Chapter 3 - Characteristics of the subseasonal Oscillation of East Asian Meiyu

Abstract

Meiyu in East Asia, including Meiyu in China, Baiu in Japan, and Changma in Korea, is the special rainy season during the northward movement of the East Asia summer monsoon. Intraseasonal oscillation (ISO) plays a key role in the onset and northward transmission of the East Asian summer monsoon, which can regulate Meiyu in East Asia. In this chapter, a new index for determining Meiyu in East Asia is put forward by combining monsoon influence with persistence characteristics of rainfall. Then the climatic characteristics of the low-frequency oscillation of the East Asian Meiyu are analyzed on the basis of the new Meiyu index. The atmospheric low-frequency evolutions associated with the East Asian Meiyu are further discussed to provide clues for subseasonal forecasts of Meiyu in East Asia. Precipitation in the East Asian Meiyu region is found to show a three-peak distribution, which is mainly affected by the 10–20 days and 30–60 days low-frequency oscillations. Precipitation anomalies in the East Asian Meiyu area are related to the background conditions influenced by stronger low-frequency signals at a longer timescale. Occurrence of peak phases of rainfall ISO over the East Asia Meiyu region is in close association with both tropical Madden and Julian oscillation and mid–high latitude ISO. Over the mid–high latitudes, the ISO wave train from the Caspian Sea as well as the ISO of cold air from the Sea of Okhotsk, respectively, eastward propagation and southwestward transport, to the northern part of the Meiyu region. Over the mid–low latitudes, northward propagation of abnormal convection at a low level forms the meridional distribution of a cyclone–anticyclone–cyclone–anticyclone pattern from the west 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 northeast propagation of the convection anomaly wave from West Indian and the Arabian Sea also contribute to the peak phase of the Meiyu region. For the valley-phase rainfall over the Meiyu region, the opposite is the case.