Trends in Seoul (1778-2004) summer precipitation

Precipitation data for Seoul, Korea, have been recorded since 1778, representing one of the world's longest instrumental measurements of daily precipitation. Trends in summer precipitation amount, intensity, and indices of climate extremes are studied. The detected trends were tested using trend-to-noise ratio test, nonparametric Mann-Kendall rank statistic and Monte Carlo simulation procedure. Over the past 227 years and in the twentieth century, the June-September total precipitation amount has a trend significant at 95% confidence level; the numbers of torrential rainy days with precipitation greater than 75 mm, the highest one-day rainfall amount, the number of extremely wet days defined as the 99 percentile of the distribution of daily precipitation, and the precipitation amount falling in the extremely wet days, all have a trend significant at 99% confidence level. An intensity index that captures the combined effects of precipitation frequency and intensity, calculated as the number of heaviest rainy days that constitute 67% of the total precipitation shows a decreasing trend significant at 99% confidence level, suggesting an increase in precipitation intensity. However, the trends calculated for the pre-1950 period (1778-1949) are all insignificant except the intensity index, suggesting that real trends occur only in the recent 55 years. Nevertheless, the pronounced recent increasing trends since 1950 are unprecedented. What give rise this trend remain elusive.