On the spatio temporal distribution of stable isotope ratios in the hydrological cycle over India and the Southern Ocean

Abstract

The isotopic composition of water vapor and precipitation at continental and oceanic sites depends on factors such as: the geographical location of the sampling site, meteorological conditions at the moisture source areas, local meteorological conditions, rainout history of air-masses along their trajectory, large-scale moisture recycling below cloud evaporation, etc. The present study aims at identification and quantification of such processes over the Indian subcontinent and the Southern Ocean based on observed stable isotopic composition in precipitation and atmospheric moisture. The oxygen and hydrogen isotopic composition of precipitation at Srinagar, Kashmir, allowed delineating the respective role of large-scale moisture transport processes vs that of other meteorological factors such as temperature and precipitation amount. The time integrated, large-scale convective activity over several days, constitutes a major factor governing the isotopic composition of precipitation at Srinagar, whereas local meteorological factors play a minor role. Over the Southern Ocean, as opposed to local factors, atmospheric pressure and air temperature majorly govern the precipitation isotopic composition. The isotopic data was further used to estimate the moisture sources contributing to the summer precipitation at Kolkata and summer/winter precipitation at Srinagar. A simple box model was developed to simulate the moisture transport and mixing to estimate the contribution of sources to precipitation at Srinagar and Kolkata. Precipitation at Srinagar, during the active period of the Western Disturbances, carries 80% of moisture from the Mediterranean region while the balance originates from the Arabian area. Adopting moisture source isotope signatures observed over a long-time span at the New viii Abstract ix Delhi station, the Rayleigh s distillation model explains the observed variability in the isotopic data at Kashmir Valley during the Indian Summer Monsoon...