Abstract
Water isotopic composition (δ18O and δD) in terrestrial proxies of past precipitation enable us to better understand and interpret variation in the Indian Summer Monsoon (ISM). Previous studies have suggested that the origin of precipitation is an important factor controlling the isotopic composition of precipitation around the Indian subcontinent; however, it is difficult to quantify using the Lagrangian approach because the approach does not satisfy the assumption of an adiabatic process over a convective area. We investigated the isotopic composition of precipitation at three sites over Bangladesh in 2010 and estimated the origins of precipitation by the Eulerian approach using an isotope-incorporating Atmospheric General Circulation Model. Our observations showed similar seasonal and intraseasonal variations in the δ18O values of precipitation among the sites, whereas the temporal characteristics of the precipitation amount differed among the sites. The isotopic composition was linked to the migration of organized convective activity around the region. The model showed that the pre-monsoon season (from mid-March to early June) was characterized by high δ18O values of precipitation originating from the Bay of Bengal and the Arabian Sea. In the ISM season (from mid-June to early October), the contribution of these sources to precipitation gradually decreased, while the contribution from the Indian Ocean increased, resulting in decreasing δ18O values of precipitation due to the enhanced rainout process during the transportation. These moisture contributed less to precipitation over Bangladesh in the post-monsoon season (from mid-October to November), whereas moisture originating from the Pacific Ocean and land surface (i.e., recycling of water) contributed to precipitation in the season. Because the recycling of water originated from past precipitation with low δ18O values in the ISM season, its contribution to precipitation reduced the δ18O values of precipitation in the ISM and post-monsoon seasons. These results suggest that the origins of precipitation and the migration of organized convective activity are the dominant factors controlling the isotopic composition of precipitation in the region. These characteristics can be used to identify monsoon onset and withdrawal based on water isotopic composition.
Highlights
The Indian Summer Monsoon (ISM), which is characterized by strong seasonality of rainfall and floods, affects regional society and agriculture around the Indian subcontinent
This finding was supported by a study based on multiple simulations of isotope-incorporating Atmospheric General Circulation Models (AGCMs) (Midhun and Ramesh 2016), which showed that the inter-annual variability in δ18O values of precipitation around the Indian subcontinent was controlled by rainout in the upstream area of the moisture flux
These results indicate that the δ18O value of precipitation over Bangladesh can be controlled by seasonal and intraseasonal variations
Summary
The Indian Summer Monsoon (ISM), which is characterized by strong seasonality of rainfall and floods, affects regional society and agriculture around the Indian subcontinent. Lekshmy et al (2014) and Chakraborty et al (2016) found low δ18O values of precipitation when air parcels passed over a convective area over the Indian Ocean This finding was supported by a study based on multiple simulations of isotope-incorporating Atmospheric General Circulation Models (AGCMs) (Midhun and Ramesh 2016), which showed that the inter-annual variability in δ18O values of precipitation around the Indian subcontinent was controlled by rainout in the upstream area of the moisture flux. This suggests that the origins of moisture and rainout in the upstream area are the dominant controlling factors of the isotopic composition of precipitation in the region
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