ENSO and Asian summer monsoon: Persistence and transitivity in the seasonal march

The lagged correlations between monthly Asian summer monsoon indices and El Nino-Southern Oscillation (ENSO) index change prominently in the middle of the summer season, based upon data from the late 1970's to late 1990's. Following this change in correlations, the traditional summer monsoon season (June-July-August-September) could be divided into two sub-periods in terms of the interannual variability. One is early summer (June), in which the variability of the Asian monsoon is strongly influenced by the anomalous state of ENSO in the previous winter. Another is mid-late summer (July-August-September), in which the Asian monsoon is related to the anomalous state of ENSO in the following winter rather than the previous winter. The precursory signals of the anomalous Asian summer monsoon which are associated with the anomalous state of ENSO in previous seasons, are valid only for the variability of monsoon in the early summer, but not for that in the whole summer season. Therefore, the drastic change of persistence of the ENSO/monsoon system occurs after the early summer, and a new anomalous state tends to start from the middle or fate summer. In this coupled system of ENSO and monsoon, the role of the western Pacific seems to be much different from the eastern Pacific. The anomalous state of sea surface temperature (SST) over the western Pacific warm pool area tends to persist from winter until the following late spring (May), and the related abnormal convective activity over that region can be maintained until the following early summer (June). Such kind of characteristics of persistence over the western Pacific is likely to have a memory effect of the anomalous ENSO state, and plays an active role in influencing the variability of the Asian monsoon in the following early summer.