Mechanism on inverse relationship between the annual-cycle's and ENSO's amplitudes

The dynamical interpretation on the inverse relationship between tropical eastern Pacific annual cycle amplitude and the El Niño-Southern Oscillation (ENSO) variability are proposed based on the pre-industrial experiment of GFDL CM2.0 spanning about 500 years. We found that the slowly varying climate states having a decadal time scale alternatively provide a favorable condition for two opposite regimes, namely ‘strong annual cycle - weak ENSO regime' and ‘weak annual cycle – strong ENSO regime'. Overall surface warming in the tropical eastern Pacific with relatively large warming in the southern hemisphere and associating deep thermocline and mixed layer in the tropical eastern Pacific is concurred with the weak annual cycle – strong ENSO regime, and the opposite case is also true for the strong annual cycle – wean ENSO regime. For example, deeper mixed layer reduces the annual cycle amplitude by decreasing surface heating efficiency in changing the mixed layer temperature, while intensifies ENSO amplitude by decreasing damping rate.