Rainfall Effect on Hawaii's Climate
The rainfall projections from the Hadley HADCM2 model show that rainfall in the vicinity of Hawaii will increase for both short and long-term simulations during the summer months. Over the short-term (years 2025-2034), rainfall (Figure 18) will increase between 1.0 to 2.0 millimeters (0.04 to 0.08 inches) per day for June, July, and August and 3.0 to 4.0 millimeters (0.12 to 0.16 inches) per day for the same months over long-term (years 2090-2099). The rainfall projections for December, January, and February for both short- and long-term show that there will be no change in precipitation during the winter, although in the long-term projections an area of reduced precipitation (0.5 millimeters or 0.020 inches per day) lies just the west of the islands (Figure 18). Except for an area southwest of the Hawaiian Islands in model A2, the A2 and B2 models project no change in the mean precipitation for the area around Hawaii from 2071-2100 (Figure 30). It is worthwhile to note that the A2 and B2 models look at year-long averages whereas the Hadley HADCM2 looks at both winter (December, January, and February) and summer (June, July, and August) month averages. So the Hadley HADCM2 model does not average out the effects of winter and summer, which are important to Hawaii.
Figure 18. Projected changes in total precipitation change for 2025-2034 and 2090-2099 in units of millimeters per day*10 (centimeters per day).
Figure 30. Mean percentage change in precipitation from the multimodel ensemble for the SRES A2 2071-2100 and mean percentage change in precipitation for SRES B2 scenario. Both scenarios are relative to mean air surface temperatures of 1961-1990.
Overall, the model projections suggest that Hawaii will have elevated precipitation (at least in the summer months of June, July, and August) in the near- (2025-2034) and long- (2090-2099) term. These projections are counter to some historic evidence that warmer periods in Hawaii may not have had elevated precipitation, but instead experienced lower rainfall and increased evaporation which would lead to a decrease in the amount of available water. On the basis of soil distribution, researchers have concluded that higher rainfall was associated with colder, and not warmer, past climates. Additionally, studies of sediments and their distribution in Hawaii suggest that higher trade wind velocities – therefore elevated orographic processes and rainfall – accompanied colder temperatures in the past.
One problem with the larger regional (e.g., the Hadley HADCM2) or global climate models is the relatively course spatial scale resolution of these models. The large scale of these models prohibits them from resolving finer scale processes. Some of these processes impact the hydrology of islands where orography is important, and thus the magnitude and spatial distribution of rainfall and evaporation. Using a hydrologic model, researchers project that a temperature rise of 3o C will result in a 10% increase in evaporation in the Pearl Harbor basin, and even if this change is accompanied by an increase of 10% in precipitation, there still may be a significant shortfall of water at the present water usage level. A 3o C temperature increase is within the range of the long-term (Figure 17) sea surface temperature increase for both winter (December, January, and February) and summer (June, July, and August) months. There is a corresponding projected increase of 3.0 to 4.0 millimeters (0.12 to 0.16 inches) per day for the long-term in precipitation for the summer months (Figure 18).
Contrary to the Hadley HADCM2 model projections, the A2 and B2 models both do not show increased annual mean precipitation from year 2071 to 2100 for Hawaii (Figure 30). For the Pearl Harbor basin, a 10% increase in the yearly precipitation over the precipitation range of the basin (500 to 6000 millimeters or 19.7 to 236 inches per year) would correspond to an increase of 50 to 600 millimeters (2.0 to 24 inches additional rainfall per year). The Hadley HADCM2 summer long-term rainfall projections of 3.0 to 4.0 millimeters (0.12 to 0.16 inches) per day (Figure 18) would correspond to an increase of rainfall by 274 to 365 millimeters (10.8 to 14.2 inches) for those 3 months during the long-term (2090-2099). Note that this increase is only over 3 summer months and not the entire year. It is unclear whether there would be increased rainfall for the remaining 9 months, but for the long-term winter month projections (December, January, February in Figure 18), there is no projected increase in precipitation. It is possible that the increased rainfall in the summer months (274 to 365 millimeters or 10.8 to 14.2 inches) over the long-term will not balance the increased evaporation over the entire year caused by the elevated temperature (3o C) leading to non-sustainability in the water resource for the island of Oahu.