The Melbourne University atmospheric general circulation model with stable water isotope tracers is used to examine the variability of isotopic ratios of precipitation and the surface ocean in the tropics for present-day (1950--1999) climate. Surface ocean isotopic ratios are simulated interactively using a one-dimensional scheme that reproduces key features of the observed tropical isotopic spatial distribution and seasonal and interannual variability. The seasonal and interannual variability of modeled isotopic ratios of tropical precipitation is strongly associated with changes in precipitation amount, in agreement with previous isotopic modeling studies. Modeled isotopic ratios of both precipitation and surface ocean water respond to El Ni¿o--Southern Oscillation (ENSO), although the spatial patterns of ENSO and monsoon isotopic responses differ from observations because of biases in the simulated tropical climate. The model captures the dependence of the interannual variability of precipitation isotopic ratios over the tropical Andes on local temperature and precipitation variability and moisture balance over the Amazon basin but fails to reproduce a significant ENSO precipitation or isotope signal over this region. Modeled precipitation isotopic ratios are significantly correlated with local precipitation amount but not with local or regional temperature at Tibetan Plateau ice core sites on interannual timescales, in disagreement with the interpretation of these ice core records as temperature proxies. Surface ocean isotopic ratios are used to calculate modeled "coral," isotopic ratios which are compared with modern coral records, reproducing observed interannual variability where precipitation is well simulated.