Changes in Precipitation Extremes in Hawaii and Taiwan Under a Warming Climate

Trends of four common climate change indices related to extreme precipitation events in Hawaii and Taiwan over the last 60 years are investigated using a nonparametric Mann-Kendall test and Sen’s method.  The former one tests whether the trend is increasing or decreasing and estimates the significance of the trend, while the latter quantifies the slope of the trend.  The first three climate change indices include the daily rainfall intensity, the annual total number of day with daily rainfall greater than a specified threshold, and the annual maximum consecutive 5-day precipitation total which can be regarded as the magnitude of intense rainfall.  The fourth index, which defines the duration of excessive dryness, is the annual maximum number of consecutive dry days. All these indices are advocated by the CLIVAR program, a component of the WMO.

For the Hawaiian Islands, Kauai and Oahu are dominated by decreasing trends for three precipitation-related indices, while increasing trends in precipitation intensity and the magnitude of intense precipitation (i.e., annual maximum consecutive 5-day precipitation) are noted over the Island of Hawaii.  Therefore, changes in precipitation extremes exhibit an east-west difference and are not necessarily uniform across the island chain.  However, the upward trend of drought conditions prevail on all the major Hawaiian Islands since the 1950s.

For Taiwan, the trends of the aforementioned indices during the typhoon season (July-October) from 21 stations are studied using the same robust nonparametric method.  Upward trends in precipitation intensity are prevalent at all stations and trends at seven out of 21 stations are statistically significant at the 5% level.  The 5-day total precipitation amounts also exhibit an increasing trend over the last 60 years.  Longer drought duration is also noted, in Southern Taiwan in particular.  This is indicative of more distinct dry-wet conditions during the typhoon season in recent years.  Daily precipitation is further partitioned into typhoon and monsoon rainfall.  Precipitation intensity induced by typhoons and monsoon systems has both increased; these two components collectively contribute to strong upward trend seen in precipitation intensity.   Given the slower rate of observed precipitation increase in response to global warming, the significant upward trend in precipitation intensity cannot be accounted for only by anthropogenic forcing.  A new hypothesis is postulated to explain the increase in typhoon rainfall intensity affecting Taiwan.