Significant Decrease In Rainfall Research Articles

Spatiotemporal variation of rainfall and occurrence of extreme events over Burundi during 1960 to 2010

The occurrence of extreme rainfall events, associated mass movements, and flash floods especially during recent years motivated the study on the spatiotemporal rainfall variability over Burundi. This study used data from distinct sources: surface observation (1981–2010), Climatic Research Unit (CRU), Global Precipitation Climate Centre (GPCC) for the period 1960–2010, and European Centre for Medium-Range Weather Forecasts (ECMWF) for the period 2001–2017. This study analyzed the main features of rainfall over the study domain and demonstrated that extreme rainfall contributed to mass movements and flash flood hazard that characterize some sections of the study domain. Analysis revealed a decrease of rainfall and number of rainy days, supported by a generally decreasing trend for annual and seasonal rainfall. The short rains that occur during September–December experienced significant variability than long rains that occur during March–May, especially from 1990s. The study further identified the west, east, and north-east parts of Burundi to be more sensitive to extreme events than other sections of the study domain. The analysis carried out the existence of high variability of rainfall which starts from 1985 with significant decrease of rainfall from 1990. The occurrence of extreme rainfall during short rains has been attributed to wind circulation anomaly due to influx of air mass from Congo basin, local topography, and local wind patterns. This study contributes to understand the spatiotemporal rainfall variability, the relationship between extreme rainfall events, and its possible impact on territory management and will help in forecasting and monitoring future extreme events over Burundi.

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.

Seasonal rainfall variability in the southern Mediterranean border: Observations, regional model simulations and future climate projections

Particular attention has been given in recent years to precipitation and variations in climate change scenarios by modeling these phenomena at different timescales. Regional climate models enable assessing the impact of climate change at the regional and local scales. This study aims, firstly, to analyze the rainfall variability in northern Algeria, and secondly, to evaluate future seasonal rainfall variability using regional climate models of the ENSEMBLE project. A statistical test and the bias method have been used for this study. Analysis of rainfall variability using the Pettitt test during the reference period (1961-1990) reveals a significant decrease of rainfall since the 1970s at the Tenes and Oran stations, located in the northwest of Algeria. This decrease occurs during the months of December and January (in winter), and March and April (in spring), and is associated to a reduction of 10-20 and 20-50 mm of daily rainfall, respectively, while the rainfall increases in the eastern region. Simulated data of 12 regional climate models have been compared to observations carried out during the reference period (1961-1990) using the bias method. Future seasonal rainfall variability was analyzed for two projection periods: 2021-2050 and 2070-2099. Most often, models underestimate the wet seasons and overestimate the dry seasons during the reference period. The models simulated a significant decrease of future rainfall in winter and spring over the two projected periods. Further investigations must be conducted to explain the underestimation of the models. It is also important to reconsider uncertainties that affect the results of this study.

The ‘Day Zero’ Cape Town drought and the poleward migration of moisture corridors

Since 2015 the greater Cape Town area (∼3.7 million people) has been experiencing the worst drought of the last century. The combined effect of this prolonged dry period with an ever-growing demand for water culminated in the widely publicized ‘Day Zero’ water crisis. Here we show how: (i) consecutive significant decreases in rainfall during the last three winters led to the current water crisis; (ii) the 2015–2017 record breaking drought was driven by a poleward shift of the Southern Hemisphere moisture corridor; (iii) a displacement of the jet-stream and South Atlantic storm-track has imposed significantly drier conditions to this region. Decreasing local rainfall trends are consistent with an expansion of the semi-permanent South Atlantic high pressure, and reflected in the prevalence of the positive phase of the Southern Annular Mode. Large-scale forcing mechanisms reveal the intensification and migration of subtropical anticyclones towards the mid-latitudes, highlighting the link between these circulation responses and the record warm years during 2015–2017 at the global scale.

RAINFALL TREND ANALYSIS OF A MINI WATERSHED: A CASE STUDY ON DEGRADING LAKES/TANKS

India as a tropical country, depends solely on south west Monsoon. Southwest monsoon spans between June and September. The present study is aimed to carry out the probable trend of rainfall and to evaluate its implications on the tanks in Hunsur taluk of Mysore district, Karnataka, India. These tanks were the livelihood sources of water for the farmers in the command area. The lakes considered under this case study have been completely dried up in the recent past. Inconsistency of rainfall is one of the factors which govern the degradation of Lakes. Trend analysis is carried on 40 years daily rainfall data (1975-2014) for monthly, seasonal and annual average values using Mann-Kendall test and Sen’s slope estimation. The analysis shows ‘no trend’ for the months of January, February, March and June. There is an ‘increasing trend’ for the months of August, October and December. However, there is a ‘decreasing trend’ for the months of April, May, July, September and November. The decadal variation indicates a very significant decrease of rainfall for the months of June and July; particularly in the recent decade (2005-2014) and hence a ‘negative trend’ in the South-West monsoon. This may be one of the reasons for the deterioration of tanks in the study area.

Rainfall patterns of Algerian steppes and the impacts on natural vegetation in the 20th century

Since 1960, the steppe regions of North Africa have been subject to an increasing desertification, including the degradation of traditional pastures. The initially dominant species (Artemisia herba-alba, Lygeum spartum and Stipa tenacissima) declined and were progressively replaced by other species (Atractylis serratuloides and Salsola vermiculata) that are more tolerant to the new conditions. It is not clear whether these changes are due to anthropogenic reasons or climatic determinism. We have carried out a statistical analysis of the climate to detect putative rainfall changes during the 20th century in the Algerian steppes based on data from 9 meteorological stations, including 2 Saharan stations (El Oued and Touggourt), 3 pre-Saharan stations (Biskra, Laghouat and Ain Sefra) and 4 steppe stations (Djelfa, Saida, Méchéria and El-Bayadh) located in the arid high plains, which represent the bioclimate diversities of the region. Previous studies suggested that significant rainfall changes for the 20th century only had records in the south of the Oran region. Most of the studies, however, looked at restricted territories over limited periods, and did not integrate the rainiest period 2004–2014. Our work is designed to integrate all the longest time series of meteorological data available for the steppe regions of Algeria. Our results confirm the spatial rainfall distribution (significant rainfall changes only recorded in the southwestern region) evidenced by previous studies, and reveal a decreasing rainfall gradient from northeastern to southwestern Algeria. Moreover, the results reveal a trend of significant decrease of rainfall in the southern Oran region, marked by two drought periods in 1980–1985 and 1999–2003. However, with the exception of the southwestern region, rainfall overall has not declined since the beginning of the 20th century. While less marked in other regions, the drought appear to have affected all territories of the Algerian steppe. Consequently, our study implies that the climate was not a leading influence in the on-going degradation of the vegetation cover of steppe landscapes. Such a vegetation evolution thus appears to be have been determined more by human activities than by climate forcing.

Future climate change enhances rainfall seasonality in a regional model of western Maritime Continent

In this study, future changes in rainfall due to global climate change are investigated over the western Maritime Continent based on dynamically downscaled climate projections using the MIT Regional Climate Model (MRCM) with 12 km horizontal resolution. A total of nine 30-year regional climate projections driven by multi-GCMs projections (CCSM4, MPI-ESM-MR and ACCESS1.0) under multi-scenarios of greenhouse gases emissions (Historical: 1976–2005, RCP4.5 and RCP8.5: 2071–2100) from phase 5 of the Coupled Model Inter-comparison Project (CMIP5) are analyzed. Focusing on dynamically downscaled rainfall fields, the associated systematic biases originating from GCM and MRCM are removed based on observations using Parametric Quantile Mapping method in order to enhance the reliability of future projections. The MRCM simulations with bias correction capture the spatial patterns of seasonal rainfall as well as the frequency distribution of daily rainfall. Based on projected rainfall changes under both RCP4.5 and RCP8.5 scenarios, the ensemble of MRCM simulations project a significant decrease in rainfall over the western Maritime Continent during the inter-monsoon periods while the change in rainfall is not relevant during wet season. The main mechanism behind the simulated decrease in rainfall is rooted in asymmetries of the projected changes in seasonal dynamics of the meridional circulation along different latitudes. The sinking motion, which is marginally positioned in the reference simulation, is enhanced and expanded under global climate change, particularly in RCP8.5 scenario during boreal fall season. The projected enhancement of rainfall seasonality over the western Maritime Continent suggests increased risk of water stress for natural ecosystems as well as man-made water resources reservoirs.

The influence of El Niño–Southern Oscillation on boreal winter rainfall over Peninsular Malaysia

Multi-scale interactions between El Nino–Southern Oscillation and the Boreal Winter Monsoon contribute to rainfall variations over Malaysia. Understanding the physical mechanisms that control these spatial variations in local rainfall is crucial for improving weather and climate prediction and related risk management. Analysis using station observations and European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim) reanalysis reveals a significant decrease in rainfall during El Nino (EL) and corresponding increase during La Nina particularly north of 2°N over Peninsular Malaysia (PM). It is noted that the southern tip of PM shows a small increase in rainfall during El Nino although not significant. Analysis of the diurnal cycle of rainfall and winds indicates that there are no significant changes in morning and evening rainfall over PM that could explain the north–south disparity. Thus, we suggest that the key factor which might explain the north–south rainfall disparity is the moisture flux convergence (MFC). During the December to January (DJF) period of EL years, except for the southern tip of PM, significant negative MFC causes drying as well as suppression of uplift over most areas. In addition, lower specific humidity combined with moisture flux divergence results in less moisture over PM. Thus, over the areas north of 2°N, less rainfall (less heavy rain days) with smaller diurnal rainfall amplitude explains the negative rainfall anomaly observed during DJF of EL. The same MFC argument might explain the dipolar pattern over other areas such as Borneo if further analysis is performed.

Temporal and spatial variability of rainfall over Greece

Recent studies have showed that there is a significant decrease in rainfall over Greece during the last half of the pervious century, following an overall decrease of the precipitation at the eastern Mediterranean. However, during the last decade an increase in rainfall was observed in most regions of the country, contrary to the general circulation climate models forecasts. An updated high-resolution dataset of monthly sums and annual daily maxima records derived from 136 stations during the period 1940–2012 allowed us to present some new evidence for the observed change and its statistical significance. The statistical framework used to determine the significance of the slopes in annual rain was not limited to the time independency assumption (Mann-Kendall test), but we also investigated the effect of short- and long-term persistence through Monte Carlo simulation. Our findings show that (a) change occurs in different scales; most regions show a decline since 1950, an increase since 1980 and remain stable during the last 15 years; (b) the significance of the observed decline is highly dependent to the statistical assumptions used; there are indications that the Mann-Kendall test may be the least suitable method; and (c) change in time is strongly linked with the change in space; for scales below 40 years, relatively close regions may develop even opposite trends, while in larger scales change is more uniform.

Relationships between atmospheric circulation indices and rainfall in Northern Algeria and comparison of observed and RCM-generated rainfall

This work aims, as a first step, to analyze rainfall variability in Northern Algeria, in particular extreme events, during the period from 1940 to 2010. Analysis of annual rainfall shows that stations in the northwest record a significant decrease in rainfall since the 1970s. Frequencies of rainy days for each percentile (5th, 10th, 25th, 50th, 75th, 90th, 95th, and 99th) and each rainfall interval class (1–5, 5–10, 10–20, 20–50, and ≥50 mm) do not show a significant change in the evolution of daily rainfall. The Tenes station is the only one to show a significant decrease in the frequency of rainy days up to the 75th percentile and for the 10–20-mm interval class. There is no significant change in the temporal evolution of extreme events in the 90th, 95th, and 99th percentiles. The relationships between rainfall variability and general atmospheric circulation indices for interannual and extreme event variability are moderately influenced by the El Nino-Southern Oscillation and Mediterranean Oscillation. Significant correlations are observed between the Southern Oscillation Index and annual rainfall in the northwestern part of the study area, which is likely linked with the decrease in rainfall in this region. Seasonal rainfall in Northern Algeria is affected by the Mediterranean Oscillation and North Atlantic Oscillation in the west. The ENSEMBLES regional climate models (RCMs) are assessed using the bias method to test their ability to reproduce rainfall variability at different time scales. The Centre National de Recherches Meteorologiques (CNRM), Czech Hydrometeorological Institute (CHMI), Eidgenossische Technische Hochschule Zurich (ETHZ), and Forschungszentrum Geesthacht (GKSS) models yield the least biased results.

An update on the rainfall characteristics of Brazil: seasonal variations and trends in 1979-2011

We studied rainfall characteristics of Brazil for the period 1979–2011. This is an update for an earlier study with data from 1958 to 1978. We compared the three consecutive wet and dry months in the two data sets. In the northern most part rainy season earlier was in April to July and now it is occurring later. In a large part of the south central region the rainy season in austral summer and dry season in austral winter did not change. However, to the east of this region rainy season in the latter data occurs earlier. Calculation of linear tendencies showed, that over the northern Amazon region there is a significant increase of rainfall. This agreed with previous results. Over this region dry season is becoming drier and wet season wetter. In the west of Northeast Brazil (NEB) there is an increase of rainfall. In southeast Brazil there is a region of highly significant decrease of rainfall in both wet and dry seasons. This is confirmed in our analysis on river discharges in Sao Paulo and Minas Gerais states. The negative tendencies in rainfall have great impact on the hydroelectric generation not only in this region but also at the national level because the hydroelectric dams furnish energy on a national level. The increase in rainfall in Northern Amazon is probably associated with Atlantic warming that resulted in higher moisture transport from east. The decrease of rainfall in southeast Brazil seems to be associated with the plummet of rain producing systems from south due to increase of mean sea level pressure in the south.

Regional Variation of Temperature and Rainfall in Bangladesh: Estimation of Trend

This study mainly focuses on exploring the regional variation of the changing patterns of temperature and rainfall in Bangladesh. The analysis is based on the temperature and rainfall variation in Bangladesh over five regions as Dhaka, Cox’s Bazar, Rajshahi, Bogra and Sylhet. The duration of the study period was chosen as 1953-2012 for Dhaka, 1948-2012 for Cox’s Bazar, 1972-2012 for Rajshahi, 1958-2012 for Bogra and 1957-2012 for Sylhet. The findings of the non-parametric Mann-Kendal test reveals that significant increase of maximum temperature has been found in Cox’s Bazar and Sylhet. Significant decrease of maximum temperature has been found in Dhaka and Bogra. Significant increase of minimum temperature has been found in Dhaka and Cox’s Bazar whereas significant decrease has been found in Rajshahi. Significant decrease of rainfall has been found in Rajshahi among the study region. The maximum temperature increases significantly by 0.021 degree Celsius per year in Cox’s Bazar and Sylhet. In case of minimum temperature highest increase is found in Dhaka by 0.049 degree Celsius followed by Cox’s Bazar (0.038 degree Celsius per year) whereas significant decrease has been found in Rajshahi by 0.047 degree Celsius per year.

Regional Variation of Temperature and Rainfall in Bangladesh: Estimation of Trend

This study mainly focuses on exploring the regional variation of the changing patterns of temperature and rainfall in Bangladesh. The analysis is based on the temperature and rainfall variation in Bangladesh over five regions as Dhaka, Cox’s Bazar, Rajshahi, Bogra and Sylhet. The duration of the study period was chosen as 1953-2012 for Dhaka, 1948-2012 for Cox’s Bazar, 1972-2012 for Rajshahi, 1958-2012 for Bogra and 1957-2012 for Sylhet. The findings of the non-parametric Mann-Kendal test revealed that significant increase of maximum temperature has been found in Cox’s Bazar and Sylhet, significant decrease of maximum temperature has been found in Dhaka and Bogra. Significant increase of minimum temperature has been found in Dhaka and Cox’s Bazar whereas significant decrease has been found in Rajshahi. Significant decrease of rainfall has been found in Rajshahi among the study region. The maximum temperature increased significantly by 0.021 Degree Celsius per year in Cox’s Bazar and Sylhet. In case of minimum temperature highest increase was found in Dhaka by 0.049 degree Celsius followed by Cox’s Bazar (0.038 degree Celsius per year) whereas significant decrease has been found in Rajshahi by 0.047 degree Celsius per year.

Satellite-based rainfall data reveal a recent drying trend in central equatorial Africa

West-central Uganda, a biodiversity hotspot on the eastern edge of central equatorial Africa (CEA), is a region coping with balancing food security needs of a rapidly growing human population dependent on subsistence agriculture with the conservation of critically endangered species. Documenting and understanding rainfall trends is thus of critical impor- tance in west-central Uganda, but sparse information exists on rainfall trends in CEA during the past several decades. The recently created African Rainfall Climatology version 2 (ARC2) dataset has been shown to perform satisfactorily at identifying rainfall days and estimating seasonal rainfall totals in west-central Uganda. Therefore, we use ARC2 data to assess rainfall trends in west-central Uganda and other parts of equatorial Africa from 1983-2012. The core variables examined were three-month rainfall variables for west-central Uganda, and annual rainfall variables and seasonal rainfall totals for a transect that extended from northwestern Democratic Republic of the Congo to southern Somalia. Significant decreases in rainfall in west-central Uganda occurred for multiple three-month periods centered on boreal summer, and rainfall associated with the two growing seasons decreased by 20 % from 1983-2012. The drying trend in west-central Uganda extended westward into the Congo rainforest. Rainfall in CEA was significantly correlated with the Atlantic Multidecadal Oscillation (AMO) at the annual scale and during boreal summer and autumn. Two other possible causes of the decreasing rainfall in CEA besides North Atlantic Ocean sea-surface temperatures (e.g.,

Observed and modelled trends in rainfall and temperature for South Africa: 1960–2010

Observed trends in seasonal and annual total rainfall, number of rain days and daily maximum and minimum temperature were calculated for a number of stations in South Africa for the period 1960–2010. Statistically significant decreases in rainfall and the number of rain days are shown over the central and northeastern parts of the country in the autumn months and significant increases in the number of rain days around the southern Drakensberg are evident in spring and summer. Maximum temperatures have increased significantly throughout the country for all seasons and increases in minimum temperatures are shown for most of the country. A notable exception is the central interior, where minimum temperatures have decreased significantly. Regionally aggregated trends for six water management zones covering the entire country are not evident for total rainfall, but there are some significant trends for the number of rain days. Temperature in these zones has increased significantly for most seasons, with the exception of the central interior. Comparison of the observed trends with statistically downscaled global climate model simulations reveals that the models do not represent the observed rainfall changes nor the cooling trend of minimum temperature in the central interior. Although this result does not rule out the possibility of attributing observed local changes in rainfall to anthropogenically forced global change, it does have major implications for attribution studies. It also raises the question of whether an alternative statistical downscaling method or dynamical downscaling through the use of a regional climate model might better represent regional and local climatic processes and their links to global change.

Analysis of time variation of rainfall in transnational basins in Iberia: abrupt changes or trends?

An analysis of recent-past changes in rainfall records from the three major transnational basins in Iberia was performed, using data from Spain and Portugal which are generally considered separately. Change point (to detect abrupt changes in rainfall) and trend analysis were performed in the basins of rivers Douro, Tagus and Guadiana for the period 1961 to 2009. Nonparametric tests (Pettitt, cusum and Mann–Kendall) were used in order not to assume a specific distribution for the data. Field significance was taken into account when applying the tests for trends and change points. The importance of spatial correlation when calculating field significance was demonstrated. As well as finding changes in rainfall which have great significance for water resources, some important issues are raised as to the nature of changes in rainfall to be expected. Significant decreases in rainfall were found for the month of February and, to a lesser extent, March. Significant increases in rainfall were found for October in the Spanish side of Douro and Tagus catchments. The variability of the NAO index was considered a likely explanation for the changes detected. It was also demonstrated that changes in rainfall cannot be interpreted as trends or change points always because the pattern of change can be more complex than these two simplistic ways of describing change. Furthermore, the magnitude of the change can be completely different depending on the type of change assumed. Therefore the quantification of the change must be made with care, as the widely used linear trend can result in an overestimation of the change and, in principle, is not coherent with the multiple alternative hypotheses (different admissible monotonic patterns) implied by the Mann–Kendall test. Copyright © 2013 Royal Meteorological Society

Has soil drying contributed to earlier grape ripening in wine regions of southern Australia?

Background and Aim A long-term trend in early ripening of winegrapes in southern Australia has been attributed to an increasing growing season temperature, soil drying linked to global warming and crop management. However, the 5 × 5-km cell size used used in the continental-scale model to derive soil properties was too coarse for the study vineyards of 0.2–16 ha. This paper aims to test the modelling conclusions using long records of annual rainfall for sites as close as possible to the experimental vineyards. Methods and Results If prolonged soil drying has occurred, it should be correlated with a decrease in annual rainfall, given that actual evapotranspiration should change little as the effect of increased atmospheric carbon dioxide concentration compensates for the effect of temperature increase on evaporation. Analysis of the longest, most complete Bureau of Meteorology records showed a highly significant decrease in rainfall (27 mm/decade) at Margaret River, but paradoxically no significant early ripening. Conversely, significant earlier ripening on the Mornington Peninsula was associated with a significant increase in rainfall (8.5 mm/decade). Conclusion Although growing season temperature and crop management to reduce yields may have contributed to earlier ripening, the case for an effect of prolonged soil drying was not supported. Significance of the Study The output of simulation models describing complex biological systems needs to be tested wherever possible against experimental data and observations. Also, the scale of modelling should be appropriate to the system being studied, especially for soil, which has marked spatial variability.

Probabilistic predictions of climate change for Australia and southern Africa using the reliability ensemble average of IPCC CMIP3 model simulations

“Reliability ensemble averaging” (REA) is applied to a multimodel ensemble of coupled atmosphere‐ocean general circulation models (AOGCMs) from the third phase of the Coupled Model Intercomparison Project (CMIP3) to produce mean and probabilistic climate change projections for Australia and southern Africa. CMIP3 was conducted in preparation for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). REA produces a weighted average of the ensemble of climate change results, based on a measure of each model's bias and each model's degree of convergence in the predicted climate change. The methodology allows an assessment of the reliability of the projected climate change, the quantification of model uncertainty range, and the production of climate change projections in probabilistic form. Temperature and precipitation changes for three emission scenarios (B1, A1B, and A2) are analyzed, with a special focus on the A2 scenario. Regional differences in temperature and rainfall changes are identified, and threshold probabilities and probability density functions (PDFs) for key subregions are produced. In summer for temperature the A2 scenario results show a narrow PDF over southwestern Australia, reflecting agreement between models, partially caused by down weighting of outlier models due to their large biases. In contrast, the PDF for southeastern Australia is much broader, and that for tropical Australia has a bimodal character. There are significant decreases in rainfall under the A2 scenario in winter in the southwest (25–30% decrease) (common in sign to all AOGCM simulations) and the southeast (15–25% decrease) of Australia. There are no changes above natural variability in summer, hence no significant changes in the magnitude of Australian monsoon rainfall. Over southern Africa the region of maximum warming is over the Kalahari Desert in summer and winter. Under the A2 scenario in summer, there is a significant increase in rainfall over Kenya and Tanzania and a significant drying (∼10–20%) over parts of Namibia and Botswana. In winter, most of southern Africa south of 15°S shows significant decreases in rainfall.

Regional Impacts of Future Land-Cover Changes on the Amazon Basin Wet-Season Climate

AbstractState-of-the-art socioeconomic scenarios of land-cover change in the Amazon basin for the years 2030 and 2050 are used together with the Regional Atmospheric Modeling System (RAMS) to simulate the hydrometeorological changes caused by deforestation in that region under diverse climatological conditions that include both El Niño and La Niña events. The basin-averaged rainfall progressively decreases with the increase of deforestation from 2000 to 2030, 2050, and so on, to total deforestation by the end of the twenty-first century. Furthermore, the spatial distribution of rainfall is significantly affected by both the land-cover type and topography. While the massively deforested region experiences an important decrease of precipitation, the areas at the edge of that region and at elevated regions receive more rainfall. Propagating squall lines over the massively deforested region dissipate before reaching the western part of the basin, causing a significant decrease of rainfall that could result in a catastrophic collapse of the ecosystem in that region. The basin experiences much stronger precipitation changes during El Niño events as deforestation increases. During these periods, deforestation in the western part of the basin induces a very significant decrease of precipitation. During wet years, however, deforestation has a minor overall impact on the basin climatology.

Summer monsoon low pressure systems over the Indian region and their relationship with the sub-divisional rainfall

In the present paper performance of the monthly sub-divisional summer monsoon rainfall is studied in association with the position of the Low Pressure System (LPS) over the Indian region. Existence of the LPS over a particular location increases the rainfall activities in certain parts of the country while decreases in some other parts. For this study, the Indian region (5°-35° N and 60° -100° E) is divided into 5° Lat. ´ 5° Long. grids. The duration of LPS is taken in terms of LPS days with respect to the location of LPS in a particular grid. Monthly total number of LPS days in each of the grids are computed during the summer monsoon season, June to September for the period 1891 – 1990. Maximum number of LPS days (more than half of the total) are observed in the latitude belt between 20°-25°N. The percentages of total LPS days in this area are higher in July and August which are peak monsoon months as compared to June and September. When there is a LPS are in the area 20°-25° N and 80°-90° E, there is significant increase in the rainfall activities in the sub-divisions along mean monsoon trough while northeast India and southeast peninsular India experience significant decrease in rainfall in the months of July and August. Owing to the movement of LPS from east to west through central India, most parts of the country, excluding northeast India and south peninsular India get good rainfall activity. Correlation coefficients between monthly LPS days over the different grids and monthly sub-divisional rainfall are computed to study the relationships. The performance of sub-divisional rainfall mostly related with the occurrence of LPS in certain grid- locations. The correlation field maps may give some useful information about rainfall performance due to LPS in a particular grid locations.