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Study of snow-monsoon relationship and changes in rainfall and temperature characteristics in India

Student Name: Ms Ashu Mamgain
Guide: Dr. Pradhan Parth Sarthi
Year of completion: 2012

Abstract:

In the recent past, there are indications of changes in the surface air temperature, extreme weather events, snow and Indian summer monsoon. This thesis analyses the above weather phenomena based on observed data and climate model simulations for the present as well as the near future. Earlier studies show a strong negative relationship between Eurasian snow cover/depth and Indian summer monsoon rainfall. Limitations of such studies are that both the parameters snow and rainfall were seasonally averaged over large areas. Indian summer monsoon has its own characteristics of evolution such as onset, active, break and withdrawal phases which have been studied extensively. However, the evolution of Eurasian snow is yet to be examined. Further, it is interesting to explore the characteristics of evolution of snow over the different regions of Eurasia and their relationship with the evolution characteristics of summer monsoon. In this thesis, a detailed examination has been done on the starting and the ending dates of snowfall over different regions of Eurasia and attempts have been made to explore any relationship with onset of Indian summer monsoon. It is observed that the regions where snowfall starts early, it ends late. Further, in those regions maximum snow depth also occurs late. In some years, more snowfall in East Eurasia is followed by less snowfall in West Eurasia. Also snow depths particularly in the northernmost and southwest regions of East Eurasia are opposite in phase. The results of this study indicate a weak relationship between snow starting dates in Eurasia and summer monsoon onset dates in the Kerala coast. However, the relationship between the northernmost Eurasian snow depth and the summer monsoon precipitation in the Peninsular India is significant.

Today, regional weather/climate models are increasingly used to study several atmospheric phenomena. The Regional Climate Model, RegCM3 has been successfully integrated to simulate the salient features of Indian summer monsoon circulation and rainfall. In this thesis, multi-member simulations are performed to identify and remove the systematic errors in the model. Changes in the frequency of extreme rainfall events simulated by RegCM3 are also examined. RegCM3 has well simulated rainfall over the Central India. Dry bias is observed over Central India and wet over Northwest and Peninsular India. Shift in mean sea level pressure is observed over the foothills of the Himalayas and Tibet. Summer monsoon active events in the model are of shorter life span that those actually observed. There are a number of years in which monsoon exhibited contrasting characteristics so far as total rainfall over India is concerned. Active and breaks in Indian summer monsoon during these contrasting years are examined in this thesis. The characteristics of model simulated active and break phases of monsoon contribute to less summer monsoon rainfall in Central India by the model. The differences in the frequency distribution between RegCM3 simulated and IMD observed rainfall explains the wet bias over Northwest, Peninsula and dry bias over Central India. RegCM3 simulated precipitation has more number of very wet and extremely wet days compared to those in IMD observed values, although their variations follow the same pattern in both.

From the climate change perspective the changes in intensities and spells of moderate and extreme temperatures and their impact on sectors like agriculture and health are very important. Results indicate significant decrease in the frequency of occurrence of cold nights in the winter months in India and in its homogeneous regions in the north except in Western Himalaya. Southern regions show drastic decrease in the frequency of cold nights relative to the period 1969–1975. A significant increasing trend in the number of warm days in summer is noticed only in the Interior Peninsula. In the entire country and in East Coast and West Coast, maximum number of warm days in summer have been noticed only during the last decade 1996–2005. Further, in the whole country maximum number of intense warm days and nights in summer are observed in the last decade. Significant increase in the number of cold days in winter is observed in the North Central and North East. Changes in the frequency of warm and cold exceedences indicate maximum warming in the West Coast as compared to all other regions. In sum, such spatial and temporal changes in the characteristics of all categories of temperature extremes broadly suggest to warming trends in large parts of India.

The possible changes in mean monsoon circulation pattern and rainfall during the period 2011– 2040 are studied with respect to the baseline period 1961–1990 under the IPCC emission scenarios A2, A1B and B1 in warmer climate. The IPCC coupled climate models simulated future projections of mean wind at 850 hPa show anomalous anticyclonic flow over Arabian Sea and anomalous easterly flow over Indian Ocean which could be responsible for weakening of Indian summer monsoon in warmer climate. At 200hPa, models simulated future projection of mean monsoon wind fields show an anomalous cyclonic flow over the north of Northwest India.

In sum, following results obtained from this thesis are worth mentioning here. Analysis of observed data shows a positive relationship between the snow starting dates in the northernmost Eurasia and onset dates of Indian summer monsoon. The regional climate model RegCM3 simulates the Indian summer monsoon rainfall reasonably well. Especially, precipitation shows very good correlation with corresponding observation in the Central India compared to the other regions. In regards to extreme temperature cases, changes in the occurrence of temperature exceedences in India indicate that decreasing trends in the frequency of cold nights are more significant and prevalent than the increasing trends in warm days. Based on the simulations of IPCC models it is found that Indian summer monsoon in the near future 2011-2040 likely to weaker under A2, A1B and B1 emission scenarios.

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