Climatologic characteristics of the intraseasonal oscillation of East Asian meiyu

Meiyu in East Asia including China, Japan and the Korean Peninsula, is the special rainy season during the northward movement of East Asian summer monsoon. Using NCEP/NCAR daily reanalysis and CMAP rainfall data, a new index of determining meiyu in East Asia is firstly put forward by combining monsoon influences with rainfall. Then, the Ensemble Empirical Mode Decomposition (EEMD), the latest signal processing method of nonlinear or non-stationary time series is adopted to indicate intraseasonal oscillation (ISO) characteristics of the climatic variation of rainfall over East Asian meiyu region as well as its relative large scale circulation. And the indicating functions of low frequency oscillation (LFO) of meiyu in East Asia to rainfall events are finally discussed, which may provide basis and reference for extended range forecasts on the intraseasonal time-scale over meiyu region in East Asia. The results suggest that: (1) The new index of East Asian meiyu defined by the spatial coverage percentage with summer monsoon influences involved, can well reflect both the climate features of meiyu rainfall in East Asia and the large-scale circulation conditions during meiyu period. (2) Climatically,the rainfall variation over East Asian meiyu region exhibits a distinct three-peak mode distribution with the peaks at the 27th, 36th and 47th pentad. The three-peak distribution of rainfall amount variation is mainly influenced by 30-60 day and 10-20 day LFO. Thereinto, 30-60 day LFO contributes more to the three-peak mode than 10-20 day LFO. (3) Occurrences of peak phases of rainfall ISO over East Asian meiyu region are in close association with both tropical and mid-high latitude LFO. Over the mid-high latitudes, LFO wave train from Caspian Sea and LFO of cold air from Sea of Okhotsk respectively eastward propagate and southwestward transport to the north part of the meiyu region. Over the mid-low latitudes, northward propagation of abnormal convection at low level forms the meridional distribution of cyclone-anticyclonecyclone-anticyclone from the western North Pacific to Northeast China. Thus, the northerly wind from the mid-high latitudes converges with the southerly wind from the mid-low latitudes over the meiyu region, which provides favorable conditions for the peak phase rainfall. Meanwhile, the northeastward propagation of the convection anomaly wave from West Indian and the Arabian Sea also contribute to the peak phase of meiyu. For the valley phase of rainfall over meiyu region, contrary is the case. (4) There exist distinct differences for probabilities of extreme rainfall events over the meiyu region among the different LFO phases of meiyu. The probability of extreme events with abnormally abundant (scarce) rainfall amount is 30% under the peak (valley) phases of meiyu LFO. Therefore, the LFO features of circulation relating to meiyu LFO have reference value for the extended range forecast of rainfall events over East Asian meiyu region.