Interdecadal Change in the Relationship between ENSO and the Intraseasonal Oscillation in East Asia

The northward-propagating intraseasonal oscillation (NPISO) during the boreal summer is closely linked to the onset/retreat and intensity of the East Asian summer monsoon (EASM). In this study, interdecadal variability in the relationships between the NPISO and El Nino-Southern Oscillation (ENSO) was investigated using long-term outgoing longwave radiation data obtained from the 40-yr ECMWF Re-Analysis (ERA-40) for a 44-yr period (1958 to 2001). It was found that before the late 1970s, the preceding winter ENSO influenced the early summer (i.e., May to June) NPISO activity, whereas after the late 1970s a strong relationship appeared during the later summertime (i.e., July to August). The May-June NPISO before the late 1970s was modulated by springtime Indian Ocean sea surface temperature warming and central North Pacific suppressed convection anomalies and was consequently related to the ENSO-induced west Pacific (WP) pattern, which shows a north-south dipole structure over the North Pacific from winter through spring. After the late 1970s, because of an anomalously strengthened Walker-Hadley circulation, Indian Ocean SST warming was significantly maintained until summer, which promoted a strong suppressed convection anomaly over the Philippine Sea during summer and consequently an enhanced western North Pacific subtropical high and Pacific-Japan (PJ) pattern.