What Causes the Postmonsoon ¹⁸O Depletion Over Bay of Bengal Head and Beyond?

Abstract

Modern observations from Bay of Bengal (BOB) head have shown that precipitation δ18O value reaches the lowest stage during late postmonsoon when the summer monsoon has weakened or retreated, complicating the traditional interpretation of low δ18O value as strong monsoon. We show that this postmonsoon 18O depletion phenomenon exists from BOB periphery to the southeast and south Tibetan Plateau. Accumulative precipitation along back‐trajectory still explains daily precipitation δ18O variability during both the monsoon and nonmonsoon, thus indicating that longer‐term weighted‐mean δ18O is biased toward synoptic conditions during precipitation events. Further, we propose that transport of moisture from the east (west) of the India‐Burma Trough leads to depleted (enriched) 18O during the postmonsoon (premonsoon). Our results reconcile the apparent contradiction but beg the consideration of potential bias and influence of different precipitation regimes in paleo archives.

What Controls the Skill of General Circulation Models to Simulate the Seasonal Cycle in Water Isotopic Composition in the Tibetan Plateau Region?

“…It also reflects the evolution of atmospheric circulation (Gao et al., 2016; Joswiak et al., 2013; Kaspari et al., 2007; X. Yang et al., 2012; X. X. Yang et al., 2018; Yu, Yao, et al., 2016). However, the interpretation of isotopic signals remains challenging due to the numerous and complex processes involved in the hydrologic cycle, such as changes in moisture sources, recycling, and transport processes (e.g., An et al., 2017; Cai & Tian, 2020; Hren et al., 2009; Kong et al., 2019; Kurita & Yamada, 2008; Midhun et al., 2018; Sengupta & Sarkar, 2006; Shi et al., 2020; Tian et al., 2007, 2008; Wu et al., 2015, 2019; Yu et al., 2008, 2014). Over the TP, the isotopic variation of precipitation reflects the integrated information of the interaction between the westerlies and monsoon, in conjunction with the local recycling.…”

Moisture Sources and Pathways Determine Stable Isotope Signature of Himalayan Waters in Nepal

“…The 7‐day long HYSPLIT trajectories (Figure 4, Text S1 in Supporting Information ) show highly seasonal patterns of source pathways, characterized by local sources from the Gangetic plain, close to the site of precipitation in pre‐ISM, and a mixture of far‐afield sources from the open ocean and Gangetic plain during the ISM (Figure 4). These changes in moisture source area and transport distance are reflected in distinct isotopic signatures during the different seasons (Tables S7 and S8 in Supporting Inforation , Figure 6a; Brunello et al., 2019; Z. Cai & Tian, 2020).…”

Controls on Stable Water Isotopes in Monsoonal Precipitation Across the Bay of Bengal: Atmosphere and Surface Analysis

“…The effects of upstream vertical air motions (i.e., the upstream accumulative convection and downward motion) on the downstream δD p are discussed in this section. The upstream accumulative precipitation and average OLR are widely used to assess the influence of the upstream accumulative convection on the downstream δD p in the low latitudes (Cai & Tian, 2020; Vimeux et al., 2011; Zwart et al., 2016). In this study, we analyzed the changes of precipitation amount and OLR along the back trajectories, and calculated the correlations between the daily δD p at the three downstream study sites and the upstream accumulative precipitation amount and upstream average OLR (Figure S11 in Supporting Information ).…”

“…The low/high δ 18 O values in these paleoclimate records were thought to reflect low/high paleo‐δ 18 O p values (δ 18 O of precipitation) driven by strong/weak ISM (Joshi et al., 2017). However, recent studies have questioned the link between ISM intensity and δ 18 O p (Cai & Tian, 2020; Midhun et al., 2018). Specifically, δ 18 O p values across most areas of South Asia show a continuous decrease throughout the monsoon season from May to September (Ahmed et al., 2020; Chakraborty et al., 2022; Islam et al., 2021; Jeelani et al., 2018; Tanoue et al., 2018), which does not track ISM intensity (highest in July–August).…”