal Function (REOF) and spectrum analysis methods using surface and bottom temperature and salinity data in February of 1976-2013 along 30°N section in the East China Sea. Result showed that temperature trends can be divided into western part and east part, salinity trend divided into western, middle and eastern part. The first mode of surface temperature presented a quasi-equilibrium trend and the range was higher in the near-shore than the offshores, first mode of bottom temperature presented a decreasing trend; surface salinity had a decreasing trend and the extent was higher in the near-shore than the offshores, the bottom salinity showed a decreasing trend in recent years. The temperature inter-annual variability related to El Niño closely; short-term shocks of salinity related to El Niño, and long-term changes had something to do with PDO.

Long-term AVHRR SST change analysis in the Taiwan Strait using the rotated EOF method

Long-term AVHRR SST change analysis in the Taiwan Strait using the rotated EOF method

High‐resolution simulation of the boreal summer intraseasonal oscillation in Met Office Unified Model

The present study investigates the fidelity and resolution sensitivity of Met Office Unified Model Global Atmosphere 3.0 in simulating the intraseasonal oscillation (ISO) of the East Asia and western North Pacific summer monsoon. Two sets of model simulations at horizontal resolutions of N216 (60 km) and N96 (130 km) forced by observed daily high‐resolution sea‐surface temperature are examined and compared with the observations. Diagnostic results show that the model can well reproduce the gross spatio‐temporal features of observed summer ISO over East Asia and the western North Pacific in terms of period, dominant rotated empirical orthogonal function (REOF) mode, variance, northward propagation, cycle evolution and vertical structure. The intraseasonal change in intensity and position of the western North Pacific subtropical high and upper troposphere westerly jet associated with the northward propagating ISOs are also reasonably captured in the model. Moreover, increasing horizontal resolution improves most aspects of the ISO simulation, especially the intensity and northward propagation of the ISO‐related convection and circulation systems. Further analysis indicates that the improvement as resolution increases is related to the weakening in background state of the East Asian summer monsoon, which is due to the realistic simulation of land–sea thermal contrast in the higher resolution model. This study suggests that enhanced model resolution can have some beneficial impacts on the ISO simulation.

Aridity over a semiarid zone in northern China and responses to the East Asian summer monsoon

AbstractIn this study, the variability of the aridity over a semiarid zone in Northern China from 1961 to 2013 and its association with the northernmost margin of the East Asian summer monsoon (EASM) and its intensity were investigated using composite analysis, correlative analysis, and unrotated and rotated empirical orthogonal function analyses. The results indicate that a nonsignificant drying trend (0.03/10 years) occurs over the entire eastern semiarid zone, accompanied by a southward retreat in the northernmost margin of the EASM (0.24°/10 years), and that drying (0.11/10 years) and southward retreat (0.28°/10 years) are most obvious on the Loess Plateau. The response of the annual variation of aridity to changes in the northernmost margin of the EASM is also compared with its responses to changes in the intensity of the EASM; the results indicate that it is more related to the former, especially over the Loess Plateau. Furthermore, an abnormal aridity event is likely to occur over the Loess Plateau in a year when the EASM margin advances to its northernmost latitude abnormally ahead of time and retreats abnormally ahead of time compared to the mean state.

Does drought in China show a significant decreasing trend from 1961 to 2009?

In recent decades, the occurrence and severity of drought in China has had devastating impact on social and economic development. The increase in drought has been attributed to global warming. We used the high-accuracy self-calibrating Palmer Drought Severity Index (scPDSI) to investigate the variation in drought in China between 1961 and 2009 using the Mann-Kendall (MK), continuous wavelet transform (CWT) and the rotated empirical orthogonal function (REOF) methods. We also analyzed the relationship between the rotated principal component time series (RPCs) and 74 circulation indices. The results revealed that: 1) all of China experienced a significant wet trend at annual and seasonal scale; an abrupt change in the drought pattern occurred around 1970 with a 2-8-year significant period; 2) eight major sub-climate regions were identified: Northwest China, Northeast-Inner Mongolia Plateau, Greater Khingan Range area, Northern Tibetan Plateau, Southern Tibetan Plateau, Central China, Huang-Huai-Hai Plain and Southeast China. Of these regions, the Southern Tibetan Plateau experienced a significant wet trend, but the Northeast-Inner Mongolia Plateau and Northern Tibetan Plateau became significantly drier. Using either annual or seasonal scales, Northwest China became significantly wetter and Central China became more arid. In addition, the period of each sub-climate region shared a significant 2-8-year band; 3) the polar vortex exhibited dominant patterns that affected most areas of China. The Pacific Decadal Oscillation had a significant influence on drought evolution, especially for Northwest China and the Huang-Huai-Hai plain. Additionally, the El Niño-Southern Oscillation also affected drought evolution, and the Central China was impacted by the Indian Ocean Dipole.

The Intraseasonal Oscillation of Eastern Tibetan Plateau Precipitation in Response to the Summer Eurasian Wave Train

Abstract This study examines the characteristics and mechanisms associated with the dominant intraseasonal oscillation (ISO) that controlled eastern Tibetan Plateau summer rainfall (ETPSR) over the period 1979–2011. The results of both power and wavelet spectrum analysis reveal that ETPSR follows a significant 7–20-day oscillation during most summers. The vertical structure of the ETPSR ISO in the dry phase is characterized by a vertical dipole pattern of geopotential height with a positive center on the eastern Tibetan Plateau (TP) and a negative center on the western TP. The wet phase shows the opposite characteristics to the dry phase. The transitions between the dry and wet phases during an ETPSR ISO cycle are related to a Rossby wave train that presents as large anomalous anticyclonic and cyclonic centers that alternate along the pathway from the eastern Atlantic to southern China via the TP. It corresponds to the evolution of the phase-independent wave-activity W, which implies an eastward/southeastward energy propagation of the ISO. The dominant modes of the daily 200-hPa geopotential height as identified by the rotated empirical orthogonal function (REOF) demonstrate that the different phases of the Rossby wave train influence the upper-level circulation over the eastern TP, which then impacts precipitation in the region. Furthermore, fluctuations in the eastern Atlantic may be the key factor for the propagation of the Rossby wave train that influences the upper-level circulation and rainfall variability over the eastern TP. Results from numerical experiments using an atmospheric general circulation model support the conclusion that the fluctuations over the eastern Atlantic contribute to the ISO of ETPSR.

Multi‐model CMIP5 and CORDEX simulations of historical summer temperature and precipitation variabilities over West Africa

ABSTRACTThe mean climatology, inter‐model variability and spatio‐temporal patterns of temperature and precipitation over West Africa from Coupled Model Intercomparison Project 5 (CMIP5), CMIP5_SUBSET [ensemble of global climate models driving COordinated Regional climate Downscaling EXperiment (CORDEX)] and CORDEX multi‐model ensembles are evaluated and intercompared for the monsoon season (June–September). We find that, while CORDEX fails to outperform the simulated mean climatology of temperature by the CMIP5 ensembles, it substantially improves precipitation and provides more realistic fine‐scale features tied to local topography and landuse. This improved performance over the region is found to depend more on the internal models physics than the driving boundary conditions and results from a more consistent and realistic simulation of monsoon precipitation across the various regional climate models (RCMs). Rotated empirical orthogonal function (REOF) analysis indicates that the CORDEX ensemble captures better the spatio‐temporal variability of both temperature and precipitation (first REOF mode), in particular depicting the warming and Sahel precipitation recovery in recent decades over West Africa. On the other hand, the spatial patterns and associated time series of the last two REOF modes in CORDEX mostly follow the CMIP5_SUBSET pointing towards a strong role of the boundary forcing in the RCM simulation of precipitation variability.

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

ABSTRACTThe 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).