High and Low Rainfall Events in Hawai'i in Relation to Large-Scale Climate Anomalies in the Pacific: Diagnostics and Future Projections

We present results from our statistical analysis regarding low and high rainfall events and their relation to large-scale climate anomalies. We focus on two aspects of the hydroclimatic conditions in Hawai'i: months with low rainfall, and the number of days with heavy rain during the wet season (November-April). Low rain events are defined as monthly rainfall amount below the 10% percentile values estimated from the empirical distribution of wet season months during a the years 1958-2007. Heavy rain events are defined on a daily basis using the 90% percentile threshold value of the empirical distribution function of daily rainfall. Our results show that: (a) The number of heavy rain events during the winter seasons displays a negative trend from 1958-2007, in particular after the mid-1970s climate shift. (b) The number of dry months during the wet season exhibits an increasing trend. (c) The trend in the heavy rain events that is observed across the rain gauge station network can be explained by shifts of the major Pacific climate modes, in particular the Southern Oscillation and its associated index (SOI) and the Pacific North American (PNA) pattern and Pacific Decadal Oscillation (PDO). (d) Statistical downscaling of future climate change projections for heavy rain events based on six IPCCC AR4 global climate model simulations show different possible scenarios, but overall small changes in the frequency of heavy rain days. (e) The same models, however, project higher probabilities of dry months in the coming decades.