Seasonal variation in isotopic composition and the origin of precipitation over Bangladesh

Abstract

Water isotopic composition (δ 18 O 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 δ 18 O 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 δ 18 O 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 δ 18 O 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 δ 18 O values in the ISM season, its contribution to precipitation reduced the δ 18 O 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.

Space- and time-varying associations between Bangladesh's seasonal rainfall and large-scale climate oscillations

“…Teleconnection analysis of Bangladesh's seasonal rains 21 correlated with the post-monsoon rain in the entire country, and the negative correlation mostly became significant in the recent epoch. Significant influence of ENSO on the post-monsoon rain in Bangladesh mostly agrees with the findings of [66], who showed that the Pacific Ocean, the BoB, and land surface were the main moisture sources inducing the post-monsoon rain in the country.…”

Spatial and temporal distributions of stable isotopes in precipitation over Thailand

“…More negative δ 18 O and δ 2 H values are usually found at higher elevations and in the continental region than the low‐lying and coastal areas, likely due to the altitude effect and continental effect (Gonfiantini et al, 2001), respectively. In addition to the effects mentioned above, extremely negative values are also correlated with deep convection, particularly in tropical regions (Ahmed et al, 2020; Tanoue et al, 2018).…”

Statistical Analysis of the Precipitation Isotope Data with Reference to the Indian Subcontinent

“…These sites are Port Blair in the Andaman Islands (data available: 2012-2018; [26,[28][29][30]), Minicoy Island in the Arabian Sea (2015-2018; [31]); Kolkata in east India (2015-2018; [26]); Darjeeling (2013-2018) and Tezpur in northeast India (2016-2018; [26]); Ahmedabad (2007) [25] and Pune in west India (2014-2018; [32,33]), Bandipora in north India (July-2016 to June-2018; [34]) and Kozhikode in south India [22]. Precipitation isotope data of a few sites in Bangladesh have also been used [29,35]. Figure 1 shows the precipitation sampling sites and the core monsoon zone of India (shaded).…”

“…This process is also likely to contribute to yielding a relatively high K value for the monsoon season. The post-monsoon season, on the other hand, gets moisture from multiple sources [35], which also makes the corresponding distribution pattern skewed in the negative direction.…”