Change of El Niño and La Niña amplitude asymmetry around 1980

Amplitude of El Niño and La Niña was significantly different during 1980–2016 but almost same during 1958–1979. The cause of this interdecadal change is investigated through an oceanic mixed-layer heat budget analysis. It was found that this interdecadal change was primarily attributed to the distinctive effects of nonlinear zonal temperature advection between the two periods. During 1980–2016 nonlinear zonal advection, working together with nonlinear meridional advection, contributes to the El Niño and La Niña amplitude asymmetry. During 1958–1979 the nonlinear zonal advection had an opposite effect. The difference in the nonlinear zonal advection between the two interdecadal periods was caused by distinctive longitudinal locations of El Niño centers. Maximum SST anomaly (SSTA) centers were confined near the coast of South America (east of 90° W) during the first period but appear near 110° W during the second period. Because of this difference, an anomalous eastward ocean surface current (caused by a positive thermocline depth anomaly during El Niño) would generate a negative (positive) nonlinear zonal advection before (after) 1980. The distinctive longitudinal locations of El Niño centers are possibly caused by the interdecadal changes of mean thermocline and high-frequency wind variability over the equatorial western-central Pacific. A hypothesis was put forth to understand distinctive initiation locations between El Niño and La Niña.