ENSO phase space dynamics with an improved estimate of the thermocline depth

The recharge oscillator model of the El Niño Southern Oscillation (ENSO) describes the ENSO dynamics as an interaction and oscillation between the eastern tropical Pacific sea surface temperatures (T) and subsurface heat content (thermocline depth; h), describing a cycle of ENSO phases. h is often approximated on the basis of the depth of the 20^oC isotherm (Z₂₀). In this study we will address how the estimation of h affects the representation of ENSO dynamics. We will compare the ENSO phase space with h estimated based on Z₂₀ and based on the maximum gradient in the temperature profile (Z_mxg). The results illustrate that the ENSO phase space is much closer to the idealised recharge oscillator model if based on Z_mxg than if based on Z₂₀. Using linear and non-linear recharge oscillator models fitted to the observed data illustrates that the Z₂₀ estimate leads to artificial phase dependent structures in the ENSO phase space, which result from an in-phase correlation between h and T. Based on the Z_mxg estimate the ENSO phase space diagram show very clear non-linear aspects in growth rates and phase speeds. Based on this estimate we can describe the ENSO cycle dynamics as a non-linear cycle that grows during the recharge and El Nino state, and decays during the La Nina states. The most extreme ENSO states are during the El Nino and discharge states, while the La Nina and recharge states do not have extreme states. It further has faster phase speeds after the El Nino state and slower phase speeds during and after the La Nina states. The analysis suggests that the ENSO phase speed is significantly positive in all phases, suggesting that ENSO is indeed a cycle. However, the phase speeds are closest to zero during and after the La Nina state, indicating that the ENSO cycle is most likely to stall in these states.