Changes in Frequency of Heavy Rain Events Over Hawaii

This study investigates the frequency of heavy rainfall events during the wet season (October-April) over Hawaii and their relationship with two Pacific climate modes, namely the PNA (Pacific North American) pattern and the ENSO (El Niño-Southern Oscillation) phase. The standard multiple linear regression technique is employed using daily rainfall reports from 12 selected locations over a period 1958-2005. The regression analysis indicates that the PNA index and the SO (Southern Oscillation) index together can explain about 18-44% of the variability in the number of heavy rain events. Further, the regression analysis reproduces the trend toward fewer heavy rain events in the years after the Pacific climate shift in the mid-1970s.

The study further applies the regression technique to the projected values of the two indices obtained from a suite of climate models used in the IPCC AR4 (2007) Documents. These models project only small changes in these indices and when applied to heavy rain events show only small changes in future heavy rain events with large uncertainty, this uncertainty stemming from disparities among the climate models.

The findings of this study portray substantial uncertainty in future projections of heavy rain events over Hawaii. A couple of other recent studies on 'extreme precipitation' over the UK (Rodda et al., 2010) and over the Zhujiang (Pearl) River basin, China (Gemmer et al., 2010) suggest that extreme precipitation events are driven more by natural climate variability and NOT by human-added CO₂ induced warming.

Additional References
Gemmer, M., Fischer, T., Jiang, T., Su, B. and Liu, L.L. 2011. Trends in precipitation extremes in the Zhujiang River basin, South China. Journal of Climate 24: 750-761.

Rodda, J.C., Little, M.A., Harvey, J.E. and McSharry, P.E. 2010. A comparative study of the magnitude, frequency and distribution of intense rainfall in the United Kingdom. International Journal of Climatology 30: 1776-1783.