Monsoon Rainfall Distribution Research Articles

Optimal rainfall threshold for monsoon rice production in India varies across space and time

Climate change affects Indian agriculture, which depends heavily on the spatiotemporal distribution of monsoon rainfall. Despite the nonlinear relationship between crop yield and rainfall, little is known about the optimal rainfall threshold, particularly for monsoon rice. Here, we investigate the responses of rice yield to monsoon rainfall in India by analyzing historical rice production statistics and climate data from 1990 to 2017. Results show that excessive and deficit rainfall reduces rice yield by 33.7% and 19%, respectively. The overall optimal rainfall threshold nationwide is 1621 ± 34 mm beyond which rice yield declines by 6.4 kg per hectare per 100 mm of rainfall, while the identifiable thresholds vary spatially across 14 states. The temporal variations in rice yield are influenced by rainfall anomalies featured by El Niño-Southern Oscillation events.

Sustainability, Livelihood Security and Resilience to Climate Variability of Small Holders through Small Onion and Jasmine Cropping in NICRA Village of Namakkal District, Tamil Nadu, India

In Vadavathur & Jambumadai village, small onion is the major vegetable crop cultivated as an annual crop in more than 400 acres and it is the only source of income. In scarce rainfall situations in National Innovations on Climate Resilient Agriculture NICRA village of Vadavathur and Jambumadai, practice of sole cropping (Small onion) is predominant but it is risky and often results in low yields or sometimes even in crop failure due to erratic monsoon rainfall and skewed distribution. To alleviate these problems in such areas of drought villages of Namakkal district crop diversification with perennial crop is a feasible option to minimize risk in crop production, ensure reasonable returns with limited care. Additional crop of onion with Jasmine is more profitable and is a key drought coping strategy especially for small and marginal farmers. Hence jasmine (20 cent) + small onion (80 cent) cultivation model is demonstrated in crop component mode in NICRA village instead of cultivating small onion as a sole crop in an area of 1 acre/ 0.4 ha. Considering the yield, jasmine flower yield was obtained in the range of 545 to 747 kg/ 20 cent and in case of small onion, the bulb yield of 4800 kg was harvested from 80 cent/season. Highest net returns of Rs.134622 / 20 cent was obtained from Jasmine cultivation compared to small onion of Rs.48,810 / 80 cent /season.

The Effect of the Climate Change Looming for Indian Summer Monsoon

In recent years, with global temperature rise, extreme weather and climate events, especially high temperature heat waves, floods and droughts, occur more frequently. Recent observational and simulation projections with paleoclimate research reported that monsoon rainfall would become uncertain in the forthcoming century. One of the major problem in predicting the ISM is the accumulation of uncertainties arising from the complex role of Anthropocene activities on the climate system. This, in turn, would result in indecisions in the projected Gross Domestic Product (GDP), sustainable growth, and economy. About 21% of the Indian GDP is contributed by Agriculture (arguably the backbone of India's Economy) which itself highly dependent on the spatial and temporal distribution of monsoon rainfall. This paper reviews the projection of the Indian Summer Monsoon (ISM) in looming climate change. Evaluating the uncertainties in future monsoon behavior predictions would facilitate informed decision-making while formulating strategies for a sustainable Economy and energy policy. Reliable predictions of future ISM in the context of climate change are required while formulating adaptation strategies for sustainable growth and water availability.

Assessment of Rainfall Variability over Karnataka state in India

Karnataka state situated in the southern part of India is surrounded by a long coast line and western ghat mountain regions on the western side. The rainfall distribution shows a large variability in different parts of the state. North interior part in the state receives less rainfall as compared to the south interior region, which receives moderate rainfall. Although coastal Karnataka witnesses a high rainfall and is one of the most important regions in the country, in recent years the state has faced drought because of less rainfall which may be attributed to the regional climate change. Because of the decreasing trend in rainfall almost all the important sectors like agriculture and water resources have been affected a lot in Karnataka. In this work, assessment of the spatial and temporal distribution of monsoon rainfall is presented in terms of the climatology, variability, trend etc. The time series analysis also indicated the increasing trend of rainfall as a whole but the trend in recent decades is very less as compared to the long term. The El-Niño Southern Oscillation (ENSO) and the regional rainfall pattern relation is also explored. The increase in heavy rainfall in recent times is observed from the analysis. This study can be useful in addressing regional climate change, understanding the local and large scale variability impacts and in can be a good input for preparing the policy formulation and pro-active disaster management.

Impact of dust aerosols on the Indian Summer Monsoon Rainfall on intra-seasonal time-scale

The Indian subcontinent is a major hotspot of aerosol loading and several studies have demonstrated the importance of interaction between Asian dust aerosols and the Indian Summer Monsoon Rainfall (ISMR). However, a few studies have suggested a declining trend in the pre-monsoon dust loading over the Indian region. The present study investigates how the decrease in dust emissions influences the intensity of the monsoon rainfall over the Indian region using a fully coupled Weather Research and Forecasting model coupled with chemistry (WRF-Chem). We have conducted two sets of simulation experiments for the June-July-August-September (JJAS) season of 2017: (1) a control experiment with the default dust aerosol emissions, and (2) a sensitivity experiment by reducing dust emissions by 60%. Our analyses show that the reduction in dust emissions significantly alters the spatial distribution of seasonal mean monsoon rainfall with enhanced rainfall over the Bay of Bengal (BoB) and eastern coastal regions, and decreased rainfall over the lower Indo-Gangetic Plain (IGP), and parts of Central India (CI). It is found that the low aerosol loading conditions alter the large-scale circulation patterns along the tropical/sub-tropical region centred between 15°N and 35°N and 40°E −130°E, and through dynamical feedback, strengthens the convective rainfall over the BoB region. As a result, the 10–20 days sub-seasonal mode of the ISMR strengthens, facilitating enhanced westward propagation of rain-bearing systems formed over northern BoB which provides substantial rainfall over vast regions of the Indian subcontinent.

Causes of decadal distributions of Indian summer monsoon rainfall during the last 1500 years

AbstractUnderstanding the distribution of Indian summer monsoon rainfall (ISMR) and the relationships with external forcing factors (EXFs) and internal climate variability (ICV) is crucial for improving the prediction accuracy of ISMR. Here, we explore and quantify the influence of EXFs and ICV on the distribution of ISMR at decadal timescales over the past 1500 years using the Community Earth System Model (CESM). We identified the major ICV and EXF modes of ISMR distributions in simulations from 501 to 1850 ad and 1851 to 2000 ad. The results show that ICV plays a dominant role in decadal ISMR changes during the pre‐industrial and the current periods. Simulations show east–west contrasts over the Indian subcontinent, with a center of rainfall variability in the high‐latitude regions around 28°N. The EXF mode, calculated from the leading mode of the singular value decomposition analysis, exhibits a nearly uniform increase (or decrease) in the distribution of ISMR over the study region. From 501 to 1850 ad, the signals of greenhouse gases (GHGs), and land use and land cover (LULC) in the ISMR at the decadal timescale are weak and difficult to isolate from the ICV. Our numerical analyses show that natural forcing was a dominant contribution (~64.1%) to the forced ISMR distribution. From 1851 to 2000 ad, natural forcing, GHGs and LULC all contributed to the forced ISMR distribution. GHGs represent a key external forcing factor affecting the distribution of ISMR and could explain 22.3% of the total variance. LULC and natural forcing factors contribute 17.5 and 16.4% to the total variance, respectively. Our results contribute to a better understanding of the mechanisms controlling decadal variability of the Indian summer monsoon.

Distribution and Prediction of Monsoon Rainfall in Homogeneous Regions of India: A Stochastic Approach

Distribution and Prediction of Monsoon Rainfall in Homogeneous Regions of India: A Stochastic Approach

Aerosol distribution during spring and summer and its relationship with Inter-Annual variability of summer monsoon rainfall over Indian region

The inter-annual variability of aerosol over Indian region and its possible implications on the Indian summer monsoon is investigated using Aerosol Optical Depth (AOD) data from Moderate Resolution Imaging Spectrometer (MODIS) during the period 2001 to 2016. An attempt has been made to link the seasonal aerosol distribution over South Asia during spring and summer season to Indian Summer Monsoon Rainfall (ISMR) distribution. AOD derived from MODIS Terra observations shows large spatial inhomogeneity and characteristic variability with distinct temporal distributions. The pre-monsoon (spring) seasons (March-April-May; MAM) is characterized by high aerosol concentration over north-west part of India along with IGP region. During the south-west monsoon (summer) seasons (June-July-August-September; JJAS), wide area of high aerosol loading along Indo Gangetic plains extending from North Arabian sea through north-west India and up to Himalayan foothills. The regional rainfall distribution and its relationship with spatial and temporal aerosol distribution are then evaluated. Further, distinct variability in the aerosol distribution with respect to excess, deficit and normal monsoon years are analysed to understand the role of aerosols on the inter-annual variability of ISMR. Monthly aerosol distribution over Central India [CI;15°-25oN;75°-88oE] and Indo Gangetic plain [IGP;25°- 350N;70°- 900E] shows increased aerosol loading during deficit rainfall years and corresponding delay in the monthly rainfall peak. This study emphasises the active role of regional aerosol loading on regulating the inter-annual variability of regional Indian summer monsoon rainfall.

High temporal variability of surface solar irradiance due to cloud enhancement effect over the Western Ghat mountains in peninsular India

Western Ghat (WG) mountains along the west coast of peninsular India play an important role in the distribution of monsoon rainfall on its windward and leeward sides. The study unravels the increase in cloudiness and also increase in the Cloud Enhancement of Solar irradiance (CES) during the dry and wet seasons in the windward (high altitude) and the leeward (low altitude) sides of WG. From May 2018 to May 2021, the global horizontal irradiance (GHI) reveals wide temporal and spatial variability with a high frequency of CES events, even exceeding the extraterrestrial value around the noontime during the wet season (May to October). The observed excess irradiance due to partial/transparent clouds compared to clear sky maxima is more than 400 W/m2 on the windward and 350 W/m2 on the leeward. With the increase in cloudy days, diffused irradiance also increased with greater values during CES events. CES induced positive shortwave cloud radiative forcing (CRF) varied up to +400 W/m2 (warming effect), while negative CRF varied up to −1000 W/m2 (cooling effect). During monsoon, the warming due to frequent CES events could locally offset a large cooling to some extent. Clouds resulting in huge irradiance variability in a day could also induce large fluctuations in renewable solar energy. Such short-term sporadic local events are averaged out in satellite data.

Indian Ocean Dipole : Assessing its impacts on the Indian Summer Monsoon Rainfall (ISMR) across North East India

The Indian Ocean Dipole (IOD), a climatic anomaly, results in sustained sea surface temperature (SST) variations between tropical western and eastern Indian Ocean temperatures. In this study, we studied the variations to inculcate the teleconnections between IOD and Indian summer monsoon rainfall (ISMR) distribution across the country for the period 1960-2020 for all the three phases of ISMR. We analyzed rainfall, SST and low-level wind circulation anomalies for the above mentioned time horizon. Positive IOD events noticeably resulted in increase in summer monsoon rainfall distribution across the country respectively while its negative counterpart led to decrease in rainfall except for the commencement phase of ISMR. The variations in SST, wind circulation and moisture movement processes across the Indian Ocean characterize significant changes in rainfall during the positive and negative phases of IOD especially during the recent decades (1991-2020). The recent time horizon also witnesses enhanced low-level equatorial jets (LEJ) across the equatorial Indian Ocean and the Arabian Sea during the positive IOD events as compared to the prior decades (1960-1990). The effect of moisture convergence zone is also analyzed which results in above rainfall conditions across northeastern and central India. Conversely, negative IOD events were found to subdue any such moisture movement mechanisms. Furthermore, and additional investigation to analyze the effect of IOD on the retreating/withdrawal monsoon across northeast India has been done and it has been observed that a stronger positive IOD is detrimental to the seasonal rainfall (May- September) over North East India (-0.7 one month lag correlation). Furthermore, the DMI index of April-May presented a clear indication of monsoon activity over the area during the withdrawal or retreating phase of the summer monsoon, i.e., during September.

Indian Ocean Dipole : Assessing its impacts on the Indian Summer Monsoon Rainfall (ISMR) across North East India

The Indian Ocean Dipole (IOD), a climatic anomaly, results in sustained sea surface temperature (SST) variations between tropical western and eastern Indian Ocean temperatures. In this study, we studied the variations to inculcate the teleconnections between IOD and Indian summer monsoon rainfall (ISMR) distribution across the country for the period 1960-2020 for all the three phases of ISMR. We analyzed rainfall, SST and low-level wind circulation anomalies for the above mentioned time horizon. Positive IOD events noticeably resulted in increase in summer monsoon rainfall distribution across the country respectively while its negative counterpart led to decrease in rainfall except for the commencement phase of ISMR. The variations in SST, wind circulation and moisture movement processes across the Indian Ocean characterize significant changes in rainfall during the positive and negative phases of IOD especially during the recent decades (1991-2020). The recent time horizon also witnesses enhanced low-level equatorial jets (LEJ) across the equatorial Indian Ocean and the Arabian Sea during the positive IOD events as compared to the prior decades (1960-1990). The effect of moisture convergence zone is also analyzed which results in above rainfall conditions across northeastern and central India. Conversely, negative IOD events were found to subdue any such moisture movement mechanisms. Furthermore, and additional investigation to analyze the effect of IOD on the retreating/withdrawal monsoon across northeast India has been done and it has been observed that a stronger positive IOD is detrimental to the seasonal rainfall (May- September) over North East India (-0.7 one month lag correlation). Furthermore, the DMI index of April-May presented a clear indication of monsoon activity over the area during the withdrawal or retreating phase of the summer monsoon, i.e., during September.

A Comparative Study on the Skill of CMIP6 Models to Preserve Daily Spatial Patterns of Monsoon Rainfall Over India

South Asian monsoon is a phenomena that plays out during June-September every year, due to the northward shift of the ITCZ which causes heavy rainfall over many countries of South Asia, including India. These rains are directly related to the lives and economic well-being of over a billion people. Indian monsoon is highly heterogeneous, due to the vast physiographic variations across the country. There is considerable interest among scientists and other stake-holders about possible future changes to Indian monsoon due to worldwide climate change. Simulations of future climate by global climate models under various scenarios can provide important clues for this. However, simulations of Indian monsoon in the historical period by global climate models under the CMIP5 family were found to be inaccurate in several aspects. Simulations by the new global climate models from the CMIP6 family are now available, and scientists are evaluating their ability to simulate Indian monsoon. In this work, we focus on one particular aspect of simulations by these models: the spatial distribution over daily rainfall over the Indian landmass during monsoon. We use a Machine Learning based probabilistic graphical model that can identify frequent spatial patterns of rainfall after creating a binary representation of rainfall. This model also helps us to identify spatial clusters, i.e., homogeneous regions within the Indian landmass with similar temporal characteristics of rainfall. We identify such frequent spatial patterns and spatial clusters from observed monsoon rainfall data, and also from simulations of monsoon rainfall by different CMIP6 models during the period 2000–2014. We evaluate the models by comparing the patterns and clusters identified from their simulations with those identified from observed data. We find that some of the CMIP6 models can simulate the spatial distribution of monsoon rainfall to a reasonable degree, but there are various limitations—most models underestimate extreme rainfall events and are unable to reproduce the regions of the landmass that are homogeneous with respect to rainfall.

Effect of monsoon onset and withdrawal on rice production in Chhattisgarh

The variability in monsoonal rainfall has affected agricultural production worldwide. Changes in the onset, withdrawal and distribution of monsoon rainfall have made a risky preposition for rice cultivation. In Chhattisgarh rice cultivation is mainly rain-fed and obviously prone to south-west monsoon variability. The aim of this research was to analyze the probable impact of monsoon onset and withdrawal variability on rice production in different agro-climatic zones of Chhattisgarh during a 29-year period (1989-2017). On the basis of correlation analysis we found that there is significant relationship between withdrawal dates of monsoon and rice production. The r-values was 0.491 (In CPZ), 0.594 (In BPZ) and 0.472 (In NHZ) for the correlation between monsoon withdrawal dates and rice production. The highest value of correlation co-efficient was found for Bastar plateau zone (r = 0.594). These co-efficient are significant at the 1% level of significance in all three agro-climatic zones for withdrawal dates of SWM (south west monsoon) and rice production during 1989-2017. In comparison of average rice production data under different categories of monsoon onset-withdrawal, we found that late withdrawal conditions are favorable for rice production and productivity.

Changes in monsoon rainfall distribution of Bangladesh using quantile regression model

Climate change is supposed to alter not only the mean and variability but also the distribution of rainfall. Changes in rainfall distribution during the monsoon months (June to September) of Bangladesh are evaluated in this study using quantile regression. Monthly rainfall data for the period 1961–2014 recorded at 18 locations distributed over the country were used for this purpose. Distributional changes of monthly rainfall showed downward convergent lines are dominant in peak monsoon months of June, July and August at 28%, 50% and 28% stations, respectively, followed by horizontally divergent lines at 17% of stations during those months. The dominating category of last monsoon month (September) rainfall was found upward divergent lines at 50% stations. The results revealed a decrease in many rainfall quantiles from June to August and increase in September in most of the stations. The increasing trend lines of September rainfall quantiles were found to become more diverse with time, which indicates an increase in rainfall extremes and the possibility of more floods which are already very common in the last month of monsoon in Bangladesh. The decrease in lower quantiles of rainfall in most of the monsoon months may cause an increase in the probability of droughts in the country. The study provided more insight on monsoon rainfall changes and improved understanding of climate change impacts on monsoon rainfall regime which can help in planning climate change adaptations in Bangladesh.

Performance of CMIP5 atmospheric general circulation model simulations over the Asian summer monsoon region

Performance of twenty-eight state of art atmosphere general circulation models (AGCMs) in simulating regional characteristic features of summer monsoon rainfall and circulation over the south Asian and the western-north Pacific regions are examined. AGCMs depict good representation of climatological spatial distribution of monsoon rainfall over both the regions. Interestingly, the year-to-year variability of the south Asian (Indian) monsoon rainfall is well simulated by the models compared to the circulation features. Whereas, for the western-north Pacific region, the AGCMs depict poor representation of monsoon rainfall variability, but circulation features are simulated with better skill. Thus suggesting that, models with good performance of monsoon rainfall variability need not necessarily have better representation of monsoon circulation features. Further analysis indicate that most models with good skill in simulating monsoon variability have better representation of monsoon-SST teleconnections with the Indo-Pacific SSTs. For the south Asian region, better simulating models have good representation of monsoon-ENSO teleconnection whereas most models have poor representation of monsoon-IOD relationship, suggesting requirement for realistic representation of these teleconnections in the models. For the western-north Pacific region, most models have unrealistic relationship of monsoon rainfall with SSTs during preceding winter and concurrent monsoon season, indicating that the poor (better) skill in simulating rainfall (circulation) probably arise from the unrealistic (realistic) representation of monsoon-SST teleconnections. Thus, the importance of realistic representation of monsoon-SST teleconnections in the climate models have been brought out for better simulation of monsoon variability over the south Asian and the western-north Pacific regions. • Performance of twenty-eight state of art atmosphere general circulation models in representing the summer monsoon rainfall and circulation over the south Asia and the western-north Pacific are examined. • Study brings out the importance of realistic representation of monsoon-SST teleconnections in the climate models for better simulation of the monsoon over the south Asia and the western-north Pacific. • The models which depict higher skill in simulating the interannual variability of monsoon have a better representation of monsoon-SST teleconnection over most of the Indo-Pacific region.

Mid-Holocene to present-day evolution of the Indian monsoon in transient global simulations

The low-frequency evolution of Indian rainfall mean-state and associated interannual-to-decadal variability is discussed for the last 6000 years from a multi-configuration ensemble of fully coupled global transient simulations. This period is marked by a shift of Indian Summer Monsoon Rainfall (ISMR) distribution towards drier conditions, including extremes, and a contraction of the rainy season. The drying is larger in simulations with higher horizontal resolution of the atmosphere and revised land surface hydrology. Vegetation–climate interactions and the way runoff is routed to ocean modulate the timing of the monsoon onset but have negligible effects on the evolution of seasonal rainfall amounts in our modeling framework in which carbon cycling is always active. This drying trend is accompanied by changes in ISMR interannual-to-decadal variability decreasing over north and south India but increasing over central India (20°–25° N). The ISMR interannual-to-decadal variability is decomposed into six physically consistent regimes using a clustering technique to further characterize its changes and associated teleconnections. From 6 to 3.8 kyr BP, the century-to-century modulations in the frequency of occurrence associated to the regimes are asynchronous between the simulations. Orbitally-driven trends can only be detected for two regimes over the whole 6–0 kyr BP period. These two regimes reflect increased influence of ENSO on both ISMR and Indian Ocean Dipole as the inter-hemispheric energy gradient weakens. Severe long-term droughts are also shown to be a combination of long-term drying and internally generated low-frequency modulations of the interannual-to-decadal variability.

Reconstruction of the East Asian Summer monsoon rainfall distribution at centennial scale during the Last Glacial Maximum

The Last Glacial Maximum (LGM), defined as the period between 23 and 19 ka, is considered a key climatic period with conditions in great contrast to the present. Study of this period may reveal aspects of global ice-sheet sensitivity to regional and global climate change which is critical in understanding climate teleconnections and feedback mechanisms. A stalagmite record from Linglong cave, Central China, in combination with other high-resolution stalagmite records from Southwest China and East China existing on the same moisture transport pathway allows a reconstruction of rainfall patterns during the LGM by making use of the δ18O differences between stalagmite records. The rainfall around both Southwest and Central East China show a prominent change centered at ~24 ka corresponding to the H2 event, one of the Heinrich events related to North Atlantic ice-rafting. Also, in Central East China around the periods at 22 and 20 ka, the rainfall is lower than the average during the whole LGM, while around 23, 21 and 19 ka the rainfall is higher than the average, which is opposite to the rainfall variation in Southwest China. This apparent dipole rainfall pattern during the LGM is similar to the modern East Asian Summer Monsoon （EASM） rainfall distribution observed during weak monsoon periods, suggesting a much weaker EASM with its front moving less far north during the LGM than at present, which may link to North Atlantic climate and shifting of the Intertropical Convergence Zone (ITCZ). Power spectral analysis reveals that the rainfall variations in Southwest China are mainly controlled by a cycle of ~1000-yr, while in Central East China, it is mainly controlled by cycles of ~1800-yr and ~ 1300 yr, which are all possible solar activity cycles, suggesting that the EASM rainfall on long term scales is dominated by external forces.

N-Alkanes and compound carbon isotope records from Lake Yiheshariwusu in the Hulun Buir sandy land, northeastern China

The Hulun Buir sandy land in northern China is located at the northern limit region of the East Asian summer monsoon (EASM) and is therefore sensitive to the extension of the front of the rainfall belt. Here we report an n-alkane and compound-specific carbon isotope record from the Holocene sediments of Lake Yiheshariwusu in the middle of the Hulun Buir sandy land. The sediments contain a suite of n-alkanes with a strong odd over even carbon number predominance, with the maximum contribution from nC31, which is a typical distribution in grassland regions. The low temperatures in this cold region greatly limit the growth of C4 plants and thus the long-chain n-alkanes in lake sediments are mainly derived from leaf wax lipids of C3 plants growing within the sandy land. In this C3-vegetation-dominated region, the δ13C27–33 value (weighted carbon isotope values of nC27– nC33) are regulated mainly by the physiological and biochemical responses of plants to water stress and are therefore interpreted as a proxy of effective precipitation or humidity. The δ13C27–33 time series shows a trend of gradually decreasing values that suggests an increase in effective precipitation since 8.5 ka (1 ka = 1000 cal yr BP). Relative droughts occurred during the intervals of 6.3–5.5, 4.1–3.6 ka, and during the last 200 years. In addition, the δ13C27–33 time series and comparable paleoenvironmental records from neighboring sites suggest opposite trends of summer monsoon rainfall between northeastern and southeastern China. We suggest that a coupled process between low and high latitudes (the western Pacific Subtropical High and the Okhotsk High) may have played a fundamental role in regulating the shift of the frontal rainfall belt and monsoon rainfall distribution in eastern China during the Holocene.

Emerging patterns in global and regional aerosol characteristics: A study based on satellite remote sensors

Understanding the global distribution pattern of aerosol particles is of paramount importance as it has applications of many fields that include, but are not limited to energy and environment, the study of aerosol particles is building blocks for nanotechnology. Therefore, the keen observation of aerosols through satellite remote sensors namely: MODIS (Moderate Resolution Imaging Spectroradiometer) and CALIOP (Cloud–Aerosol Lidar with Orthogonal Polarization) is important. The newly launched MODIS collection 6.1 (C061) aerosol product are containing three different aerosol inversion algorithms such as the Dark Target (DT) algorithm for land, the DT algorithm for the ocean, and the Deep Blue (DB) algorithm for land. In this study, we have used a combined aerosol product based on DT and DB to present emerging patterns in global and regional aerosol characteristics over short-term (2001–2007) and long-term (2001–2018) observation periods. The CALIOP, a Lidar instrument on aboard of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite which presents the 3-dimensional view of aerosol climatology, is also used during special cases to investigate the aerosol distribution patterns. Based on MODIS observations the aerosol distribution patterns show strong seasonal differences. The seasonal characteristics vary with latitude and longitude across the globe. The regional characteristics clearly showed their region-specific behavior. Interestingly, MODIS observations captured high concentrations over the regions traced in the developing countries particularly South and East Asia, where observations show strong seasonality. The averaged MODIS AOD trend over global is found slightly negative. On regional scales, positive trends found over the Arabian Peninsula, Africa (Sahel), Eastern China, Northern part of India. These regions are next to major monsoon regions. The increasing concentration of aerosols over these regions may modulate the monsoon rainfall distribution pattern. In contrast, the negative trend observed over Eastern America, Europe, the southeast part of Africa as well as in the associated dust outflow over the North Atlantic.

Characteristics of Observed Meteorological Drought and its Linkage with Low-Level Easterly Wind Over India

Meteorological drought in India arises due to significant deficiency of rainfall for abnormal periods over an area. The large spatial and temporal variability of Indian summer monsoon rainfall (ISMR) over the Eastern Gangetic Plain (EGP) of India triggers meteorological drought (further leading to agricultural and hydrological drought), with widespread effects on both agricultural production and water resources over the area. To assess meteorological drought over agro-climatic zones of the states of Uttar Pradesh (UP), Bihar and West Bengal (WB) in the EGP, high-resolution gridded rainfall data (1961–2013) at resolution of 0.25° × 0.25° of India Meteorological Department (IMD) and u, v wind at 850 hPa at resolution of 0.25 × 0.25° of ERA-40 (1961–2002) of the European Centre for Medium-Range Weather Forecasts (ECMWF) is considered. Over the agro-climatic zones, the seasonality index (SI) of summer monsoon rainfall, spatial and temporal distribution of the 4-month Standardized Precipitation Index (SPI-4), frequency and probability of drought occurrence is estimated. The severe drought-prone zones are found to be over agro-climatic zones 6, 8 and 10 of UP; 1, 2 and 3B of Bihar with more than 50% probability of drought occurrence. At a 95% confidence level, a significant decrease in rainfall (for the period 1961–2013) is found over these zones. Over the EGP, a low-level easterly wind at 850 hPa in July is shifted towards foothills of the Himalaya, while in August it is weakened during drought conditions. This low-level easterly wind may be responsible for less moisture incursion over the Gangetic Plain from the Bay of Bengal, and may be the probable cause of less rainfall over the EGP, leading to meteorological drought.