While Asian monsoon (AM) changes have been clearly captured in Chinese speleothem oxygen isotope (δ¹⁸O) records, the lack of glacial-interglacial variability in the records remains puzzling. Here, we report speleothem δ¹⁸O records from three locations along the trajectory of the Indian summer monsoon (ISM), a major branch of the AM, and characterize AM rainfall over the past 180,000 years. We have found that the records close to the monsoon moisture source show large glacial-interglacial variability, which then decreases landward. These changes likely reflect a stronger oxygen isotope fractionation associated with progressive rainout of AM moisture during glacial periods, possibly due to a larger temperature gradient and suppressed plant transpiration. We term this effect, which counteracts the forcing of glacial boundary conditions, the moisture transport pathway effect.

We present precipitation isotope data (δ²H and δ¹⁸O values) from 19 stations across the tropics collected from 2012 to 2017 under the Coordinated Research Project F31004 sponsored by the International Atomic Energy Agency. Rainfall samples were collected daily and analysed for stable isotopic ratios of oxygen and hydrogen by participating laboratories following a common analytical framework. We also calculated daily mean stratiform rainfall area fractions around each station over an area of 5° x 5° longitude/latitude based on TRMM/GPM satellite data. Isotope time series, along with information on rainfall amount and stratiform/convective proportions provide a valuable tool for rainfall characterisation and to improve the ability of isotope-enabled Global Circulation Models to predict variability and availability of inputs to fresh water resources across the tropics.

The intertropical convergence zone (ITCZ) plays a key role in regulating tropical hydroclimate and global water cycle through changes in its convection strength, latitudinal position, and width. The long-term variability of the ITCZ, along with the corresponding driving mechanisms, however, remains obscure, mainly because it is difficult to separate different ITCZ variables in paleoclimate proxy records. Here, we report a speleothem oxygen isotope (δ ¹⁸ O) record from southwestern Sulawesi, Indonesia, and compile it with other speleothem records from the Maritime Continent. Using the spatial gradient of speleothem δ ¹⁸ O along a transect across the ITCZ, we constrain ITCZ variabilities over the Maritime Continent during the past 30,000 y. We find that ITCZ convection strength overall intensified from the last glacial period to the Holocene, following changes in climate boundary conditions. The mean position of the regional ITCZ has moved latitudinally no more than 3° in the past 30,000 y, consistent with the deduction from the atmospheric energy framework. However, different from modern observations and model simulations for future warming, the ITCZ appeared narrower during both the late Holocene and most part of the last glacial period, and its expansion occurred during Heinrich stadials and the early-to-mid Holocene. We also find that during the last glacial and deglacial period, prominent millennial-scale ITCZ changes were closely tied to the variability of the Atlantic meridional overturning circulation (AMOC), whereas during the Holocene, they were predominantly modulated by the long-term variability of the Walker circulation.

Monthly precipitation samples from Singapore were collected between 2013 and 2019 for stable isotope analysis to further our understanding of the drivers of tropical precipitation isotopes, in particular, ¹⁷O‐excess. δ¹⁸O ranges from –11.34‰ to –2.34‰, with a low correlation to rainfall (r = –0.31, p = 0.014), suggesting a weak amount effect. d‐excess is relatively consistent and has an average value of 10.89‰ ± 3.45‰. Compared to high‐latitude regions, ¹⁷O‐excess in our samples generally falls in a narrower range from 2 to 47 per meg with an average of 21 ± 11 per meg. Moreover, ¹⁷O‐excess shows strong periodic variability; spectral analysis reveals 3‐month, 6‐month, and 2.7‐year periodicities, likely corresponding to intraseasonal oscillations, monsoons, and the El Niño–Southern Oscillation (ENSO), respectively. In contrast, d‐excess shows no clear periodicities. Although spectral analysis only identifies 6‐month periodicity in the δ¹⁸O time series, δ¹⁸O tracks the Nino3.4 sea surface temperature variability; the average δ¹⁸O value (–5.2‰) is higher during El Niño years than ENSO neutral years (–7.6‰). Therefore, regional convection associated with monsoons and ENSO has different impacts on δ¹⁸O, d‐excess, and ¹⁷O‐excess. ¹⁷O‐excess and d‐excess are anticorrelated and do not relate to the relative humidity in moisture source regions. Extremely low humidity and drought conditions in moisture source regions would be required to account for high ¹⁷O‐excess. Processes during transport and precipitation likely modify these two parameters, especially ¹⁷O‐excess, which no longer record humidity conditions of moisture source regions. Our findings will be useful for further modeling studies to identify physical processes during convection that alter d‐excess and ¹⁷O‐excess.