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. At Kolkata, the Bay of Bengal is a major moisture contributor to precipitation supplying overall 65-75% of the total precipitation during the south west and the continental contribution varies from 25-35%. Over the Himalayan region, a discrepancy was noted between the measured isotopic composition of precipitation and that simulated by an isotope enabled general circulation model. The model underestimates the number of precipitation days as well as the magnitude of isotopic values. This may be attributed to deficiencies in the meteorological inputs or inadequate estimates of other parameters used in the model, as distinct hydrological parameterizations do lead to large differences in isotopic responses. The Craig and Gordon models are used to calculate the isotopic composition of evaporation flux in many isotope enabled general circulation models. The parameters governing the isotopic composition of evaporation flux can be considered separately or simultaneously in the Traditional and the Unified Craig-Gordon (CG) models. These models were tested for different molecular diffusivity ratios and ocean surface conditions to identify the parameterizations that best match the observed relationships between the meteorological parameters and the isotopic composition of water vapor. The model that matches best is used to calculate the isotopic composition of the local evaporation along the Southern Ocean sampling transect. The advected end member isotopic composition is assumed from the previous observations along the path followed by the air trajectories. Solving in a two-component mixing framework, the relative contribution of advected and evaporated vapor along the sampling transect is established. The advected moisture is a prominent component of the ambient vapor and its contribution becomes greater with increasing latitudes. South of 65 S the amount of moisture present in the atmosphere is less and is majorly local in origin with a small mixing of light Antarctic moisture. Results from this study should be useful to tune the models, and further help in the interpretation of regional paleo-climate data based on isotope proxies.