A rainfall index for the State of Hawaii and for each of the major islands is developed from more than 100 climate stations with long-period records (>50 yrs). A strong decline in rainfall is apparent in the last 30 years or so of the record for the state as a whole, and is present also for most of the islands. The precipitation changes are examined in terms of possible causal mechanisms, and statistical significance of the change is evaluated using different parametric and non-parametric tests. Changes in different daily and monthly quantiles are also examined.

The recent decline in Hawaiian Island rainfall approximately coincides with the mid-1970s climate change in the Pacific, as depicted by tropical sea surface temperatures (SST). The change may be related to an intensification of the Pacific sector Hadley circulation in northern winter and spring, which implies stronger subsidence over the Islands during that time. We consider the possible impacts of these changes on the State's water resources.