Speleothem oxygen isotopes (δ^(18)O) are often used to reconstruct past rainfall δ^(18)O variability, and thereby hydroclimate changes, in many regions of the world. However, poor constraints on the karst hydrological processes that transform rainfall signals into cave dripwater add significant uncertainty to interpretations of speleothem-based reconstructions. Here we present several 6.5 year, biweekly dripwater δ^(18)O time series from northern Borneo and compare them to local rainfall δ^(18)O variability. We demonstrate that vadose water mixing is the primary rainfall-to-dripwater transformation process at our site, where dripwater δ^(18)O reflects amount-weighted rainfall δ^(18)O integrated over the previous 3–10 months. We document large interannual dripwater δ^(18)O variability related to the El Niño–Southern Oscillation (ENSO), with amplitudes inversely correlated to dripwater residence times. According to a simple stalagmite forward model, asymmetrical ENSO extremes produce significant offsets in stalagmite δ^(18)O time series given different dripwater residence times. Our study highlights the utility of generating multiyear, paired time series of rainfall and dripwater δ^(18)O to aid interpretations of stalagmite δ^(18)O reconstructions
