The impact of mid-and high-latitude Rossby wave activities on the mediumrange evolution of EAP event in the pre-rainy period of South China.

Based on the NCEP-DOE AMIP-Ⅱ daily reanalysis data (1979-2005), the evolution of the EAP event during the pre-rainy period of South China is studied on the mediumrange time scale. It is shown that: (1) The life cycles of positive and negative EAP events share some similarities: in the middle and upper troposphere, Rossby wave packets emanating from the northeast Atlantic or Europe propagate toward East Asia along the Eurasian continent waveguide and finally give rise to the three anomaly centers of EAP event over East Asia. Among the three anomaly centers, Rossby wave packets propagate from the high-latitude anomaly center toward the middle-latitude and subtropical ones. The enhancement and maintenance of the subtropical anomaly center is closely associated with the existence of subtropical jet waveguide and with the incoming of Rossby wave packets from the upstream. In the lower troposphere, Rossby wave packets emanating from Asia subtropical area toward East Asia. (2) Among the three anomaly centers of EAP events in the middle and upper troposphere, the western Pacific subtropical one appears latter most. (3) Yet, the positive and negative EAP events could not be regarded as “mirror” each other. For the positive event, the positive height anomaly center around the Scandinavia Peninsula keep its strength and position during the mature period, and the Rossby wave packets thus propagate persistently toward East Asia, facilitating a longer ripe time of the positive event. As for the formation of negative EAP event, however, the incoming Rossby wave energy from the upstream contribute to both the enhancement and southeastward movement of the negative anomaly belt from the Yenisei River to the Bering Strait and the positive anomaly center around Mongolia. At the peak time, those two anomaly circulations become the northeast Asia and middle-latitude anomaly centers of the negative event, respectively. The energy dispersion of Rossby wave packets is relatively fast due to the predominant zonal circulation in the extratropics, causing a shorter mature period of the negative event. (4) During the pre-rainy period of South China, the prevalence of the EAP event significantly affects the rainfall over the region south of the Yangtze River. The positive (negative) EAP event tends to cause the positive (negative) precipitation anomalies of the region south of the Yangtze River, which is different from the early research based on the monthly mean data.