The interdecadal changes in the spatial structure of the South Pacific Oscillation and its implication for ENSO diversity

Abstract The South Pacific Oscillation (SPO), characterized by a north-south dipole-like pattern of sea level pressure anomalies, is one of the key factors in understanding tropical-extratropical interactions in South Pacific. We show that in boreal summer (June–July–August), the center of the northern lobe sea level pressure anomalies in the SPO is shifted to the east gradually after the 1960–70s. This study focuses on the relationship between the boreal summer SPO and following winter El Niño–Southern Oscillation (ENSO) diversity before and after the eastward shift of the SPO’s subtropical lobe. The eastward shift of the SPO’s subtropical lobe altered both the seasonal footprint mechanism and the trade wind charging mechanism associated with the SPO, thus profoundly influenced the ENSO diversity. It is revealed that when the northern lobe of the SPO shifts to the west of its average location, it tends to strengthen the EP El Niño mainly via the seasonal footprint mechanism. but after the SPO’s northern lobe shifts to the east of its average location, it tends to promote the development of CP El Niño mainly via the trade wind charging mechanism. The changes in the spatial structure of convection over the tropical Pacific and Indian Ocean may be one of the possible causes for the eastward shift in the SPO’s northern lobe. The findings in the present study have implications for a better understanding of ENSO diversity.