Speleothem oxygen isotope records in arid regions are typically interpreted as indicators of the total precipitation amount and/or its seasonal balance. Such studies rarely address the potential influence of groundwater mixing processes on δ¹⁸O variability of cave dripwater. Here, we develop a model of oxygen isotopes in dripwater, which we compare to water and stalagmite measurements from Cave of the Bells, Arizona. We simulate moisture flux from surface to cave as a two-layer "leaky-bucket" model. In observations and the model, fed with modern climate data, dripwater δ¹⁸O is most comparable to that of winter precipitation. We show that seasonality and duration of the regional summer monsoon affect how much summer precipitation reaches the cave. We employ a Monte Carlo method to specify statistically realistic ranges for input climate variables and produce time series and variance spectra of cave drips. The spectra of our synthetic δ¹⁸O series exhibit a high degree of variance at decadal to multidecadal frequencies, despite being driven by synthetic data that includes only a seasonal cycle. This suggests that some background level of variance in speleothem δ¹⁸O records could be due to nonclimatic processes, such as subsurface water storage and mixing. Interpreting climatic vs. nonclimatic controls on speleothem δ¹⁸O variance could be achieved by replicating records from different caves.