The cooler SST's result in only a few percent reduction in evaporation. The primary reason why the atmosphere is so much drier in unfavourable IOD (and/or ENSO) is the introduction of large scale sinking air and stability. This dries out the atmosphere from above. Any cold pool/cut off upper level low will reverse this tendency, at least locally near the location of the system. Some research on IOD impacts suggest that along the east coast of Australia a +ve IOD enhances easterly flows and increases moisture supply and rainfall. Only in a fairly thin strip near the coast and on the eastern slopes of the ranges which are far more sensitive to surface level trade wind moisture supply, and/or orographic benefits from an easterly regime.