Summary and Conclusions

Transcription

1 Summary and Conclusions This Chapter summarizes the results obtained in the earlier chapters based on the analysis of observed data and climate models simulations for the present and future emission scenarios. In this thesis extensive analysis have been done on the characteristics of Eurasian snow depth with respect to Indian summer monsoon rainfall, RegCM3 simulated long term data based on Indian summer monsoon season, summer and winter observed temperatures and IPCC model generated past and future climate scenarios. The analysis is supported by an understanding of underlying changes in climate variables and associated atmospheric circulation. Negative relationship between Eurasian snow cover/depth and Indian summer monsoon rainfall is well documented. Further studies suggest strong positive correlation of East Eurasian snow depth and negative correlation of West Eurasian snow with Indian summer monsoon rainfall. Station observations from Historical Soviet Daily Snow Depth version-2, IMD gridded rainfall dataset and onset dates over Kerala coast are considered here for the period First of all East and West Eurasian regions are selected on the basis of this strong positive and negative correlations, respectively. West and East Eurasian zones are further divided into six zones based on the latitudes and evolution of snowfall. Statistical analysis has been done on snow depth in different zones of Eurasia. Detailed examination has been done on the starting and the ending dates of snowfall in all the stations of Eurasia and an attempt has been made to explore possible relationship with the onset of Indian summer monsoon in order to make use of these findings in long range monsoon prediction. Snow depths in different zones of Eurasia are also related with summer monsoon rainfall in five homogeneous regions in India. Simulations of IIT Delhi T80L18 spectral GCM from a sensitivity experiment are considered to re-examine snow-monsoon relationship for the years 1975 and 1979, the low and high spring snow years, respectively. Further, high resolution IPCC MIROC simulated snow depths are compared with the observations for base line period ( ) and snow depth projections for the near future ( ) in East and West Eurasia. Climatologically, snow depths in the northernmost zones of Eurasia are around three times more than those in the respective southern zones. In southern zones, snow depths are less than the mean over the whole of Eurasia. Analysis shows that snow starts falling at stations located in the northeast of Eurasia and gradually propagates towards southwest. Stations with early starting date of snowfall have late ending dates and also have late occurrence of maximum snow depth. The duration of snowfall in the northern zones is longer and hence there snow depths persist for relatively longer period than in the southern zones. Snow depth varies to a great extent over different stations in West and East Eurasia. Relatively high coefficients of variation in the snow depths are observed in south and southwest stations in West Eurasia than in any other station. In some years, more snowfall in East Eurasia is accompanied by less snowfall in the West Eurasia and vice versa. Also, in some years snow depths in the northernmost and southwest zones in East Eurasia are opposite in phase. The relationship between snow starting dates in Eurasia and onset dates of summer monsoon is weak. However, the northernmost zones of East and West Eurasia have significant correlations with seasonal summer rainfall in

2 Northwest, West Central and Peninsular India. The most robust relationship is with the Peninsular India. Indian summer monsoon rainfall has been examined in detail by using nine-member ensemble seasonal integrations of a regional climate model RegCM3 at 55km resolution over the period NCEP reanalysis data are used for validation of temperature, mean sea level pressure and wind field simulations. Rainfall simulations are compared with observations from IMD, APHRODITE, CRU, GPCP and CMAP gridded data. Climatology of variables such as rainfall, temperature, mean sea level pressure and wind at lower and upper level are analyzed to evaluate the model performance. An attempt has been made to identify the causes behind model bias. Four pairs of contrasting monsoon rainfall years are considered for the analysis of active and break events in RegCM3 simulated Indian summer monsoon. Frequency distributions of IMD observed and RegCM3 simulated precipitation and surface maximum and minimum temperatures are compared. For extreme rainfall analysis over a considered period of 28 years, very wet and extremely wet days precipitation indices are considered. Comparison of IIT Delhi T80L18 spectral GCM simulations at 150km resolution and RegCM3 in contrasting monsoon years is also made. RegCM3 simulated precipitation is well captured over the Central and Northeast India with slightly underestimation over some of the parts. RegCM3 has overestimated rainfall over the southern Peninsular and Northwest India. A dipole like structure is observed over the Indian Ocean. Northern parts of the Indian Ocean have wet bias whereas the southern parts have dry bias. At 500 hpa, RegCM3 has a warm bias of about 1.5 o C over the Himalayan and Tibetan regions. Temperature in the remaining parts of the domain is well captured by the model. The pressure gradient develops towards the Northwest of India up to the southern plains of Pakistan and Afghanistan in RegCM3 simulations which is similar to the observations. RegCM3 simulates another low pressure regime at the foothills of Himalayas and Tibet which is deeper than the previous. This low pressure system is reverse in the case of reanalysis. As a consequence the rainfall is not well distributed over the Indian land. RegCM3 simulated maximum surface temperature shows strong cold bias over the entire Indian region. However, the minimum temperature is close to the observation except over the Himalayas. The direction of the Somali jet and easterly jet and Tibetan anticyclonic position are well simulated by RegCM3. However, difference fields show that RegCM3 simulated wind at 850 hpa is stronger than the reanalysis wind over the southern Peninsula and Central India. Over the Indian Ocean, near to the equator and along the Kerala coast, RegCM3 simulates stronger low level wind fields. RegCM3 simulates stronger easterly component of Tibetan anticyclone than observation over the Bay of Bengal and southernmost Peninsular India. However, westerly is weaker to the north of 30 N latitude. The temporal correlation coefficients (CCs) between area weighted RegCM3 rainfall and IMD observed rainfall over India as a whole in June, July, August, September and JJAS are 0.53, 0.67, 0.61, 0.15 and 0.5, respectively. Annual time variations in Central India JJAS rainfall depict some improvement in CCs. Except in September, simulated rainfall in rest the months

3 TERI University Ph.D. Thesis, Summary and Conclusions have been well correlated with the observation. The CCs between RegCM3 and IMD area weighted mean surface temperatures over Indian land for June, July, August, September and the JJAS season as a whole are 0.62, 0.69, 0.35, 0.78 and 0.66, respectively. Best correlation between RegCM3 simulated and IMD observed JJAS rainfall and temperature is found over central India. The positive and negative phases of rainfall particularly during contrasting monsoon seasons, , , , and are comparable with corresponding IMD observed values. RegCM3 simulated active and break spells in these four pairs of contrasting years are less in number than those in IMD observations. Monsoon breaks in the RegCM3 are of longer life span that those actually observed. Mean summer monsoon rainfall during the months June to September in two contrasting monsoon years 2002 and 2003 simulated by two different models; IITD spectral GCM and RegCM3 are compared. RegCM3 results are much closer to the actual measurements of IMD compared to the GCM results. Frequency of occurrence of rainfall less than 5mm is underestimated by RegCM3 with different magnitudes in all the three selected zones Northwest, Central and Peninsular India. Frequency of occurrence of more than 20mm rainfall are underestimated over the Central India zone and overestimated in Peninsular India. 5-10mm rainfall frequency is overestimated in Northwest and Central India. These differences in frequency distribution between RegCM3 simulated and IMD observed rainfall could explain the wet bias over Northwest, Peninsula and dry bias over Central India up to some extent. Variations in very wet and extremely wet days show that correlations between RegCM3 and IMD frequency of occurrence of rainfall extremes are weak. Frequency distribution of minimum temperature is well in phase with observation compared to maximum temperature. Maximum temperature shows 10 o C shift towards the colder domain. However, both warm days and nights are well simulated by the model with an exception of some systematic bias. Nevertheless, smoothed curves obtained from binomial filtering are well in phase with each other. The extremes of temperature are always of great concern due to their direct influence on human being and surrounding environment. In the recent past, the changing pattern, frequency and magnitudes of extreme temperature events across the world are well reported. Studies suggest that extreme temperature events are increasing over the world. Some such inference over India has been so far based on observed temperature values. However, these observations are not uniform in space and time duration. Currently, a very good quality dataset of daily maximum and minimum temperatures for the period is prepared by the IMD at a resolution of 1 o x1 o over the Indian land points. This dataset is used to examine the changes in frequency of occurrence and episodes of temperature in seven homogeneous regions and over the whole of the country. The year to year variations in seasonal mean, maximum and minimum temperatures in pre-monsoon, monsoon, post-monsoon and winter seasons are analyzed. Extreme temperature indices such as warm days and nights in summer and cold days and nights in winter are divided into three different intensities based on its severity. These temperature exceedences are examined in detail at inter-annual and inter-decadal scales. Further, long and short spells of warm days and cold nights are also considered. An attempt has also been made to analyze variations in temperature, simulated by RegCM3 in order to find out the possibility to generate future projections of temperature extremes using high resolution RegCM3 driven with GCMs output.

4 For validation of model output temperature variations, corresponding NCEP reanalysis fields and IMD observed data are used. One of the important results of this study is the significant decreasing trends in the frequency and spells of cold nights for the period in the country as a whole and in all the regions in the north except Western Himalaya. In this region the number of cold days show decreasing trend. Southern regions in India show drastic decrease in the frequency of cold nights in winter with respect to the period Relatively more warm days in the summers of the last decade is noticed in southern regions as compared to the northern regions. Increase in the frequency of warm nights in summer is found in two northern regions North West and North East and in the two southern regions, West Coast and Interior Peninsula. Cooling tendency in winter over two northern regions, North Central and North East has also been noticed due to significant increase in the frequency of cold days. Changes in the frequency of warm and cold exceedences indicate maximum warming in the West Coast as compared to all other regions. Results show that frequencies of occurrence of the strongest category of warm days and nights such as TX99p and TN99p are maximum during the decade as compared to earlier decades when India is considered as one complete unit. Regions in the south India show rise in the frequency of warm days in summer whereas regions in the north India do not indicate any change in the frequency of occurrence of warm days. Nevertheless, decrease in the number of winter cold nights leads to relatively more warming in the northern regions of the country than in the south. Overall results indicate that the decreasing trends in the frequency of cold nights are more significant and prevalent than the increasing trends in warm days in India. Simulation of Indian summer monsoon rainfall and its variability by a global model at different time scales is always a challenging task. The global models are unable to capture rainfall pattern over complex terrains with good confidence. In the fourth assessment report of IPCC it is reported that the pattern and magnitude of Indian summer monsoon rainfall are likely to change under warmer climate. It is mainly based on simulations of IPCC models under different forced scenarios. In this study, selected high resolution GCM simulations with average latitudinal surface resolution varying from 1.1 to 2.0 o are used. These models are CCSM3, ECHAM5, GFDL2.1, MIROC3.2 (hires) and UKMO-HadGEM1. The marker scenarios namely, A2, B1 and A1B are used to make projections of future climate. Changes in monsoon rainfall, temperature and wind circulations are studied in the simulations of IPCC AR4 models under forced scenarios during with respect to the base line period To understand the possible changes in mean JJA circulation and rainfall during with respect to the baseline period under the emission scenarios A2, A1B and B1, the future projected changes in mean JJA temperature at 500hPa, wind at 850hPa and 200hPa and rainfall are analyzed in detail. Relatively less warming is noticed over the Himalayan and Tibetan regions compared to other parts of India in all considered model projections under A2, A1B and B1 scenarios. The model simulated future projection of mean wind at 850hPa under A2, A1B and B1 scenarios shows an 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

5 TERI University Ph.D. Thesis, Summary and Conclusions climate. At 200hPa, models simulated future projections of mean monsoon wind fields show an anomalous cyclonic flow over the north of Northwest India, Himalayan and Tibetan regions that leads to develop a westerly flow over the Indian Ocean and may cause changes in the distribution of ISMR in parts of India. The future projected ISMR under A2, B1 and A1B scenarios show deficit and excess of rainfall over the lower part of western and eastern coast of India in HadGEM1, ECHAM5, and MIROC (hires) simulations. Low emission scenarios, nearly from all models, show deficit of rainfall over West Coast, Central Northeast and parts of North East India. Some of the important results obtained from this thesis are concluded here. Observational analysis shows a positive relationship between the snow starting dates in the northernmost Eurasia and onset dates of Indian summer monsoon. Regional climate model RegCM3 simulates the Indian summer monsoon rainfall reasonably well especially in the Central India compared to the other regions. 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 likely to be weaker under A2, A1B and B1 emission scenarios. In future, snow starting dates and its connection with Indian summer monsoon onset should be re-examined using more northernmost Eurasian stations data with continuous and longer records including the recent years. Tropical convective heating anomalies may force the anomalous mid-latitude circulation and large scale mid-latitude circulation affects land temperature. The phenomenon of mid-latitude circulation is not properly understood yet and more investigation is required. Further study and modeling experiments are needed verify and understand the mechanisms associated with the opposite polarity between snow depths in northern and southern Eurasia and also the inverse correlation between East and West Eurasia snow depths. More sensitivity experiments using IIT Delhi spectral GCM can also be set during contrasting monsoon cases in order to investigate influence of snow on monsoon. In case of regional climate study over several Indian regions, there is a need to use the state-of-the-art model RegCM4 and tune it for the Indian summer monsoon. A number of sensitivity experiments can be conducted in order to understand and correct the model bias.