Variability and singularity of Seoul, South Korea, rainy season (1778-2004)

The 227-yr daily precipitation record gathered for Seoul, South Korea, represents one of the longest instrumental measurements, which provides an exceptional opportunity for detecting climate singularity (a property of phase locking to annual cycle) of extreme weather events and multidecadal-centennial variability of the rainy season structure. From late June to early September, the occurrence of heavy rain events shows a climatological quasi-biweekly oscillation. The rainy season characteristics, including the dates of onset, retreat, summit, and the duration, all show significant centennial variations. The rainy season summit shows a tendency toward delayed occurrence, which changed from the 37th pentad (P37; 30 June-4 July) during the 1778-1807 period to P44 (4-8 August) during the 1975-2004 period. The amplitude of the interannual (2-6 yr) variation of summer precipitation shows a prominent fluctuation with a 50-yr rhythm. A notable climatological break (around 9-13 August) divides the rainy season into a changma (Korean for continuous rain period) and a post-changma period. The major modes of subseasonal variability of the rainy season are characterized by an advanced changma and an enhanced post-changma, respectively. The former is dominated by biennial variation, whereas the latter has a major 5-yr spectral peak, suggesting that the processes leading to their variability are different. The occurrence of severe drought events exhibits a 4-yr spectral peak along with large power on a centennial time scale, while the severe flood events have a spectral peak at 3 and 19 yr, respectively. The remarkable climate variability in Seoul rainfall suggests that trends detected by using a 50-yr-or-shorter precipitation record likely reflect natural variability.