G. B. Pant

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.

Past monsoon rainfall variations in peninsular India recorded in a 331-year-old speleothem

An actively growing stalagmite collected from a cave located in the hills of the Western Ghats in the Uttar Kannada District of Karnataka, India, has been studied for stable isotope ratios of oxygen and carbon, width of growth layers and grey-level changes. Distinct carbonate layers, alternate coarse and compact, are seen in cross-section. Each couplet of compact and coarse layer is found to represent a single year. A total of 331 such couplets has been counted, indicating that the stalagmite started growing in AD 1666 and continued until it was sampled. Stable isotope ratios of oxygen (δ18O) and carbon (δ13C) show variations ranging from-13.6 to-7.9%° and from-2.7 to 1.6%°, respectively. We have reconstructed past rainfall changes of the cave site using the‘amount effect’ in &δ18O of rain. Speleothemδ18O and instrumental rainfall data from the associated climate subdivision show a significant correlation (r =-0.62, decadal average). Several sharp spikes of enrichment and depletion in 18O are indicative of the past deficiency and excess in rainfall. Most of the severe drought years recorded independently by meteorological observations are found registered in the stalagmite layers. During the 331-year-period, rainfall was highest at Añ 1666 and lowest around AD 1900. The stalagmite-generated past rainfall record can serve as a reasonable proxy for testing monsoon models.

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.

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.

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.

About the observed and future changes in temperature extremes over India

An attempt is made in the present study to analyse observed and model simulated temperature extremes over Indian region. Daily maximum and minimum temperature data at 121 well-distributed stations for the period 1970–2003 have been used to study the observed changes in objectively defined values of temperature extremes. In addition, an assessment of future scenarios of temperature extremes associated with increase in the concentration of atmospheric greenhouse gases is done using simulations of a state-of-the-art regional climate modelling system known as PRECIS (Providing Regional Climate for Impact Studies) performed to generate the climate for the present (1961–1990) and future projections for the period 2071–2100. Observational analysis done with 121 stations suggests the widespread warming through increase in intensity and frequency of hot events and also with decrease in frequency of cold events. More than 75% stations show decreasing trend in number of cold events and about 70% stations show increasing trend in hot events. Percentage of stations towards the warming through intensity indices of highest maximum temperature, lowest minimum temperature is 78 and 71% stations, respectively. Remaining stations show opposite trends, however, most of them are statistically insignificant. Observational analysis for India as a whole also shows similar results. Composite anomalies for monthly temperature extremes over two equal parts of the data period show increase (decrease) in the frequency of hot (cold) events for all months. In general, PRECIS simulations under both A2 and B2 scenarios indicate increase (decrease) in hot (cold) extremes towards the end of twenty-first century. Both show similar patterns, but the B2 scenario shows slightly lower magnitudes of the projected changes. Temperatures are likely to increase in entire calendar year, but the changes in winter season are expected to be prominent. Diurnal temperature range is expected to decrease in winter (JF) and pre-monsoon (MAM) months.

Pre-monsoon maximum and minimum temperatures over India in relation to the summer monsoon rainfall

The pre-monsoon thermal field over the Indian landmass has an important bearing on the land‐sea heating contrast in the region, consequently influencing the establishment, advance and overall performance of the Indian summer monsoon rainfall. This paper examines the relationship between the pre-monsoon thermal field over India and the following summer monsoon rainfall, in order to identify possible predictors for long-range forecasting of Indian summer monsoon rainfall. Based on the spatial patterns of correlations of monsoon rainfall with maximum and minimum temperatures at 121 stations well distributed over India, during the recent period 1951‐80, two predictors showing a significant contribution to the variance in monsoon rainfall have been identified. They are (i) March minimum temperature in east peninsular India and (ii) May minimum temperature in west central India. These two predictors have performed very well in terms of their significant contribution to the multiple regression models during 1951‐1987, vis-a `-vis several other known predictors. They have also shown a consistently significant relationship with the monsoon rainfall during the recent period, from the mid-1940s till the end of the data period. A stepwise regression model for long-range forecasting of all-India summer monsoon rainfall, involving three regional predictors, has been developed, and has shown a multiple correlation of 089. # 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 1115‐1127 (1997)

Long-Term variability of the indian summer monsoon and related parameters

The long-term variability of the Indian summer monsoon rain-fall and related regional and global parameters are studied. The cubic spline is used as a digital filter to smooth the high frequency signals in the time series of the various parameters. The length of the data series varies from 95 to 115 years during the period 1871-1985. The parameters studied within the monsoon system are: (a) monsoon rainfall of the country as a whole; (b) number of break-monsoon days during July and August; (c) number of storms/ depressions in Bay of Bengal and Arabian Sea during summer monsoon season; and (d) dates of onset of summer monsoon over South Kerala Coast. The parameters studied outside the monsoon system are: (a) the Wright’s Southern Oscillation Index (June-July-August); (b) the January mean Northern Hemispheric surface air temperature anomaly; and (c) the East-equatorial Pacific sea surface temperature anomaly.In order to examine the variability under various degrees of the smoothing, the series are filtered with splines of 50% variance reduction frequency of one cycle per 10, 20 and 30 years. It is observed that the smoothed time series of the parameters within the monsoon system comprise a common slowly varying component in an episodic manner distinctly showing the excess and deficient rainfall epochs. The change of intercorrelations between the time series with increasing degree of smoothing throws some light on the time scales of the dominant interactions. The relation between Southern Oscillation and East equatorial Pacific sea surface temperature and the Indian summer monsoon seems to be dominant on the interannual scale. The low frequency variations are found to have significantly contributed to the instability of the correlations of monsoon rainfall with parameters outside the monsoon system.

Premonsoon Ridge Location over India and Its Relation to Monsoon Rainfall

Abstract The location of the 500-hPa ridge axis during April over India is one of the most important long-range predictors for the summer monsoon rainfall. This paper presents a comprehensive analysis on its space-time variability during the premonsoon season and its relation with the monsoon rainfall. Data on the daily latitudinal locations of the 500-hPa ridge axis along three longitudes during March, April, and May, as well as all-India rainfall and subdivisional monsoon rainfall for the period 1967–90, have been used. The analysis involves correlations between the running means of the premonsoon ridge locations over windows of 15, 21, and 31 days, and the subsequent monsoon rainfall. The ridge location in March shows negative correlation with the all-India summer monsoon rainfall, while that in April shows positive correlation. The anticorrelation of the March ridge was more dominant with the monsoon rainfall of the peninsular India, while the positive correlation of the April ridge was more dominant ...