Trends in return levels of 24-hr precipitation extremes during the typhoon season in Taiwan

This study is to investigate changes in maximum 24-hr precipitation for 20 stations during the typhoon season (July-October) and how the El Nino-Southern Oscillation (ENSO) may modulate precipitation extremes in Taiwan. Based on the non-parametric Mann-Kendall method and Sens's test, 15 out of 20 stations (three fourth) exhibited an upward trend from 1958 to 2013. Results of the field significance test suggest that the significant increasing trend is not caused by random variability. The method of the non-stationary generalized extreme value distribution (NGEV) is then applied to determine temporal changes in return levels. Results show that a large majority of stations are marked by an increasing trend in the three chosen return levels (2, 20, and 100 years) over the last 56 years. Therefore, more intense typhoon producing seasonal maximum 24-hr precipitation has been observed in Taiwan. The waiting time for an extreme event to occur has shortened considerably in recent years. For stations located in western/central Taiwan, an El Nino (La Nina) event favours low (high) precipitation extremes. It is the opposite for stations in northern and eastern Taiwan. Thus, an east-west regional difference in precipitation extremes across Taiwan is noted. A NGEV model based on both time and ENSO as covariates is also applied. Inter-annual variations influenced by ENSO are more dominant than long-term trend in return levels for most stations in western/central Taiwan.