Abrupt change in the characteristics of the cloud zone in subtropical east asia around the middle of may

An abrupt seasonal change in cloud features around the East Asian cloud band (EA-band) was found in the middle of May or late May in all the years of 1982 to 1987 by using two indexes derived from GMS infrared data, middle- and high-level cloud amount (MHc-amount) and the appearance frequency of the high-level cloud region (Hc-frequency) for a 10- or 15-day period. The change occurred over a short period, less than 5 days (transition period: TP), nearly simultaneously with the onset of the "Southeast Asian summer monsoon" about one month before the onset of the Indian monsoon. The main results are as follows: 1) The MHc-amount and Hc-frequency increase considerably in the coastal region of South China after the TP. The greater part of the Hc-frequency are associated with the deep convective clouds. This change is associated with the enhancement of the low-level southwesterly wind, an decrease in thickness of baroclinic layer below the 500 mb level and an increase in specific humidity there and in the region to the south of the EA-band. 2) MHc-amount and Hc-frequency in the EA-band also increase to the east of 13°E after the TP, accompanied by the enhancement of the southwesterly wind in its southern part and the southward shift of a subtropical high pressure area west of 150°E. 3) As for the year-to-year variation, the distribution of the area with large MHc-amount in the EA-band before the TP seems to be greatly influenced by the amount of confluence between the southwesterly wind in the north of the subtropical high pressure area and westerly wind north of the EA-band. 4) MHc-amount over Central China after the TP is about 60% in the year of stronger baroclinicity there, but is about 20% in the year of weaker baroclinicity. 5) Only in 1987, the EA-band and a subtropical high pressure area east of 130E shifted northward after the TP, which are the opposite characteristics to all the other years.