Interdecadal changes in the nonlinearity of ENSO

The El Niño events (warm) are often stronger than La Niña events (cold). This asymmetry was found as the result of the strong nonlinear dynamic warming in the equatorial upper ocean through the ocean-atmosphere interaction, which has been increased since the late 1970s. In addition, the nonlinear principal component of the thermocline anomalies in the tropical Pacific showed that the evolution of ENSO cycle changed since the late 1970s; for the pre-1980s, the ENSO cycle associated with the thermocline is less asymmetrical then that during the post-1980s so that the nonlinearity of the ENSO cycle has become stronger since the late 1970s.