Stable water isotopologues in the Indian summer monsoon rainfall

Stable oxygen isotope ratios (18O) of tree cellulose and speleothem carbonate are useful proxies for past monsoon rain in many tropical regions, as in such region a decrease in rain 18O accompanies an increase in rainfall on a monthly time scale. This amount effect varies spatially; therefore a local calibration, with actual measurements of rain amount and its 18O is required. Such observations, however, are quite limited in space and time. This thesis is aimed to improve the understanding of factors that control the 18O of Indian monsoon rainfall. For the present study water vapor samples were collected from the marine atmosphere over the Bay of Bengal (BoB) and rainfall sampled from Central and Northern India. The multiple simulations from isotope enabled General Circulation Models (GCM) are also used to understand the variability of the 18O of the Indian Summer Monsoon (ISM) rainfall on daily to inter-annual time scales. Measurements of stable isotopic compositions ( 18O and D) of water vapor collected from the BoB helps characterize both ISM vapor and North East Monsoon (NEM) vapor. This study shows that vapor 18O is higher during ISM compared to NEM with higher d-excess during NEM. This seasonal deference is possibly due to the seasonality in sea surface conditions, change in circulation pattern and changes in the type of rain forming systems (monsoon depression during ISM vs. tropical cyclones during NEM). The stable isotopic composition of water vapor estimated using Craig and Gordon model with the closure assumption (i.e., evaporation from the BoB is the only source of vapor) matches well with the measured values during non rainy days of ISM, whereas, it shows a large deviation from the model estimate during NEM season. The deviation from model estimate is negatively correlated with the rainfall along parcel trajectory (upstream rainfall) during both the seasons. The convective downdraft associated with tropical cyclones during NEM also plays major role in the lowering of vapor 18O. During ISM 2013, rain water samples were collected on a daily basis from six different cities (Ahmedabad, Bhopal, New Delhi, Kanpur, Varanasi and Dhanbad) spread over central and northern India and stable isotopic analyses were carried out. A weak amount effect (negative correlation between local rain and its 18O) is observed at ve out of the six stations, which explains 7-22 % of intrapersonal variation of 18O of rain. The nudged simulations from an isotopeenabled General Circulation Model (IsoGSM) is compared with the observations. Though the model has some limitation in simulating the accurate spatio-temporal pattern of rainfall, the model simulated rain 18O is in good agreement with the observations. This study suggests strong control of moisture transport pathways on daily rain 18O at Ahmedabad, Bhopal and New Delhi. At New Delhi this effect is observed on intraseasonal to interannual timescales. Many isotope enabled General Circulation Models (GCM) are used to aid the interpretation of rainfall-18O based proxies; nevertheless, all such simulations taken together remained to be evaluated against observations over the Indian Summer Monsoon (ISM) region. This study also examine ten such GCM simulations archived by the Stable Water Isotope INtercomparison Group, phase 2 (SWING2). The spatial patterns of simulated ISM rainfall and its 18O are in good agreement with the limited observations available. Simulations nudged with observed wind ends show better skill in reproducing the observed spatio-temporal pattern of rainfall and its 18O. A large discrepancy is observed in the magnitude of the simulated amount effect over the Indian subcontinent between the models and observations, probably because models simulate the spatial distribution of monsoon precipitation dierently. Nudged simulations show that interannual variability of rainfall 18O at proxy sites are controlled by either regional (rather than local) rainfall or upstream rain out. Interannual variability of rainfall 18O over the East Asian region is well correlated with El Nino Southern Oscillation (ENSO) while it is only weakly correlated over the Indian sub-continent.by M. MidhunPh.D