SEASONAL PHASE LOCK OF INTRASEASONAL VARIATION DURING THE ASIAN SUMMER MONSOON

By using the climatological mean daily value from the 10-year objective analysis dataset and the 12-year OLR data, the enhancement of the Asian summer monsoon activity, which occurs intermittently with a period of 30-60 days, is examined. In particular, this work describes the seasonal phase lock of the intraseasonal variation (ISV). In the northern hemisphere summer, the enhancement of the convective activity and the low-level monsoon westerlies is evident along 10-degrees-N around early June and middle July over the Indian Ocean and around middle June and late July to early August and early-middle September over the western Pacific. The enhancement is also found around late May and middle July over the equatorial Indian Ocean. The first enhancement in late May and early June over the Indian Ocean corresponds to the climatological monsoon onset, dated June 1 in the South India and in southern Japan. The second enhancement in late July over the western Pacific is accompanied by a rather abrupt change. The monsoon westerlies in the lower latitude suddenly extend to the western Pacific up to 150-degrees-E. The penetration of the westerlies to the subtropical western Pacific is associated with the eastward propagation of the ISV from the Arabian Sea and the equatorial Indian Ocean in middle July. The westerlies over the subtropical western Pacific result in cyclonic circulation over the western Pacific and anti-cyclonic circulation over Japan, which brings the withdrawal of the Baiu and the summer season in Japan.