Interannual Variability of Spring Intraseasonal Variability and Mei-Yu Onset

The Mei-yu onset over the middle-lower reaches of Yangtze River Basin from mid-June to Mid-July in China is associated with the transition of weather regimes with the evolution of the East Asia summer monsoon. The Mei-Yu onset date is closed related to the amount of rainfall and the rainy season length as well as its intensity. In this study, it is found that the Mei-Yu onset date is significantly associated with the strength of the intraseasonal oscillation (ISO) in East Asia in spring. A strong ISO during the spring season leads to an early onset of Mei-yu, while a weak spring ISO is associated with a late Mei-yu onset over the middle-lower reaches of Yangtze River Basin. The onset of Mei-yu is closely related to opposite convective phases with active ISO convection over the South China Sea and depressed convection in coastal southern China. In the Z500 field, the blocking highs over the high latitude in both Lake Baikal and the Sea of Okhotsk are present with an early Mei-yu onset, and an abnormal low trough is detected over the high latitude region in East Asia with a late Mei-yu onset.

An examination of the precursory signals associated with early onsets of Mei-yu reveals that, during strong ISO years, the abnormal warm seas surface temperature (SST) over the Indian Ocean at the beginning of June plays a key role on the enhancement and westward extension of the western-Pacific subtropical High, which leads to increased southwesterlies and active convection over the middle-lower reaches of Yangtze River Basin. In weak ISO years, the abnormal warm SST over the Indian Ocean appears much later around the end of June, and the active convection over the middle-lower reaches of Yangtze River Basin corresponding to the onset of the Mei-yu season is originated from a westward propagation from the tropical western pacific. A statistical forecast model is established to assess the predictability of the onset dates of Mei-yu.