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.

中文翻译：

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孟加拉国同位素组成的季节性变化和降水的起源

水同位素组成（δ ¹⁸O和δD）在过去的降水的地球代理中使我们能够更好地理解和解释印度夏季风（ISM）的变化。先前的研究表明，降水的起源是控制印度次大陆周围降水同位素组成的重要因素。但是，由于拉格朗日方法不能满足对流区域绝热过程的假设，因此难以量化。我们调查了2010年孟加拉国三个地点的降水同位素组成，并使用结合了同位素的大气总循环模型通过欧拉方法估算了降水的成因。我们的观察显示，在δ类似的季节和季节内变化¹⁸各地点之间的降水量为O值，而各地点之间的降水量的时间特征不同。同位素组成与该区域周围对流活动的迁移有关。该模型表明，预季风季节（从三月中旬至六月初）的特点是高δ ¹⁸从孟加拉湾和阿拉伯海沉淀发起的O值。在ISM季节（从六月中旬到十月初），这些来源对降水的贡献逐渐下降，而来自印度洋的贡献增加，导致δ下降¹⁸由于运输过程中降雨过程的增强，降水的O值。在季风后季节（从10月中旬到11月），这些水分对孟加拉国降水的贡献较小，而来自太平洋和陆地表面（即水的循环利用）的水分对季节的降水贡献很大。由于水的循环利用来源于过去的沉淀与低δ ¹⁸在ISM赛季O值，其降水的贡献减少δ ¹⁸ISM和季风后季节的降水O值。这些结果表明，降水的起源和有组织的对流活动的迁移是控制该地区降水同位素组成的主要因素。这些特征可用于根据水同位素组成识别季风爆发和撤出。