Onset and Withdrawal of the Western North Pacific Summer Monsoon in 2000s

This work examines peculiarities of onset and withdrawal of western North Pacific summer monsoon (WNPSM) in 2000s. In most of earlier works, the monsoon onset and withdrawal were discussed using pentad data. In order to reveal the latest monsoon variability, this work examines the onset and withdrawal of the WNPSM in 2000s (1998-2011) using daily data. In particular, this work puts emphasis on the data of tropical rainfall measuring mission (TRMM) and comparison with the results in previous studies revealing the features in 1980s (1980-1989). First, the region of WNPSM is defined by large precipitation increase from June to July in the western North Pacific. Using harmonics analysis, the onset and withdrawal of WNPSM, i.e., the rainy season in this region, are determined. The results clearly show that the onset in 2000s is about one month earlier than that in 1980s. However, the difference in the days of withdrawal is small between 2000s and 1980s. These results suggests that the WNPSM has a tendency to be longer. Around the onset days, a large tropical cyclone (TC) tends to appear in the WNPSM region in the western North Pacific. After this large TC's passage, many TCs pass and develop in this area. Therefor, the first large TC in this region would determine the onset like a jump. The onset days in 2000s become earlier because of earlier appearance of large TCs in this region. The pentad analysis similar to that in previous works reveals that the occurrence of heavy precipitation in the WNPSM region is 3-4 pentads earlier in 2000s. This early occurrence of heavy precipitation corresponds to the earlier rise in sea surface temperature (SST) reaching 29 degrees about one month. In fact, the lag correlations between precipitation around onset days and earlier SST are high. However, the correlations with SST after onset are low. SST before onset is crucial for the precipitation event determining the onset. It is shown that the onset of WNPSM becomes earlier with the increase of SST in the western North Pacific.