A Contrast of the East Asian Summer Monsoon–ENSO Relationship between 1962–77 and 1978–93

Abstract

Using station rainfall data and the NCEP?NCAR reanalysis, the authors investigate changes in the interannual relationship between the east Asian summer monsoon (EASM) and El Niño?Southern Oscillation (ENSO) in the late 1970s, concurrent with the Pacific climate shift. The present study focuses on decaying phases of ENSO because changes in developing phases of ENSO are less significant. Remarkable changes are found in the summer rainfall anomaly in northern China and Japan. From pre- to postshift period, the summer rainfall anomaly in eastern north China during decaying phases of El Niño changed from above to below normal, whereas that in central Japan changed from negative to normal. Consistent with this, the barotropic anticyclonic anomaly over the Japan Sea changed to cyclonic; the associated anomalous winds changed from southerly to northerly over the Yellow Sea?northeastern China and from northeasterly to northwesterly over central Japan. The change in the ENSO?related east Asian summer circulation anomaly is attributed to changes in the location and intensity of anomalous convection over the western North Pacific (WNP) and India. After the late 1970s, the WNP convection anomaly is enhanced and shifted to higher latitudes due to increased summer mean SST in the Philippine Sea. This induces an eastward shift of an anomalous low pressure from east Asia to the North Pacific along 30°?45°N during decaying phases of El Niño. Thus, anomalous winds over northeastern China and Korea switch from southeasterly to northeasterly. Before the late 1970s, an anomalous barotropic anticyclone develops over east Asia and anomalous southerlies prevail over northeastern China during decaying phases of El Niño. This may relate to anomalous Indian convection through a zonal wave pattern along 30°?50°N. After the late 1970s, anomalous Indian convection weakens, which reduces the impact of the Indian convection on the EASM.