What Causes the Postmonsoon <sup>18</sup>O Depletion Over Bay of Bengal Head and Beyond?
Zhongyin Cai, Lide Tian
Abstract
Abstract Modern observations from Bay of Bengal (BOB) head have shown that precipitation δ 18 O value reaches the lowest stage during late postmonsoon when the summer monsoon has weakened or retreated, complicating the traditional interpretation of low δ 18 O value as strong monsoon. We show that this postmonsoon 18 O depletion phenomenon exists from BOB periphery to the southeast and south Tibetan Plateau. Accumulative precipitation along back‐trajectory still explains daily precipitation δ 18 O variability during both the monsoon and nonmonsoon, thus indicating that longer‐term weighted‐mean δ 18 O 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) 18 O 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.