Linkage between Interannual Variation of the East Asian Intraseasonal Oscillation and Mei-Yu Onset

The Mei-Yu onset over the middle-lower reaches of the Yangtze River Valley (MLYRV) varies considerably from early June to mid-July, which leads to large interannual changes in rainy season length, total summer rainfall and flooding potential. Previous studies have investigated the impact of El Niño/Southern Oscillation (ENSO) on the Mei-Yu onset. This study shows that a strong (weak) East Asian and Western North Pacific (EAWNP) intraseasonal oscillation (ISO) in spring leads to an early (late) onset of Mei-Yu over the MLYRV, and this ISO-Mei-Yu relationship is attributed to different types of ENSO in the preceding winter. A strong EAWNP ISO in spring is related to an eastern Pacific ENSO (EP-ENSO) in the previous winter, and negative sea surface temperature (SST) anomalies in the eastern Indian Ocean and the South China Sea (SCS) in May, which can cause an early onset of the South China Sea summer monsoon that also favors an early Mei-Yu onset. In contrast, a weak EAWNP ISO in spring is associated with a central Pacific ENSO (CP-ENSO) before April, but with an EP-ENSO after April, and positive SST anomalies in both the eastern Indian Ocean and the SCS in May. A statistical forecast model combining the intensity of spring EAWNP ISO, CP-ENSO and EP-ENSO indices shows a high prediction skill of the observed Mei-Yu onset date.