Fig. 1: General circulation and rainfall patterns in Southeast Asia.

Precipitation and circulation patterns during (a) the summer monsoon season and (b) the autumn/winter monsoon season (NEM) in Southeast Asia (red star indicates GNIP (Global Network of Isotopes in Precipitation) station in Dong Hoi central Vietnam, ca. 70 km from Thien Duong Cave used in this study). During summer, strong southwesterlies prevail over mainland Southeast Asia, leading to intense rainfall during the summer monsoon. Between mid-September and December strong northeasterly winds carry moisture into central Vietnam. δ¹⁸O (VSMOW) and rainfall recorded at the GNIP station in central Vietnam (c), showing low values in summer and high values in winter. Rainfall and δ¹⁸O data are from 2014 to 2021 and error bars give standard error of the mean. Effective precipitation and amount-weighted δ¹⁸O were calculated from the GNIP data and TerraClimate dataset⁸⁸ d. Precipitation is shown in green and δ¹⁸O in orange. Shading in (a) and (b) shows long-term mean monthly precipitation rate from the GPCP (Global Precipitation Climatology Project) Version 2.3 Combined Precipitation Dataset⁸⁹, covering 1981 to 2010. The arrows indicate surface wind strength at 10 m using the u and v components of the ERA5 data set⁹⁰. Circles show the location of proxy records discussed in this work: D Dongge Cave (China)⁹¹, LH Lake Huguang Maar (China)¹⁶, TM Tham Doun Mai Cave (Laos)⁴³, TD Thien Duong Cave (Vietnam, red star), K Klang Cave (Thailand)⁹², BA Bukit Assam Cave (Borneo)⁴¹, T Tangga Cave (Sumatra)^25,74, LL Liang Luar Cave, Flores (Indonesia)⁹³.