K. Rupa Kumar

Global observed changes in daily climate extremes of temperature and precipitation

A suite of climate change indices derived from daily temperature and precipitation data, with a primary focus on extreme events, were computed and analyzed. By setting an exact formula for each index and using specially designed software, analyses done in different countries have been combined seamlessly. This has enabled the presentation of the most up-to-date and comprehensive global picture of trends in extreme temperature and precipitation indices using results from a number of workshops held in data-sparse regions and high-quality station data supplied by numerous scientists world wide. Seasonal and annual indices for the period 1951-2003 were gridded. Trends in the gridded fields were computed and tested for statistical significance. Results showed widespread significant changes in temperature extremes associated with warming, especially for those indices derived from daily minimum temperature. Over 70% of the global land area sampled showed a significant decrease in the annual occurrence of cold nights and a significant increase in the annual occurrence of warm nights. Some regions experienced a more than doubling of these indices. This implies a positive shift in the distribution of daily minimum temperature throughout the globe. Daily maximum temperature indices showed similar changes but with smaller magnitudes. Precipitation changes showed a widespread and significant increase, but the changes are much less spatially coherent compared with temperature change. Probability distributions of indices derived from approximately 200 temperature and 600 precipitation stations, with near-complete data for 1901-2003 and covering a very large region of the Northern Hemisphere midlatitudes (and parts of Australia for precipitation) were analyzed for the periods 1901-1950, 1951-1978 and 1979-2003. Results indicate a significant warming throughout the 20th century. Differences in temperature indices distributions are particularly pronounced between the most recent two periods and for those indices related to minimum temperature. An analysis of those indices for which seasonal time series are available shows that these changes occur for all seasons although they are generally least pronounced for September to November. Precipitation indices show a tendency toward wetter conditions throughout the 20th century.

Diurnal asymmetry of surface temperature trends over India

Diurnal asymmetry of the recent surface warming, reported for several continental areas, is believed to be related to changes in cloudiness, humidity, atmospheric circulation patterns, winds and soil moisture. This paper presents linear trend analyses of maximum and minimum temperature data at 121 stations in India during the period 1901–87. While the mean temperature trends over India are similar to the global and hemispheric trends, there are marked differences in their diurnal manifestation from those reported for other areas. The increase in the mean temperatures over India is almost solely contributed by the maximum temperatures, with the minimum temperatures remaining practically trendless, leading to an increase in the diurnal range of temperatures. These trends do not show any significant urban or altitude bias. There are some differences in the trends on smaller spatial and temporal scales, but the increase of maximum temperature is predominant over a major part of India, particularly in winter and post-monsoon seasons.

Evidence of Secular Variations in Indian Monsoon Rainfall–Circulation Relationships

Abstract Detailed correlation analysis of the all-India monsoon rainfall and mean sea-level seasonal pressure at Bombay (19°N, 73°E) up to three lags on either side of the monsoon wren during the last 30 years (1951–80) indicates a systematic relationship. The winter-to-premonsoon (March, April, May–Deceinber, January, February; MAM–DJF) seasonal pressure tendency at Bombay shows a correlation coefficient (CC) of −0.70 (significant at 0.1% level) with the Indian monsoon rainfall. Further examination of this relationship over a long period of 144 years (1847–1990), using sliding correlation analysis, reveals some interesting features. The sliding CCs were positive before 1870, negative during 1871–1900, positive in the years 1901–40, and again negative later on, showing systematic turning points around the years 1870, 1900, and 1940. In light of other corroborative evidence, these climatic regimes can be identified as “meridional monsoon” periods during 1871–1900 and after 1940, and as “zonal monsoon” peri...

Extreme Daily Rainfall Events and Their Impact on Ensemble Forecasts of the Indian Monsoon

The Indian summer monsoon rainfall is the net result of an ensemble of synoptic disturbances, many of which are extremely intense. Sporadic systems often bring extreme amounts of rain over only a few days, which can have sizable impacts on the estimated seasonal mean rainfall. The statistics of these outlier events are presented both for observed and model-simulated daily rainfall for the summers of 1986 to 1989. The extreme events cause the wet-day probability distribution of daily rainfall to be far from Gaussian, especially along the coastal regions of eastern and northwestern India. The gamma and Weibull distributions provide good fits to the wetday rainfall distribution, whereas the lognormal distribution is too skewed. The impact of extreme events on estimates of space and time averages can be reduced by nonlinearly transforming the daily rainfall amounts. The square root transformation is shown to improve the predictability of ensemble forecasts of the mean Indian rainfall for June 1986‐89.

Homogeneous Indian Monsoon rainfall: Variability and prediction

The Indian summer monsoon rainfall is known to have considerable spatial variability, which imposes some limitations on the all-India mean widely used at present. To prepare a spatially coherent monsoon rainfall series for the largest possible area, fourteen subdivisions covering the northwestern and central parts of India (about 55% of the total area of the country), having similar rainfall characteristics and associations with regional/global circulation parameters are merged and their area-weighted means computed, to form monthly and seasonal Homogeneous Indian Monsoon (HIM) rainfall series for the period 1871–1990. This paper includes a listing of monthly and seasonal rainfall of HIM region. HIM rainfall series has been statistically analysed to understand its characteristics, variability and teleconnections for long-range prediction.HIM rainfall series isfound to be homogeneous, Gaussian distributed and free from persistence. The mean (R) rainfall is 757 mm (87% of annual) and standard deviation (S) 119 mm, with a Coefficient of Variation (CV) of 16%. There were 21 dry (K, -<R S) and 19 wet (Ri R + S) years during 1871–1990. There were clusters of frequent negative departures during 1899–1920 and 1965–1987 and positive departures during 1942–1961. The recent three decades show very high rainfall variability with 10 dry and 6 wet years. The decadal averages were alternatively positive and negative for three consecutive decades, viz., 1871–1900 (positive); 1901–1930 (negative); 1931–1960 (positive) and 1961–1990 (negative) respectively. Significant QBO and autocorrelation at 14th lag have been found in HIM rainfall series.To delineate the changes in the climatic regime of the Indian summer monsoon, sliding correlation coefficients (CCs) between HIM rainfall series and (i) Bombay msl pressure, (ii) Darwin msl pressure and (iii) Northern Hemisphere surface air temperature over the period 1871–1990 have been examined. The 31-year sliding CCs showed the systematic turning points of positive and negative CCs around the years, 1900 and 1940. In the light of other corroborative evidences, these turning points seem to delineate ‘meridional’ monsoon regime during 1871–1900 and 1940–1990 and ‘zonal’ monsoon regime during 1901–1940. The monsoon signal is particularly dominant in many regional and global circulation parameters, during 1951–1990.Using the teleconnections ofHIM series with 12 regional/global circulation parameters during the recent 36-year period 1951–86 regression models have been developed for long-range prediction. In the regression equations 3 to 4 parameters were entered, explaining upto 80% of the variance, depending upon the data period. The parameters that prominently enter the multiple regression equations are (i) Bombay msl pressure, (ii) April 500 mb Ridge at 75°E, (iii) NH temperature, (iv) Nouvelle minus Agalega msl pressure and (v) South American msl pressure. Eleven circulation parameters for the period 1951–80 were subjected to Principal Component Analysis (PCA) and the PC’s were used in the regression model to estimate HIM rainfall. The multiple regression with three PCs explain 72% of variance in HIM rainfall.

ENSO‐monsoon relationships in a greenhouse warming scenario

Recent studies based on observed climatic data indicate weakening of the relationship between El Nino-Southern Oscillation (ENSO) and Indian summer monsoon rainfall, possibly due to global warming. Transient climate change simulations of a coupled ocean-atmosphere GCM (ECHAM4/OPYC3) project a change towards enhanced ENSO activities in the tropical Pacific, as well as increase in mean monsoon rainfall and variability over India. However, the interannual correlations between the two are strong throughout the 240 year simulation. Analysis of monsoon viz-a-viz ENSO in the model simulations suggest a diminished impact of warm ENSO (El Nino) events on monsoon, while the impact of cold ENSO (La Nina) events remains unchanged in the scenario. Anomalous warming over the Eurasian landmass as well as enhanced moisture conditions over the Indian monsoon region in the global warming scenario have possibly contributed to the weakening of the impact of warm ENSO events on monsoon.

RING‐WIDTH VARIATIONS IN CEDRUS DEODARA AND ITS CLIMATIC RESPONSE OVER THE WESTERN HIMALAYA

Tree-ring anlaysis of Cedrus deodara from three different sites of western Himalaya has been carried out. The chronologies include 47 cores (26 trees) from Manali, 33 cores (18 trees) from Kufri (Shimla) and 25 cores (13 trees) from Kanasar forest sites. Moderately high values of common variance exhibited by all three chronologies indicate the great potential of the species for dendroclimatic studies. Response function and correlation analyses using the above tree-ring-width data and Shimla climate show a significant negative relationship with summer temperature and positive relationship with summer precipitation. Based on these results, calibration equations have been developed for different periods, and appropriately verified using independent data, to reconstruct the summer (March–April–May) temperature at Shimla. The reconstruction has extended the temperature record of the region back to the eighteenth century.

Dendroclimatic reconstruction of summer precipitation at Srinagar, Kashmir, India, since the late-eighteenth century

The extensive coniferous forests of the western Himalayas provide great potential for dendroclimatic research. Abies, Cedrus, Picea and Pinus are the main tree genera of the region, consisting of many old and living trees. The annual growth-ring patterns of these trees, which can be precisely dated, contain valuable high-resolution information on climate variability over the past few centuries. This paper presents an analysis of the ring-width data of Abies pindrow and Picea smithiana based on samples collected from forest sites around Pahalgam in the Kashmir Valley in 1982. The non- climatic variations have been filtered out of the raw ring-width series using a cubic spline smoothing, and ring-width index series have been obtained. The mean ring-width index chronologies have been prepared by averaging the indices of 13 samples for P. smithiana and eight samples for A. pindrow. The maximum lengths of mean ring-width index chronologies are 208 years (1775-1982) for P. smithiana and 371 years (1612-1982) for A. pindrow. Response functions have been developed using the instrumental record of monthly mean temperature and rainfall at Srinagar during the period 1893-1982. Both ring- width chronologies indicate a significant negative response to summer temperature and a significant positive relationship with summer precipitation. Calibration and verification analysis has been carried out for different combinations of months using a multiple species ring-series network and the summer precipitation at Srinagar has been reconstructed back to the eighteenth century.

Long-term variations of surface air temperature at major industrial cities of India

Local temperature is one of the major climatic elements to record the changes in the atmospheric environment brought about by industrialization and urbanization. Long-term variations of seasonal and annual surface air temperature at six major industrial cities of India (Calcutta, Bombay, Madras, Bangalore, Pune and Delhi) have been studied, using data for the past 86 to 112 yr. Comparative analysis of the temperature data of six nonindustrial stations has also been done. The long-term change in the temperature has been evaluated by linear trend. Calcutta, Bombay and Bangalore have shown significant warning trend, while Delhi has shown a cooling trend. The trends at Madras and Pune are not significant. The nonindustrial stations did not show significant trends. In general, there was either a cooling tendency or cessation of warming, after the late 1950s at most of the industrial cities.

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.