N. A. Sontakke

On Climatic Fluctuations and Environmental Changes of the Indo-Gangetic Plains, India

Paralleling the Southern Himalayan Province, the Indo-GangeticPlains region (IGPR) of India (geographical area ∼ 6,00,000 km2) is veryimportant for the food security of South Asia. Due to numerous factors inoperation there is widespread apprehension regarding sustainability offragile ecosystems of the region. Literature provides detailed documentation of environmental changes due to different factors except climatic. The present study is intended to document the instrumental-period fluctuations of important climatic parameters like rainfall amounts (1829–1999), severe rainstorms (1880–1996) and temperature (1876–1997) exclusively for the IGPR. The summer monsoon rainfall over western IGPR shows increasing trend(170 mm/100-yr, significant at 1% level) from 1900 while over central IGPR it shows decreasing trend (5 mm/100-yr, not significant) from 1939 and over eastern IGPR decreasing trend (50 mm/100-yr, not significant) during 1900–1984 and insignificant increasing trend (480 mm/100-yr, not significant) during 1984–1999. Broadly it is inferred that there has been a westward shift in rainfall activities over the IGPR. Analysis suggests westward shift in the occurrence of severe rainstorms also. These spatial changes in rainfall activities are attributed to global warming and associated changes in the Indian summer monsoon circulation and the general atmosphericcirculation. The annual surface air temperature of the IGPR showed rising trend (0.53 ° C/100-yr, significant at 1% level) during 1875–1958 and decreasing trend (–0.93 ° C/100-yr, significant at 5% level) during 1958–1997. The post-1958 period cooling of the IGPR seems to be due to expansion and intensification of agricultural activities and spreading of irrigation network in the region. Lateral shift in the river courses is an environmental hazard of serious concern in the IGPR. In the present study it is suggested that meteorologic factors like strength and direction of low level winds and spatial shift in rainfall/climatic belt also play a significant role along with tectonic disturbances and local sedimentological adjustments in the vagrancy of the river courses over the IGPR.

Instrumental period rainfall series of the Indian region (AD 1813—2005): revised reconstruction, update and analysis

Sontakke and Singh (The Holocene 6, 315—31, 1996) developed instrumental period summer monsoon (June—September total) rainfall series (1813—1995) for six homogeneous zones and all of India using 306 raingauge stations. This reconstruction has been revised and updated. Besides reconstructing backward and updating to 2005 the longest possible summer monsoon rainfall series (1813—2005), post-monsoon (October— December total) and annual rainfall series have also been developed for seven homogeneous zones: North Mountainous India (NMI), Northwest India (NWI), North Central India (NCI), Northeast India (NEI), West Peninsular India (WPI), East Peninsular India (EPI), South Peninsular India (SPI), and the whole country using data from 316 well-spread stations. The different series are reported here. The underlying mechanism of the possible cause of the recent decreasing trend in monsoon rainfall and increasing trend in post-monsoon rainfall is described.

RAINFALL VARIABILITY OVER BANGLADESH AND NEPAL: COMPARISON AND CONNECTIONS WITH FEATURES OVER INDIA

In this study monthly rainfall data for 14 stations over Bangladesh for the period 1901-1977 are used to investigate and understand the interannual variability of the summer monsoon rainfall. Monthly, seasonal, and annual spatial rainfall patterns, and the spatial patterns of variability, are presented. Dominant structures of seasonal rainfall are determined through the empirical orthogonal functions. A homogeneous series for All Bangladesh Monsoon Rainfall is prepared and its temporal characteristics are studied. It is observed that the standardized rainfall for this series shows random fluctuations up to 1963, thereafter the standardized values are much above the normal values. Further the rainfall variations over Bangladesh are not related to large-scale variables such as the Northern Hemisphere surface temperature, Darwin pressure tendency, and the subtropical ridge over the Indian region. However, the rainfall variations over Bangladesh are related well with rainfall variations over north-east India. Similar analysis is done for the Nepal region by examining the monthly rainfall data over Kathmandu for a 105 year period (1851-1900, 1921-1975). Results reveal that Nepal rainfall is well related with rainfall variations over northern and central parts of India.

Construction of All-India Summer Monsoon Rainfall Series for the Period 1844–1991

Abstract An all-India summer monsoon rainfall series for the instrumental period of 1844–1991 has been constructed using a progressively increasing station density to 1870, and one that is fixed thereafter at a uniformly distributed 36 stations. The statistical scheme accounts for the increasing variance contributed to the all-India series by the increasing number of stations during the period 1844–1870. An interesting outcome of this study is that a reliable estimate of summer monsoon rainfall over India can he obtained using only 36 observations.

Longest instrumental regional and all-India summer monsoon rainfall series using optimum observations: reconstruction and update

Long period June-to-September total rainfall series are vital for the study of summer monsoon/climatic variability over the Indian region. Owing to high spatial variability in rainfall, the representa tiveness of the summer monsoon rainfall series for the entire country is limited. In order to develop an effective system for monsoon rainfall studies, the country has been divided into six zones, named as North West India (NWI), North Central India (NCI), North East India (NEI), West Peninsular India (WPI), East Peninsular India (EPI) and South Peninsular India (SPI). Fluctuation, empirical orthogonal function (EOF) and cluster analyses of the summer monsoon rainfall of the period AD 1871-1984 of 306 raingauges are carried out for this classifi cation. Updating the different zonal rainfall series on a real-time basis from all the gauges is a difficult task. In the present study, an objective technique is applied to select a subset of gauges whose mean showed the highest correlation with the all-gauges mean series; a total of 116 raingauge stations (19 for NWI, 27 for NCI, 15 for NEI, 18 for WPI, 14 for EPI and 23 for SPI) maintained by the India Meteorological Department (IMD) have been identified for updating the different zonal series. The different zonal series are extended as far as possible backward (prior to AD 1871) by applying an objective technique to the available gauges. The NEI and the EPI series are extended back to 1848, the NWI to 1844, the NCI to 1842, the WPI to 1817 and the SPI to 1813. Though of limited practical value, the all-India series is widely used in studies of long-range monsoon forecasting, teleconnections and large-scale climate dynamics. The all-India monsoon rainfall series from AD 1871 onwards is therefore calculated from the area-weighted mean of the six zones. The series is also extended back to AD 1813 by applying the objective technique on the available gauges. The different recon structed series for the longest instrumental period are reported.

Surface and upper air temperatures over India in relation to monsoon rainfall

SummaryThe relationship between the all-India summer monsoon rainfall and surface/upper air (850, 700, 500 and 200 mb levels) temperatures over the Indian region and its spatial and temporal characteristics have been examined to obtain a useful predictor for the monsoon rainfall. The data series of all-India and subdivisional summer monsoon rainfall and various seasonal air temperatures at 73 surface observatories and 9 radiosonde stations (1951–1980) have been used in the analysis. The Correlation Coefficients (CCs) between all-India monsoon rainfall and seasonal surface air temperatures with different lags relative to the monsoon season indicate a systematic relationship.The CCs between the monsoon rainfall and surface-air temperature of the preceding MAM (pre-monsoon spring) season are positive over many parts of India and highly significant over central and northwestern regions. The average surface air temperature of six stations i.e., Jodhpur, Ahmedabad, Bombay, Indore, Sagar and Akola in this region (Western Central India, WCI) showed a highly significant CC of 0.60 during the period 1951–1980. This relationship is also found to be consistently significant for the period from 1950 to present, though decreasing in magnitude after 1975. WCI MAM surface air temperature has shown significant CCs with the monsoon rainfall over eleven sub-divisions mainly in northwestern India, i.e., north of 15 °N and west of 80 °E.Upper air temperatures of the MAM season at almost all the stations and all levels considered show positive CCs with the subsequent monsoon rainfall. These correlations are significant at some central and north Indian stations for the lower and middle tropospheric temperatures.The simple regression equation developed for the period 1951–1980 isy = − 183.20 + 8.83x, wherey is the all-India monsoon rainfall in cm andx is the WCI average surface air temperature of MAM season in °C. This equation is significant at 0.1% level. The suitability of this parameter for inclusion in a predictive regression model along with five other global and regional parameters has been discussed. Multiple regression analysis for the long-range prediction of monsoon rainfall, using several combinations of these parameters indicates that the improvement of predictive skill considerably depends upon the selection of the predictors.

Relationship between all-India summer monsoon rainfall and southern oscillation/eastern equatorial Pacific sea surface temperature

The interannual variability of all-India summer monsoon (June to September) rainfall and its teleconnections with the southern oscillation index (SOI) and sea surface temperature (SST) anomaly of the eastern equatorial Pacific ocean have been examined for the period 1871–1978 for different seasons (i.e., winter, spring, summer and autumn). The relationship (correlation coefficient) between all-India summer monsoon rainfall andSOI for different seasons is positive and highly significant. Further examination of 10-, 20- and 30-year sliding window lengths’ correlations, brings out the highly consistent and significant character of the relationships. The relationship between all-India monsoon rainfall andSST for different seasons is negative and is significant at 1 % level or above. Drought years are characterised by negative anomalies ofSOI and positive anomalies ofSST and vice versa with flood years. The relationship betweenSOI andSST is negative and significant at 0.1 % level.The relationships between all-India summer monsoon rainfall,SOI and sst are expected to improve our understanding of the interannual variability of the summer monsoon.

An index of summer monsoon rainfall excess over India and its variability: 1871–1978

SummaryTo assess the excess in the activity of the summer monsoon (June to September) over India during the period 1871 to 1978, an index based on the percentage area with a specified percentage seasonal rainfall excess and termed as the Monsoon Excess Index (MEI) has been used. On examination of MEI series it is found that it is homogeneous and random and has a high variability. The distribution of MEI is positively skewed and Gamma probability model is observed to be a good fit to the series. Considering the year in which the MEI value exceeds the nineth decile, i.e. 35, of the Gamma distribution fitted to the MEI series, as year of large-scale excess over India, such years have been identified during the period 1871–1978. There are ten such years. These excess years are found to be randomly distributed in time.ZusammenfassungZur Schätzung der Exzesse der Aktivität des Sommer-Monsuns (Juni–September) über Indien in der Periode 1871–1978 wird ein auf den prozentuellen Flächenanteil mit in Prozenten angegebenem jahreszeitlichen Regenüberschuß basierender Index verwendet; der als Monsun-Exzess-Index (MEI) gezeichnet wird. Die Untersuchung der MEI-Reihe hat ergeben, daß diese homogen ist und eine große Veränderlichkeit aufweist. Die Häufigkeitsverteilung der MEI-Werte ist positiv schief und ein Gamma-Modell gibt eine gute Anpassung an die Reihe. Ein Jahr, in dem der MEI-Wert das neunte Dezil (d.i. 35) der der MEI-Reihe angepaßten Gamma-Verteilung überschreitet, wird als großräumiger Niederschlagsexzess über Indien betrachtet. In der Periode 1871–1978 wurden zehn solche Jahre festgestellt. Diese Exzeß-Jahre weisen eine zeitliche Zufallsverteilung auf.

Optimization of the raingauges for a representative all-India and subdivisional rainfall series

SummaryAn objective approach similar to the forward selection of independent variables in the multiple linear regression has been attempted to optimize the network of raingauges for the summer monsoon rainfall (June–September total) series (1871–1984) of India as well as its 29 selected meteorological subdivisions prepared involving the data of 306 raingauges. For the all-India monsoon rainfall series twenty seven gauges entered the selection whose mean showed the correlation coefficient (CC) of 0.9869. Keeping in view the difficulties of getting data from all the 306 gauges, 35 India Meteorological Department (IMD) gauges with mean showing CC of 0.9898 have been identified for updating this series. The constructed all-India monsoon rainfall series for the period 1871–1992 using 35 selected observations is presented. It was interesting to note that the set of 35 gauges selected for the monsoon total has shown equally promising results for the all-India monsoon monthly (June–September) as well as the annual rainfall series.For the 29 subdivisional monsoon rainfall series, however, in total 188 IMD-gauges (62% of the total of 306 gauges) entered the selection. For 17 subdivisions the CC exceeded 0.98, for 3 subdivisions it varied between 0.97 and 0.98, for 5 subdivisions between 0.96 and 0.97 and for the remaining 4 subdivisions between 0.90 and 0.94. They showed equally encouraging results for the monsoon monthly and annual rainfall series for the different subdivisions.Limitations and implications of the optimization technique are also briefly discussed.

Monitoring Physiographic Rainfall Variation for Sustainable Management of Water Bodies in India

About 90% of the natural disasters are of meteorological or hydrometeorological origin. Disasters such as earthquakes and tsunamis are occasional, but rainfall related hazards are frequent and needs constant monitoring. The annual average rainfall over India is 1166 mm out of which 78% is received in the hydrological wet season (mostly summer monsoon rainfall) of 135 days from 30 May to 11 October (Ranade et al., 2007) and is widespread over the country with large spatial variability due to large variation in intensity and frequency of rain-inducing disturbances such as monsoon depressions/lows, monsoon trough position, and physiography. There is a large intra-seasonal variability in this hydrological wet season. Rainfall during the remaining period though restricted to limited areas (i.e. during winter over northernmost and east peninsular India, during summer over northeast and southwestern India and during post-monsoon season over south peninsula, east coast and northeastern part of India) is crucial.