We present the use of a stable water isotope tracer scheme in the Melbourne University General Circulation Model (MUGCM) to test the isotopic response to different climate conditions and reexamine the use of present day relationships to reconstruct past tropical climates from isotopic records. The isotopic ratios in precipitation are modelled using the atmospheric MUGCM, and the surface ocean isotopic ratios are calculated using a slab ocean scheme.

Oxygen isotope ratios in precipitation are correlated with local surface temperature at high latitudes and with precipitation amount in the tropics, making isotopic records an invaluable tool for the reconstruction of past climate. Oxygen isotope ratios in tropical marine records provide information about past sea surface temperatures (SSTs), the hydrological balance and global changes in sea level. However interpretation of isotopic climate records requires an understanding of the complex interaction between temperature, atmospheric circulation, hydrology and oxygen isotope ratio changes. The use of an isotopic tracer scheme in a climate model allows such interactions to be examined for a variety of climate conditions.

The ability of the MUGCM to capture present day isotopic climate signals is tested by carrying out a simulation of recent (1950-1999) climate using the GISST2.3b SST and sea ice monthly data as boundary conditions. Comparison with isotopic records from observing stations verifies the model's ability to reproduce observed seasonal isotopic variability in the tropics as well as interannual signals associated with El Nino-Southern Oscillation and monsoon variability. Comparison between simulated coral isotopic ratios and isotopic records from coral samples over the same time period shows that the model correctly captures the observed dependence of coral isotopic ratio on SST and local hydrological balance.

A simulation of Last Glacial Maximum (LGM) climate using CLIMAP boundary conditions is presented investigating the sensitivity of isotopic ratios in precipitation to large scale changes in climate. The isotopic ratios in precipitation are compared with records from tropical and polar ice cores while the isotopic ratios in the surface ocean are compared with coral and foraminifera records. The change in modelled isotope-climate relationships under LGM conditions is used to quantify the limitations of the use of present day relationships to reconstruct temperature, precipitation and SST from isotopic proxy records.