Using the mean pentad Outgoing Longwave Radiation (OLR) data observed from the NOAA satellites and the conventional weather observations for the 1991 Mei–yu season, the diagnostic study of the characteristics of the OLR is conducted. The paper describes the OLR and its anomalies pattern and the characteristics of propagation of the intraseasonal variation (ISV) of OLR during the 1991 flood Mei–yu periods. It is revealed that the variation of the ITCZ and the subtropical high are related to the evolution of the Mei–yu. The relationship between the anomalies of OLR over Tibet Plateau and the flood Mei–yu is also revealed. The results show that the earlier ITCZ occurrence in the western Pacific and the positive OLR anomalies (snow cover less than normal) over Tibet Plateau in winter are the important indications for the earlier onset date of the 1991 Mei–yu. Finally, by analyzing the features of the OLR pattern for the active and break phases of the 1991 Mei–yu and comparing with that of Indian monsoon, it is found that their onset dates are different from each other and their active phases either same or contrary. It is suggexted that the complication of the Mei–yu in China is mainly due to the interaction between the middle latitude cold air and two kinds (east Asia and south Asia) of monsoon events.
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