Monsoon Drought Research Articles

Identification of seasonal drought and its impacts on cropping season in Bangladesh

PurposeThis study aims to identify seasonal drought using standardized precipitation index (SPI). The following specific objectives are to generate result and identify seasonal drought and determine different scale of seasonal drought and its impacts on cropping season.Design/methodology/approachSeasonal SPI was calculated using long-term rainfall data for three seasons. The SPI was calculated using the formula and it is effective for the determinants. This study showed the functional relationship between drought duration, frequency and drought time scale using the SPI. SPI=X−X¯σ.FindingsSeasonal drought occurs more frequently in Bangladesh that affects crops and the agricultural economy every year. More severe drought was recorded during the Kharif-1 and Kharif-2 seasons and most crops were affected in these two seasons. No severe or moderate drought was recorded during the Rabi season. The results showed that monsoon crops were severely affected severely by extreme and severe droughts during the Kharif-2 season. Eventually, the people remain jobless during the monsoon, and they experience food shortages like monga. Several obstacles were recorded during the season, including delayed preparation of land, sowing, transplanting and other farming activities because of monsoon droughts. This study revealed that very frequently, mild dryness occurs in winter, but crop loss is minimal. The scale and occurrence of extreme droughts are more frequent during monsoons and reduce crop yields, affecting livelihoods in the study area. Seasonal drought affects cropping patterns as well as reduce crop yields.Originality/valueThe outcome of this study derived from the secondary data and field data.

A Pre‐Monsoon Signal of False Alarms of Indian Monsoon Droughts

AbstractCurrent knowledge suggests a drought Indian monsoon (perhaps a severe one) when the El Nino Southern Oscillation and Pacific Decadal Oscillation each exhibit positive phases (a joint positive phase). For the monsoons, which are exceptions in this regard, we found northeast India often gets excess pre‐monsoon rainfall. Further investigation reveals that this excess pre‐monsoon rainfall is produced by the interaction of the large‐scale circulation associated with the joint phase with the mountains in northeast India. We posit that a warmer troposphere, a consequence of excess rainfall over northeast India, drives a stronger monsoon circulation and enhances monsoon rainfall over central India. Hence, we argue that pre‐monsoon rainfall over northeast India can be used for seasonal monsoon rainfall prediction over central India. Most importantly, its predictive value is at its peak when the Pacific Ocean exhibits a joint positive phase and the threat of extreme drought monsoon looms over India.

Monitoring and Assessment of Drought using Remote Sensing and association rules

Drought is a natural threat that exists in all climatic zones around the globe. There is a need to categorize drought events and the probability of occurrence for better planning and management of relief and rehabilitation. In this study, drought monitoring indices namely the Standard Precipitation Index (SPI) and Vegetation Condition Index (VCI) were used to analyse the observed variability of monsoon droughts over Andhra Pradesh State. Precipitation data between 1991-2019 was used to evaluate the SPI and to evaluate the VCI from NDVI data collected from 2011 to 2019 using multi-temporal Terra MODIS Vegetation Indices Product (MOD13Q1). In this analysis, more often drought events occurred in 3 and 6 months SPI during monsoon season. In this study, data mining techniques (such as the Association Rules) are used to explain the association between VCI and SPI to predict the probability of occurrence of drought. The association rules formed by the VCI and the 3-month SPI with 77 percentage of confidence and 1.11 of lift indicate the higher accuracy of the rules and the effect on vegetation ford rainfall accumulation. This research incorporated the various software and dataset levels used to predict the probable occurrence and severity of drought using the current situation. The analysis revealed the advantages of NDVI and rainfall for indices of spatial and multitemporal drought to identify and forecast the characteristics of drought.

Regional tree-ring oxygen isotope deduced summer monsoon drought variability for Kumaun-Gharwal Himalaya

The precipitation in the Kumaun-Gharwal (Uttarakhand) Himalaya (KGH) is predominantly regulated by the Indian summer monsoon (ISM) and has been declining over the last century. However, because of limited historical data, it is difficult to place this recent decreasing trend in the context of pre-anthropogenic influences and understand a fuller range of hydroclimate scenarios for the region. Thus, we developed a 508-year regional tree-ring stable oxygen isotope (δ18OTR) record for multiple coniferous taxa (Abies spectabilis, Cedrus deodara and Picea smithiana) for the KGH, which spanned 1508–2015 CE. The δ18OTR record explained 35.8% of instrumental June–August Palmer Drought Severity Index (JJAPDSI) variance modelled from a nearby meteorological station. The JJAPDSI reconstruction shows regional coherency with both instrumental and proxy-based independent hydroclimatic records providing confidence that it is a reliable proxy to assess the long term hydroclimatic variability of the region. We identified decreasing strength of JJAPDSI reconstruction in KGH during recent decades indicating intensified drought. This is in the context of pre-anthropogenic influences and understand a fuller range of hydroclimate scenarios for the region. The δ18OTR also has coherency with Pacific and Indian Ocean sea surface temperature variability illustrating the broader teleconnections that influence the regional hydroclimate.

Climate warming and summer monsoon breaks drive compound dry and hot extremes in India

Considering the severe impacts of compound dry and hot extremes, we examine the primary drivers of CDHEs during the summer monsoon in India. Using ERA5 reanalysis, we show that most of the CDHEs in India occur during the droughts caused by the summer monsoon rainfall deficit. Despite a decline in the frequency of summer monsoon droughts in recent decades, increased CDHEs are mainly driven by warming and dry spells during the summer monsoon particularly in the Northeast, central northeast, and west central regions. A strong land-atmospheric coupling during droughts in the summer monsoon season leads to frequent CDHEs in the Northwest and southern peninsular regions. Furthermore, regional variations in land-atmospheric coupling cause substantial differences in the CDHE occurrence in different parts of the country. Summer monsoon rainfall variability and increased warming can pose a greater risk of compound dry and hot extremes with severe impacts on various sectors in India.

Protracted Indian monsoon droughts of the past millennium and their societal impacts

Protracted droughts lasting years to decades constitute severe threats to human welfare across the Indian subcontinent. Such events are, however, rare during the instrumental period (ca. since 1871 CE). In contrast, the historic documentary evidence indicates the repeated occurrences of protracted droughts in the region during the preinstrumental period implying that either the instrumental observations underestimate the full spectrum of monsoon variability or the historic accounts overestimate the severity and duration of the past droughts. Here we present a temporally precise speleothem-based oxygen isotope reconstruction of the Indian summer monsoon precipitation variability from Mawmluh cave located in northeast India. Our data reveal that protracted droughts, embedded within multidecadal intervals of reduced monsoon rainfall, frequently occurred over the past millennium. These extreme events are in striking temporal synchrony with the historically documented droughts, famines, mass mortality events, and geopolitical changes in the Indian subcontinent. Our findings necessitate reconsideration of the region's current water resources, sustainability, and mitigation policies that discount the possibility of protracted droughts in the future.

An assessment of the impact of Indian summer monsoon droughts on atmospheric aerosols and associated radiative forcing at a semi-arid station in peninsular India

A continuing increase in droughts/floods in Asian monsoon regions and worsening air quality due to aerosols are the two biggest threats to the health and well being of over 60% of the world's population. This study focuses on in-situ observations of atmospheric aerosols and their impact on shortwave direct aerosol radiative forcing (SDARF) during the southwest monsoon season (June–September) from 2015 to 2020 over a semi-arid station in Southern India. The Standardized precipitation index (SPI) is used to identify the droughts and normal monsoon years. Based on the SPI index, 2015, 2016, and 2018 were considered the drought monsoon years, while 2017, 2019, and 2020 were chosen as the normal monsoon years. During the drought monsoon years (normal monsoon years), the monthly mean black carbon (BC) was 1.17 ± 0.25 (0.72 ± 0.18), 1.02 ± 0.31 (0.64 ± 0.17), 1.02 ± 0.38 (0.74 ± 0.28), and 1.28 ± 0.35 μg/m3 (0.88 ± 0.21 μg/m3), for June, July, August and September respectively. The lower BC concentration during the normal monsoon years is mainly due to the enhanced wet-removal rates by high rainfall over the measurement location. In July, there was a high ventilation coefficient (VC) and low concentration of BC, while in September, low VC, and a high concentration of BC was observed in both the drought and the normal monsoon years. In addition, a plane-parallel radiative transfer model was used to estimate shortwave direct aerosol radiative forcing for composite and without BC at various surfaces, including the surface (SUF), atmosphere (ATM), and top of the atmosphere (TOA). During the drought monsoon years (normal monsoon years), the estimated monthly mean ATM forcing was 17.6 ± 2.4 (13.9 ± 2.1), 17.5 ± 7.5 (12.7 ± 4.4), 17.2 ± 4.0 (13.5 ± 1.9), and 17.4 ± 2.8 Wm−2 (14.6 ± 0.7 Wm−2) for June, July, August, and September, respectively. During the drought monsoon years, the estimated BC forcing was substantially larger (8.8 ± 2.6 Wm−2) than that of normal monsoon years (6.0 ± 1.5 Wm−2). It indicates the important role of absorbing BC aerosols during the drought monsoon years in introducing additional heat to the lower atmosphere, particularly over peninsular India.

Famines and likelihood of consecutive megadroughts in India

Consecutive failures in the summer monsoon rainfall led to widespread and severe droughts with profound implications for agricultural activities in India. However, the likelihood of successive megadroughts in India’s past and future climate remain poorly understood. Using Palmer Drought Severity Index (PDSI) from the Monsoon Asia Drought Atlas (MADA), we show that the major famines that affected millions of people during 1200–2018 were linked with summer monsoon droughts. Four megadroughts covering more than 40% of the country occurred for two consecutive summer monsoon seasons during 1200–2018. The most recent and severe megadrought occurred in 2002–2003. Simulations from the Community Earth System Model (CESM) for the last millennium (850–2005) ensemble (LME) show that the likelihood of two and three-year consecutive megadroughts during the summer monsoon is about 0.7 and 0.3 events per 100 years, respectively. Large ensemble simulations from CESM (CESM-LE) show a decline in the frequency of megadroughts in the future. Summer monsoon megadroughts are strongly associated with the warm sea surface temperature (SST) anomalies over the Pacific Ocean in the past and future climate. Substantial warming under the projected future climate can cause megadroughts under near-normal precipitation during the summer monsoon season. Despite the projected decline in the likelihood of the summer monsoon megadroughts under the warming climate, megadroughts in the future can have considerable implications for India’s food production and water availability.

Mid-Late Holocene Stalagmite δ18O and δ13C Records in Naduo Cave, Guizhou Province, China

Global warming and climate anomalies have attracted worldwide attention. The study of global climate change has received increasing attention from all countries and fields worldwide. Paleoclimate research is an important way to understand past global change and environmental evolution and to simulate and predict future climate development. A stalagmite ND3 collected in Naduo Cave was used to reconstruct the history of local climate and environmental changes from 0.55 to 5.07 ka BP based on the data of 13 230Th ages and 642 groups of oxygen and carbon stable isotopes. First, according to correlation analysis, δ18O and δ13C were significantly correlated (correlation coefficient r = 0.308, n = 318, P < 0.001 ) during the 5.07–2.00 ka BP period. However, during the period of 2–0.55 ka BP, there was no significant correlation P > 0.05 . The δ18O and δ13C data indicate that the climatic environment changed asynchronously during the period of 2.00–0.55 ka BP. During the period of 5.07–2.00 ka BP, the influence of human activities was weak, and δ18O and δ13C indicate similar climatic and environmental conditions, both of which changed in the same direction (positive correlation). In other words, when δ18O was positive, it indicated weak summer monsoons and lower precipitation, which led to declines in vegetation, weakened biological activity, and decreased soil CO2 and positive δ13C. The reverse patterns were also true. Since 2.0 ka BP, the intensity of human activities and the transformation and influence of surface vegetation have increased, and native vegetation has been destroyed in large quantities. Therefore, the climatic and environmental significance indicated by δ13C and δ18O has been well demonstrated. Second, the δ18O records showed that stalagmite ND3 responded to the weak monsoon drought events of 4.2 ka BP and 2.8 ka BP in the Holocene in a discontinuous deposition manner, which brings up new directions for future research.

Multivariate analysis of concurrent droughts and their effects on Kharif crops—A copula‐based approach

AbstractApart from creating an ecological imbalance, drought events could affect an agrarian country's economy and food security by reducing crop yields. The antecedent meteorological droughts could prolong into hydrological and (or) agricultural droughts and may co‐exist as concurrent droughts. The current study aims to comprehensively study Indian concurrent droughts, their effects on crop yield, and possible teleconnection with ENSO (El Niño–Southern Oscillation), adopting a copula‐based multivariate approach. The copula functions can replicate the correlation among the variables and keep the dependence structure intact. The concurrent drought characteristics are computed using a multivariate standardized drought index that incorporates the three primary drought indices using the Gaussian copula. Some of the severe concurrent drought years such as 2002, 1987, 1972, and 1965 caused considerable yield losses in Kharif season crops of groundnut, millet, and rice. This prompts to construct quad‐variate models involving the crop yield and the three drought indices using the vine copulas that perform better than the elliptical and symmetric Archimedean copula. Though the isolated forms of droughts could cause mild yield losses, the probability of concurrent droughts causing high to exceptional losses is more. Further, the ENSO teleconnection with the concurrent monsoon droughts is analysed and mapped. The above‐normal warming of the Nino 3.4 region over the tropical Pacific during the months leading up to the monsoon could signal concurrent monsoon droughts in the areas under the Ganga‐Brahmaputra basin at a probability of around 45%. These results could be helpful in drought mitigation measures and policymaking.

Regional and Seasonal Precipitation and Drought Trends in Ganga–Brahmaputra Basin

Satellite-based precipitation products can be a better alternative of rain gauges for hydro-meteorological studies in data-poor regions. This study aimed to evaluate how regional and seasonal precipitation and drought patterns had changed in the Ganga–Brahmaputra Basin between 1983 and 2020 with PERSIANN-CDR precipitation data. The spatial pattern of winter drought, monsoon drought, and Standardized Precipitation Index (SPI) calculated for different time scales were evaluated using principal component analysis. Ganga–Brahmaputra is one of the most populated river basins that flows through different geographical regions. Rain gauges are heterogeneously distributed in the basin due to its complex orography, highlighting the significance of gridded precipitation products over gauge observations for climate studies. Annual and monthly precipitation trends between 1983 and 2020 were evaluated using the original and modified Mann–Kendall trend test, and annual precipitation in the basin was found to be declining at a rate of 5.8 mm/year. An increasing trend was observed in pre-monsoon rainfall, whereas precipitation exhibited a decreasing trend for other months. Results of the Pettitt test showed precipitation time series was inhomogeneous and changepoint occurred around 2000. Decreasing trends of SPI indicated increasing frequency and intensity of drought events. Winter drought showed a clear spatial pattern in the basin; however, SPIs calculated for different time scales and monsoon drought had complex spatial patterns. This study demonstrates the applicability of satellite-based PERSIANN-CDR precipitation data in climate research in the Ganga–Brahmaputra Basin.

Spatiotemporal variability of rainfall and its effect on hydrological regime in a tropical monsoon-dominated domain of Western Ghats, India

Study regionKerala, Southern Western Ghats, India Study focusA better understanding of the trends in rainfall, its distribution and characteristics is of paramount importance, for effective management of water resources, especially, in regions with marked spatiotemporal variability in rainfall. The present study examines the spatiotemporal changes in rainfall pattern and its effect on the hydrological regime of a tropical monsoon dominated region in Western Ghats, India. New hydrological insights for the regionWeakening of monsoon rains was observed in the region, with decreasing trend in monsoon season, increasing trend in pre-monsoon season as well as an accelerated change in rainfall pattern during the recent decades. Monsoon droughts are becoming more prominent as evidenced the increasing trend in total number of dry and prolonged dry spells. Seasonality Index of rainfall consistently showed decreasing trend indicating that the rainfall distribution is becoming more symmetric over the years. No significant relationship was observed between ENSO events and rainfall characteristics indicating that ENSO episodes do not considerably affect the variability of rainfall in the region. Effect of changes in rainfall characteristics have a strong bearing on the hydrological regime of the region, which was evident in the declining trend of streamflow and groundwater level. The study calls for a revised plan for water management, adaptive cropping pattern and reservoir operations in the state of Kerala.

Factors responsible for consecutive deficit Indian monsoons during 2014 and 2015

Increased frequency of droughts in the recent decade (2002, 2004, 2009, 2014, 2015 being drought years) and the severity of their impact makes drought prediction an inherent component of forecasts for drought mitigation and preparedness. The research and development activities cannot prove their full potential unless they can produce a long lead skillful prediction of extreme conditions like flood or drought. During 2014 and 2015, India experienced deficit monsoon for two successive years. The present study explores the various factors responsible for the droughts during 2014 and 2015 and how well the state-of-the-art coupled model capture the consecutive droughts. The study shows that not only tropical sea surface temperatures (SSTs) but also extratropical SSTs can influence the rainfall over India. Extratropical SSTs impacted the rainfall during 2014 by modulating the strength and location of subtropical jet and tropospheric temperature gradient. On the other hand, tropical SSTs over Pacific Ocean influenced the rainfall during 2015 by modulating the atmospheric teleconnections via Walker circulation. The present-day models considered in this study could not capture consecutively the Indian monsoon droughts and the associated atmosphere and ocean conditions.

Effect Of Climate Change On Himalayan Regions

The climate change is one of the biggest challenges in the whole world. According to current situation the temperature of world can be anything between 1.5-5.80C.The warming rates of Himalaya is higher than rest of world. The rate of retreating of Himalayan glacier is six times greater than retreating rate of other glaciers of the world. The water supply the Asian countries like India, Bangladesh, Bhutan, China, Pakistan, Nepal, Afghanistan is done by the rivers which originate from Himalaya and the life circle, economic growth, agriculture and other requirement of life which is related to water of these countries is effected to the rivers which comes from Himalaya so it effected with any changes if happen in glaciers of Himalayas. The population of these countries is increases mainly of china and India at very fast rate, so in future their requirement in future will increase and the pressure on Himalayas will increase which is affected with continuing change climate change. Change climate has affected the Himalayas from various sides its glacier rate has accelerated, ecosystem and biodiversity has disturb in continuing pattern the flora and fauna is effected the species which were founded in low elevation regions in past decades is shifting towards upper elevation because of increase temperature this will create the problem of resources where they are shifting and competition is start for spacing we have no idea how long they will survive .Climate change also affected the alpine ecosystem this is most important by the point of view of medicine because 62%of medical plants are found from these parts. If this will continue affected then it will create great problem in the field of medicine. The increase temperature is favourable for female and transgender which is danger for their future production. The agricultural activity of above mention Asian countries is dependent on rivers which come from Himalaya due to retreating rate of glaciers of Himalayas there is chances of flood in future and due to change pattern of monsoon drought condition will come in future both drought and flood will create problems of live, diseases, food etc.

Influence of the Eastern Pacific and Central Pacific Types of ENSO on the South Asian Summer Monsoon

Based on observational and reanalysis data, the relationships between the eastern Pacific (EP) and central Pacific (CP) types of El Nino-Southern Oscillation (ENSO) during the developing summer and the South Asian summer monsoon (SASM) are examined. The roles of these two types of ENSO on the SASM experienced notable multidecadal modulation in the late 1970s. While the inverse relationship between the EP type of ENSO and the SASM has weakened dramatically, the CP type of ENSO plays a far more prominent role in producing anomalous Indian monsoon rainfall after the late 1970s. The drought-producing El Nino warming of both the EP and CP types can excite anomalous rising motion of the Walker circulation concentrated in the equatorial central Pacific around 160°W to the date line. Accordingly, compensatory subsidence anomalies are evident from the Maritime Continent to the Indian subcontinent, leading to suppressed convection and decreased precipitation over these regions. Moreover, anomalously less moisture flux into South Asia associated with developing EP El Nino and significant northwesterly anomalies dominating over southern India accompanied by developing CP El Nino, may also have been responsible for the Indian monsoon droughts during the pre-1979 and post-1979 sub-periods, respectively. El Nino events with the same "flavor" may not necessarily produce consistent Indian monsoon rainfall anomalies, while similar Indian monsoon droughts may be induced by different types of El Nino, implying high sensitivity of monsoonal precipitation to the detailed configuration of ENSO forcing imposed on the tropical Pacific.

Season of the drought

Indian Monsoon The Indian monsoon is a critical source of water for hundreds of millions of people, and when it fails to deliver its normal quantity of rain, enormous human, economic, and ecological costs can be incurred. Monsoon droughts are not always seasonal, however. Borah et al. found that nearly half of all monsoonal droughts were subseasonal and characterized by a steep decline in late-season rainfall. Moreover, this type of subseasonal drought appears to be related to a distinct cold anomaly in the North Atlantic Ocean, raising the possibility that monsoon droughts may be more predictable. Science , this issue p. [1335][1] [1]: /lookup/doi/10.1126/science.aay6043

Indian monsoon derailed by a North Atlantic wavetrain.

The forecast of Indian monsoon droughts has been predicated on the notion of a season-long rainfall deficit linked to a warm equatorial Pacific. Here we show that nearly half of all droughts over the past century differ from this paradigm in that they (i) occur when Pacific temperatures are near-neutral and (ii) are subseasonal phenomena, characterized by an abrupt decline in late-season rainfall. This severe subseasonal rainfall deficit can be associated with a Rossby wave from mid-latitudes. Specifically, we find that the interaction of upper-level winds with an episodic North Atlantic vorticity anomaly results in a wavetrain that curves toward East Asia, disrupting the monsoon. This atmospheric teleconnection offers an avenue for improved predictability of droughts, especially in the absence of telltale signatures in the Pacific.

Volatility in Rainfall and Predictability of Droughts in Northwest Bangladesh

This study was conducted to evaluate the variability, trends, volatility, and transition patterns of rainfall in drought-prone northwest Bangladesh. Daily rainfall recorded at five stations for the period 1959–2018 were used for this purpose. Non-parametric tests of variability changes, a modified Mann–Kendall trend test, innovative trend analysis (ITA), a generalized autoregressive conditional heteroscedasticity (GARCH)–jump model, and a Markov chain (MC) were used to assess the variability changes, trends, volatility, and transitions in rainfall to understand the possibility of the persistence of droughts and their predictability. The results showed an overall decrease of variability in annual and seasonal rainfall, but an increase in mean pre-monsoon rainfall and a decrease in mean monsoon rainfall. This caused a decrease in pre-monsoon droughts, but few changes in monsoon droughts. The ITA and rainfall anomaly analysis revealed high temporal variability and, thus, rapid shifts in rainfall regimes, which were also supported by the volatility dynamics and time-varying jumps from the GARCH–jump model and the rapid changes in drought index from the MC analysis. Therefore, the lack of drought in recent years cannot be considered as an indicator of declining droughts in the region.

Equatorial Indian Ocean Response during Extreme Indian Summer Monsoon Years Using Reliable CMIP5 Models

Monsoon droughts are characterized by the weakening of surface winds over the equatorial Indian Ocean and increased upper ocean heat storage, thus providing a sustained heat source for deep convection. This in turn increases rainfall over the eastern equatorial Indian Ocean and weakens the regional Hadley circulations, along with the subsidence of these circulations over the Indian subcontinent. To simulate reliable future climate projections, it is necessary to simulate the observed features during extreme monsoon events. In this study we examine the ability of climate models to simulate extreme rainfall events and their associated observed features over the Equatorial Indian Ocean (EIO) in the historical simulations of reliable IPCC CMIP5 models. Here we analyzed the Sea Surface temperature (SST), Ocean vertical temperature and rainfall anom alies during extreme rainfall events in the time period 1951–2005. In addition, we utilized reliable CMIP5 models which can simulate the Indian summer monsoon (ISM) and tried to study ocean features during extreme years. Drought years produce an abnormal su bsurf ace warming (0.5–1.5°C) over the Eastern Equatorial Indian Ocean (EEIO). This subsurface warming changes the wind pattern and causes drought in land regions of India. At the same time during excess monsoon years there is an abnormal cooling over the EEIO.

Drought Characterization Using Drought Indices and El Niño Effects

Droughts are extreme meteorological and hydrological events having severe impacts on the natural environment and socioeconomic conditions of the affected region. Drought characterization is vital in planning, design, and management of water resources systems. Drought indices such as standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI) are generally used as a tool for monitoring changes in drought conditions. The variability of monsoon droughts over India is significantly influenced by the tropical sea surface temperature anomalies. A major portion of the drought variability is influenced by the El Nino–Southern Oscillation (ENSO). Drought characterization for Indo Gangetic alluvial plain demonstrates the efficacy of the present work. The values of SPI and SPEI are found in all the possible ranges representing mild, moderate, and severe drought events in the study area. Most drought events are observed in Raebareli District, as both SPI and SPEI indices showed 27 and 23 severe events. Also, correlation is found between observed drought periods, ENSO episodes, and drought indices.