During the last glacial period, eustatic lowering of sea level exposed large areas of continental shelves that are presently submerged. By a series of direct numerical simulations we investigate the sensitivity of the atmosphere-ocean system to exposure of the extensive Sunda shelf in the western tropical Pacific. We demonstrate that this specific forcing factor triggers increased convection and upper level divergence over the shelf itself, an increased Walker circulation over the far western Pacific, and subsidence over the central and eastern Pacific that weakens the surface easterlies. A comparison of atmosphere-only with coupled atmosphere-ocean integrations demonstrates that dynamic coupling of the ocean enhances the equatorial response, with cooling and drying of the tropics north and south of the Intertropical Convergence Zone. Exposure of the shelf therefore represents one mechanism by which glacial climates are affected by eustatic sea level changes. These changes constitute a fundamental shift in the El Ni¿o background state.