Temporal Variability Of Drought Research Articles

Quantifying Multi-hazards and Impacts Over Different Growth Periods of Maize: A Study Based on Index Construction

AbstractOwing to the complexity and variability of global climate, the study of extreme events to ensure food security is particularly critical. The standardized precipitation requirement index (SPRI) and chilling injury index (ICi) were introduced using data from agrometeorological stations on the Songliao Plain between 1981 and 2020 to identify the spatial and temporal variability of drought, waterlogging, and low-temperature cold damage during various maize growth periods. Compound drought and low-temperature cold damage events (CDLEs) and compound waterlogging and low-temperature cold damage events (CWLEs) were then identified. To measure the intensity of compound events, the compound drought and low-temperature cold damage magnitude index (CDLMI), and compound waterlogging and low-temperature cold damage magnitude index (CWLMI) were constructed by fitting marginal distributions. Finally, the effects of extreme events of various intensities on maize output were examined. The findings demonstrate that: (1) There were significant differences in the temporal trends of the SPRI and ICi during different maize growth periods. Drought predominated in the middle growth period (MP), waterlogging predominated in the early growth period (EP) and late growth period (LP), and both drought and waterlogging tended to increase in intensity and frequency. The frequency of low-temperature cold damage showed a decreasing trend in all periods. (2) The CDLMI and CWLMI can effectively determine the intensity of CDLEs and CWLEs in the study area; these CDLEs and CWLEs had higher intensity and frequency in the late growth period. (3) Compared to single events, maize relative meteorological yield had a more significant negative correlation with the CDLMI and CWLMI.

Spatiotemporal monitoring of drought using satellite precipitation products: The case of Borena agro-pastoralists and pastoralists regions, South Ethiopia

Drought is increasingly affecting farmers in agro-pastoralist and pastoralists region. It is one of the most harmful natural disasters that significantly affects rain-fed agriculture in developing countries. Drought assessment is an important component of drought risk management. This study used CHIRPS rainfall data to monitor the characteristics of drought in Borena Zone in southern Ethiopia. The standardized precipitation index (SPI) is used to calculate the magnitude, intensity, and severity of drought during the rainy season. Results show that severe and extreme droughts were detected in the first rainy season (March to May) and second wet season (September to November). Severe and extreme droughts were detected in the first rainy/wet season in 1992, 1994, 1999, 2000, 2002–2004, 2008,2009, 2011, 2019–2021. The spatial and temporal variability of drought is highly influenced by El Nino-Southern Oscillation (ENSO) in Ethiopia. Results revealed that most of the first rainy season was dry. 2011 was the driest year during the first wet season. Drought risk events in the first wet season were greater than in the second wet season. Results show that drought more frequently occurred in the northern and southern part in the first wet season. In the second rainy season extreme drought was detected in 1990, 1992, 1993, 1994, 1996, and 1997. The results of this study will promote the importance of early warning measures, drought risk management, and food security management in the study area.

Spatiotemporal Variability of Drought and its Relationships to ENSO and IOD Indices in Somaliland

Drought is one of Somaliland’s most prevalent natural hazards, causing serious socioeconomic and environmental harm. This study investigated the spatial and temporal variability of drought and its relationships with El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) indices using the Standardized Evapotranspiration Index (SPEI) from 1981 to 2020. The Mann-Kendall trend test and Sen's slope estimator were used to assess the trends of annual and seasonal SPEI time series. The Empirical Orthogonal Function (EOF) was employed to examine the dominant modes of the SPEI series, and Pearson and partial correlation analyses were performed to investigate the associations between significant modes of drought variability and ENSO and IOD indices. The results demonstrated a statistically significant downward trend of SPEI (increasing drought) at 99% confidence level. The EOF analysis indicated two spatially distinct zones of drought variability in the west and east of the country. Drought variability had a statistically significant negative correlation with ENSO in the summer and winter seasons, and with IOD in the winter season and annual time series. The findings of this study will provide important information for drought risk assessment, mitigation, and predictability in Somaliland.
 Keywords: Drought, Standardized Precipitation Index, ENSO, IOD, Somaliland

Detecting Drought Variability by using Two-Dimensional Correlation Analysis

The Standardized Precipitation Index (SPI) has been recognized as the standard index that should be used for quantifying and reporting meteorological drought. This study aims to present the application of the two-dimensional correlation method for determining the spatial and temporal variability of drought among the SPI series. Heatmaps were used as a new way of visualizing SPI series which enables visual inspection of dry and wet periods. The developed method was applied on 13 meteorological stations in Central Anatolia. Significant variations in drought behaviour were found in the investigated region even for the stations close to each other.

Temporal Variability of Drought in Nine Agricultural Regions of China and the Influence of Atmospheric Circulation

In recent decades, the severe drought across agricultural regions of China has had significant impact on agriculture. The standardized precipitation evapotranspiration index (SPEI) has been widely used for drought analyses; however, SPEI is prone to be affected by potential evapotranspiration (PET). We thus examined the correlations between soil moisture anomalies and the SPEI calculated by the Thornthwaite, Hargreaves, and Penman–Monteith (PM) equations to select the most suitable for drought research. Additionally, the Mann–Kendall and wavelet analysis were used to investigate drought trends and to analyze and the impact of atmospheric circulation on drought in China from 1961 to 2018. The results showed that (1) PET obtained from the PM equation is the most suitable for SPEI calculation; (2) there were significant wetting trends in Northern China and the whole Chinese mainland and most of the wetting mutation points occurred in the 1970s and 1980s and the significant inter-annual oscillations period in the Chinese mainland was 2–4 years; (3) the Chinese mainland and Northern China are strongly influenced by West Pacific Trade Wind, while Western Pacific Subtropical High Intensity and Pacific Subtropical High Area have primary impact on Southern China.

Long‐term variability and trends in meteorological droughts in Western Europe (1851–2018)

AbstractWe analysed long‐term variability and trends in meteorological droughts across Western Europe using the Standardized Precipitation Index (SPI). Precipitation data from 199 stations spanning the period 1851–2018 were employed, following homogenisation, to derive SPI‐3 and SPI‐12 series for each station, together with indices on drought duration and severity. Results reveal a general absence of statistically significant long‐term trends in the study domain, with the exception of significant trends at some stations, generally covering short periods. The largest decreasing trends in SPI‐3 (i.e., increasing drought conditions) were found for summer in the British and Irish Isles. In general, drought episodes experienced in the last two or three decades have precedents during the last 170·years, emphasizing the importance of long records for assessing change. The main characteristic of drought variability in Western Europe is its strong spatial diversity, with regions exhibiting a homogeneous temporal evolution. Notably, the temporal variability of drought in Western Europe is more dominant than long‐term trends. This suggests that long‐term drought trends cannot be confirmed in Western Europe using precipitation records alone. This study provides a long‐term regional assessment of drought variability in Western Europe, which can contribute to better understanding of regional climate change during the past two centuries.

Evidence for intensification of meteorological droughts in Oman over the past four decades

Spatial and temporal variability of drought in Oman was assessed for the period 1979–2014 using the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) at both seasonal (3-month) and annual (12-month) timescales. Changes in drought characteristics (e.g. frequency, duration, intensity, severity, and spatial extent) were detected for six distinct climatic regions delineated using a Principal Component Analysis (PCA). Results suggest a statistically significant increase in the frequency and severity of drought during the past four decades. Nonetheless, drought exhibited pronounced decadal variability, with more frequent and severe droughts occurring after 1997/1998. Also, results demonstrate that the areas influenced by extreme and very extreme droughts witnessed a statistically significant increase on the order of 0.13% and 0.18%/decade, respectively, compared to −0.06%/decade for the areas impacted by mild droughts. Spatially, there has been a prevalent drying trend in central and eastern regions of Oman, while a tendency towards wetter conditions was observed in northern Oman (especially in the Batinah plain). This study also investigates the time-varying dependency between climatic drivers of drought and large-scale circulation patterns using the cumulative annual mean (CAM) method. Results indicate an acceleration of drying after 1997/1998, mainly due to the warming sea surface temperature (SST) anomalies over the Atlantic Ocean in recent decades and also the shift of the Pacific Ocean circulation patterns (e.g. PDO and El Nino) from the positive to the negative mode at the end of the 1990s. These configurations are favorable for significant changes in the thermodynamic drivers of drought (i.e. air temperature and relative humidity), but have less influence on aerodynamic drivers (i.e. wind speed). The significant temperature rise in recent decades, combined with a significant decline of rainfall and relative humidity, can explain the prolonged drought episode lasting from 1998 to 2014. A comprehensive assessment of drought variability in a poorly-studied water-stressed region like Oman is of particular concern for stakeholders and policy-makers. This study provides meaningful conclusions and a basis for water resources management, agricultural and energy production, ecological protection, and contribute to more reliable drought mitigation strategies at the regional and national scales.

Long-term variability of drought indices in the Czech Lands and effects of external forcings and large-scale climate variability modes

Abstract. While a considerable number of records document the temporal variability of droughts for central Europe, the understanding of its underlying causes remains limited. In this contribution, time series of three drought indices (Standardized Precipitation Index – SPI; Standardized Precipitation Evapotranspiration Index – SPEI; Palmer Drought Severity Index – PDSI) are analyzed with regard to mid- to long-term drought variability in the Czech Lands and its potential links to external forcings and internal climate variability modes over the 1501–2006 period. Employing instrumental and proxy-based data characterizing the external climate forcings (solar and volcanic activity, greenhouse gases) in parallel with series representing the activity of selected climate variability modes (El Niño–Southern Oscillation – ENSO; Atlantic Multidecadal Oscillation – AMO; Pacific Decadal Oscillation – PDO; North Atlantic Oscillation – NAO), regression and wavelet analyses were deployed to identify and quantify the temporal variability patterns of drought indices and similarity between individual signals. Aside from a strong connection to the NAO, temperatures in the AMO and (particularly) PDO regions were disclosed as one of the possible drivers of inter-decadal variability in the Czech drought regime. Colder and wetter episodes were found to coincide with increased volcanic activity, especially in summer, while no clear signature of solar activity was found. In addition to identification of the links themselves, their temporal stability and structure of their shared periodicities were investigated. The oscillations at periods of approximately 60–100 years were found to be potentially relevant in establishing the teleconnections affecting the long-term variability of central European droughts.

Spatial and Temporal Variability of Drought and Precipitation Using Cluster Analysis in Xinjiang, Northwest China

Analyses of drought and precipitation require long-term historical data and reasonable drought index to ensure reliable monitoring and prediction, especially for the Xinjiang Uygur Autonomous Region, where is sensitive and vulnerable to drought disasters. Drought characteristic was expressed using the observational precipitation data and calculated precipitation threshold at 105 meteorological stations from 1979 to 2014. The results showed that the variation of drought and precipitation was not spatially uniform. Analysis in sub-areas was conducted based on mutli-statistical methods. The historical drought and precipitation situation in Xinjiang is better characterized by three clusters. Cluster 1 is the driest, cluster 2 has a clear increasing trend of precipitation, while cluster 3 is the wettest with the mean annual precipitation approximates to 300 mm. In addition, the precipitation concentration and distribution is becoming uniform, there is a faster rate of increasing precipitation in cool-season rather than in warm-season. The results provide critical support for the drought disasters management and mitigation, it also provide a base for in-depth investigation on the possible mechanisms of regional drought.

Evolutional characteristics of hydro-meteorological drought studied using standardized indices and wavelet analysis

The study presents a more recent historical analysis of hydro-meteorological droughts in the Olifants Basin, South Africa from 1980 to 2013 using the Standardized Precipitation Index (SPI), the Standardized Precipitation Evapotranspiration Index (SPEI) and the Streamflow Drought Index (SDI). Time series of SPI, SPEI and SDI were analyzed for drought intensity and duration using wavelet analysis. Results indicate different periodicity and temporal variability of drought with significant meteorological drought incidence in 1991–2004 within the 2–8-year band and hydrological drought in 1996–2004 within the 3–5-year band. Spearman correlation analysis revealed a significant correlation (p < 0.01) between SPI and SPEI irrespective of the timescale. However, the strongest correlation occurred at the same timescale for SPI and SPEI: 3-month timescale (r = 0.904, p < 0.01), 6-month timescale (r = 0.936, p < 0.01), 9-month timescale (r = 0.946, p < 0.01) and 12-month timescale (r = 0.947, p < 0.01). A significant relation also existed between SDI and SPI (r = 0.731, p < 0.01) as well as between SDI and SPEI (r = 0.631, p < 0.01). Findings further indicate varying degrees of drought ranging from near normal to extremely dry conditions during 1980–2013. It is expected that strategies and policy actions on drought factor in the recurring nature of droughts so as to minimize the negative impact on water resources and other human–hydro-ecological systems.

Spatio temporal variability of drought and its trend in Marathwada

Spatio temporal variability of drought and its trend in Marathwada

Understanding the global hydrological droughts of 2003–2016 and their relationships with teleconnections

Droughts often evolve gradually and cover large areas, and therefore, affect many people and activities. This motivates developing techniques to integrate different satellite observations, to cover large areas, and understand spatial and temporal variability of droughts. In this study, we apply probabilistic techniques to generate satellite derived meteorological, hydrological, and hydro-meteorological drought indices for the world's 156 major river basins covering 2003–2016. The data includes Terrestrial Water Storage (TWS) estimates from the Gravity Recovery And Climate Experiment (GRACE) mission, along with soil moisture, precipitation, and evapotranspiration reanalysis. Different drought characteristics of trends, occurrences, areal-extent, and frequencies corresponding to 3-, 6-, 12-, and 24-month timescales are extracted from these indices. Drought evolution within selected basins of Africa, America, and Asia is interpreted. Canonical Correlation Analysis (CCA) is then applied to find the relationship between global hydro-meteorological droughts and satellite derived Sea Surface Temperature (SST) changes. This relationship is then used to extract regions, where droughts and teleconnections are strongly interrelated. Our numerical results indicate that the 3- to 6-month hydrological droughts occur more frequently than the other timescales. Longer memory of water storage changes (than water fluxes) has found to be the reason of detecting extended hydrological droughts in regions such as the Middle East and Northern Africa. Through CCA, we show that the El Niño Southern Oscillation (ENSO) has major impact on the magnitude and evolution of hydrological droughts in regions such as the northern parts of Asia and most parts of the Australian continent between 2006 and 2011, as well as droughts in the Amazon basin, South Asia, and North Africa between 2010 and 2012. The Indian ocean Dipole (IOD) and North Atlantic Oscillation (NAO) are found to have regional influence on the evolution of hydrological droughts.

Caractérisation à l’aide du SPI de la Sécheresse climatique dans le bassin versant de Ziz (Sud-Est, Maroc)

This paper analyzes the spatial and temporal variability of drought in winter season in an arid geographical gradient located in the eastern edge part of the Moroccan High Atlas especially at the Ziz watershed. Drought period between 1975 and 2013 were analyzed using the Standardized Precipitation Index (SPI) over 12 months for a 38-year scale. The results indicate that the most important drought by intensity and duration occurred during the 1982/84, 1983/84, 2000/01, and 2004/05 periods are generalized on all stations of the basin with a minimum SPI reached -2.77 at the Taous station. The results indicate that drought in winter have been tied to the dominance of dry circulation with the exception of the frequency of thermodynamic and north-eastern disturbances which are considered unstable aerological conditions. Which occur frequently between February and April of each year for the Errachidia station, Hassan Adakhil Dam, Foum Zaabal, Tillicht, M'Zizl, Arfoud and Taouz.

Spatial and Time Variability of Drought Based on SPI and RDI with Various Time Scales

The spatial and temporal variability of droughts were studied for the Northeast Algeria using SPI and RDI computed with monthly precipitation data from 123 rainfall stations and CFSR reanalysis monthly temperature data covering the period 1979–80 to 2013–14. The gridded temperature data was interpolated to all the locations having precipitation data, thus providing to compute SPI and RDI with the time scales of 3-, 6- and 12-month with the same observed rainfall data. Spatial and temporal patterns of droughts were obtained using Principal Component Analysis in S-Mode with Varimax rotation applied to both SPI and RDI. For all time scales of both indices, two principal components were retained identifying two sub-regions that are similar and coherent for all SPI and RDI time scales. Both components explained more than 70% and 74% of drought spatial variability of SPI and RDI, respectively. The identified sub-regions are similar and coherent for all SPI and RDI time scales. The Modified Mann-Kendall test was used to assess trends of the RPC scores, which have shown non-significant trends for decreasing drought occurrence and severity in both identified drought sub-regions and all time scales. Both indices have shown a coherent and similar behavior, however with RDI likely showing to identify more severe and moderate droughts in the southern and more arid sub-region which may be due to its ability to consider influences of global warming. Results for RDI are quite uniform relative to time scales and show smaller differences among the various climates when compared with SPI. Further assessments covering the NW and NE of Algeria using longer time series should be performed to better understand the behavior of both indices.

Spatial and temporal analysis of drought variability at several time scales in Syria during 1961–2012

This paper analyses the observed spatiotemporal characteristics of drought phenomenon in Syria using the Standardised Precipitation Index (SPI) and the Standardised Precipitation Evapotranspiration Index (SPEI). Temporal variability of drought is calculated for various time scales (3, 6, 9, 12, and 24months) for 20 weather stations over the 1961–2012 period. The spatial patterns of drought were identified by applying a Principal Component Analysis (PCA) to the SPI and SPEI values at different time scales. The results revealed three heterogeneous and spatially well-defined regions with different temporal evolution of droughts: 1) Northeastern (inland desert); 2) Southern (mountainous landscape); 3) Northwestern (Mediterranean coast). The evolutionary characteristics of drought during 1961–2012 were analysed including spatial and temporal variability of SPI and SPEI, the frequency distribution, and the drought duration. The results of the non-parametric Mann–Kendall test applied to the SPI and SPEI series indicate prevailing significant negative trends (drought) at all stations. Both drought indices have been correlated both on spatial and temporal scales and they are highly comparable, especially, over a 12 and 24month accumulation period. We concluded that the temporal and spatial characteristics of the SPI and SPEI can be used for developing a drought intensity - areal extent - and frequency curve that assesses the variability of regional droughts in Syria. The analysis of both indices suggests that all three regions had a severe drought in the 1990s, which had never been observed before in the country. Furthermore, the 2007–2010 drought was the driest period in the instrumental record, happening just before the onset of the recent conflict in Syria.

English

Analysis of spatial and temporal drought variability in the upper Tana River basin using Palmer Drought Severity Index (PDSI) was conducted. The drought is critical for formulation of mitigation measures in the river basin. A monthly temporal and 90-m spatial resolution was applied. This was achieved within ArcGIS environment. Climatic data for 1970 to 2010 was used for computation of the PDSI while the missing data sets were filled using Artificial Neural Networks (ANNs). The results of PDSI for dry and wet seasons at meteorological stations indicate that the time series plots for the PDSI values for dry season are generally lower than those for the wet seasons. The PDSI values for meteorological stations located at the lower elevation of the basin are lower than those located at higher elevation. On the other hand, spatially distributed drought severity based on PDSI show that the ranges of maximum and minimum drought severity values in 1970 are -0.868 to -0.804 and -0.675 to -0.610 respectively. These values of drought severity occur respectively in the north-western and south-eastern areas of the basin. PDSI values increased from the range -0.675 to -0.610 in 1970 and from -1.087 to 0.957 in 2010 for the north-eastern areas of the upper basin. The south eastern areas of the basin are more prone to drought risks than north-western parts. Use of the PDSI reflects the spatial heterogeneity and temporal variability of drought across the basin. The drought assessment offer technical approach for comprehensive understanding of drought for effective drought-induced disaster mitigation and its management, with a view to reducing adverse effects on livelihoods. Key words: Palmer Drought Severity Index (PDSI), drought severity, upper Tana River basin, monthly resolution, drought-induced disaster.

Spatio‐temporal characteristics of Indonesian drought related to El Niño events and its predictability using the multi‐model ensemble

ABSTRACTSpatio‐temporal drought characteristics related to weak, moderate, and strong El Niño events in the Indonesian region were investigated. The empirical orthogonal function (EOF) patterns of normalized monthly precipitation and Standardized Precipitation Index (SPI) for the period 1950–2010 were analysed; the influence of El Niño events are better represented by SPI. The effects of El Niño events on meteorological drought were investigated based on a composite analysis. The effects of El Niño were more obvious during June–July–August and September–October–November, while they were not significant in March–April–May especially during weak and moderate El Niño events. Spatial distributions of affected areas varied by season, intensity of El Niño events, and local factors. During moderate and strong El Niño events, meteorological drought was more common in southern Sumatra, Java, Bali, Nusa Tenggara, southeastern Kalimantan, Sulawesi, Maluku, and Papua. Based on the spatial and temporal variability of droughts, the monthly or seasonal meteorological drought outlook can be improved by using SPI focusing more on the regions and seasons that are affected by El Niño events. The multi‐model ensemble (MME) forecast of SPI performed better for the target months of September to November even at medium lead times. The months are consistent with the period of serious El Niño effects on drought, implying that serious drought conditions during the most harshly affected season can be predicted using the MME forecast.

Spatial and temporal variability of Standardized Precipitation Index over Indochina Peninsula

Indochina Peninsula has abundant water resources; however, most of the rain falls during the wet season. An arid condition is quite pronounced throughout the dry season. The majority of population depends on the agriculture as the main source of livelihood income. It is, therefore, important to study the drought and wetness over the region because crops are vulnerable to extreme climatic conditions. We used gridded precipitation APHRODITE and Standardized Precipitation Index (SPI) to evaluate the spatial and temporal variability of drought and wetness over Indochina peninsula. Nonparametric Modified Mann-Kendall (MMK) trend test was applied to determine the SPI trends over this region. There is a decrease in precipitation over a large part of Indochina during winter (dry season) and an increasing pattern during summer (rainy season). The increasing trend of SPI indicates an increase in wet condition over most parts of Indochina peninsula except for Red River Delta in Vietnam, central parts of Vietnam/Laos and western parts of Cambodia.

Changes in the frequency and severity of hydrological droughts over Ethiopia from 1960 to 2013

Here we present an analysis of drought occurrence and variability in Ethiopia, based on the monthly precipitation data from the Climate Research Unit (CRU-v3.22) over the period from 1960 to 2013. The drought events were characterized by means of the Standardized Precipitation Index (SPI) applied to precipitation data at a temporal scale of 12 months. At the national scale, the results reveal a statistically significant decrease in the severity of droughts over the 54-year period, a pattern that is mostly attributed to a statistically significant decrease in the frequency of high intensity drought episodes (i.e., extreme and very extreme droughts), compared to moderate droughts. To assess the general patterns of drought evolution, a principal component analysis (PCA) was applied to the SPI series. PCA results indicate a high spatial heterogeneity in the SPI variations over the investigated period, with ten different spatially well-defined regions identified. These PCA components accounted for 72.9% of the total variance of drought in the region. These regions also showed considerable differences in the temporal variability of drought, as most of the regions exhibited an increase in wetness conditions in recent decades. In contrast, the regions that receive less than 400 mm of annual precipitation showed a declining trend, with the largest changes occurring over Afar region. Generally, the highly elevated regions over the central Ethiopian Highlands showed the weakest changes, compared to the lowlands. This study confirms the local character of drought evolution over Ethiopia, providing evidence for policy makers to adopt appropriate local policies to cope with the risks of drought. Over Ethiopia, the detailed spatial assessment of drought evolution is required for a better understanding of the possible impacts of recurrent drought on agriculture, food production, soil degradation, human settlements and migrations, as well as energy production and water resources management across Ethiopia.

The influence of circulation weather patterns at different spatial scales on drought variability in the Iberian Peninsula

Europe has suffered several extreme weather events which were responsible for considerable ecological and economic losses in the last few decades. In Southern Europe, droughts are one of the most frequent extreme weather events, causing severe damages and various fatalities. The main goal of this study is to determine the role of Circulation Weather Types (CWT) on spatial and temporal variability of droughts by means the new multi-scalar Standardized Precipitation Evapotranspiration Index (SPEI). This study aims also to identify the main CWTs associated with winter and summer droughts over different regions of Iberian Peninsula. During the period between 1950 and 2012, the most frequent CWTs were found to be the Anticyclonic (A), Cyclonic (C), North (N) and Northeast (NE) types. The trend analysis for winter season shows a clear increase of frequency CWTs associated to dry events (A, East and Southeast) and a decrease of frequency of C and northern types while in summer a clear decrease of NE is observed. The spatial patterns of correlation between SPEI and CWT show large patterns of negative correlations with winter frequencies of A and eastern weather types, while the reverse occurs with C and western types. This feature is highlighted on a regional approach. The NE type presents negative correlations Central, Northwestern and Southwestern regions during winter and positive correlations in Eastern region during summer. In opposition, the West type presents positive correlations in all regions (except Eastern region) during winter and does not present significant correlations during summer. In general, the predominant CWT associated to winter or summer drought conditions differs greatly between regions. The winter droughts are associated mainly with high frequency of E types and low frequency of W types for all areas, while the summer drought in eastern sectors are linked with low frequency of C type, as well as the western regions are related with the N type.