The dynamics of the El Niño–Southern Oscillation diversity in the recharge oscillator framework

Abstract

This study investigates the observed El Niño-Southern Oscillation (ENSO) dynamics for the eastern Pacific (EP) and central Pacific (CP) events in reference to the canonical ENSO (T). We use the recharge oscillator (ReOsc) model concept to describe the ENSO phase space and polar coordinate statistics, based on the interaction of sea surface temperature (T) and thermocline depth (h), for the different types of ENSO events. We further look at some important statistical characteristics, such as power spectrum and cross-correlation as essential parameters for understanding the dynamics of ENSO. The results show that the dynamics of the EP and CP events are very different from each other and from the canonical ENSO events. The canonical ENSO (T) events fit closest to the idealised ReOsc model and has the most clearly oscillating ENSO phase space, suggesting it is the most predictable ENSO index. The EP phase space evolution is similar to the canonical ENSO, but the phase transitions are less clear, suggesting less of an oscillatory nature and that the index is more focussed on extreme El Niño and discharge states. The CP phase space, in turn, does not have a clear propagation through all phases and are strongly skewed towards the La Niña state. The interaction between CP and h are much weaker, making the mode less predictable. Wind forced shallow water model simulations show that the CP winds spatial pattern do not force significant h tendencies, strongly reducing the delayed negative feedback, which is essential for the ENSO cycle.

ENSO phase space dynamics in CMIP models

“…For the ENSO phase space analysis, we are following the approach by Dommenget and Al-Ansari (2022), Dommenget et al (2023), and Priya et al (2024). We normalise the monthly means of T and h by dividing them by their standard deviations, resulting in a non-dimensional presentation of the variables, T n and h n , respectively.…”

“…The ReOsc model describes the variability of ENSO as an interaction between the tendencies of SST anomaly in the eastern equatorial Paci c (T) and equatorial mean thermocline depth anomaly (h), with the latter representing a proxy for the subsurface equatorial heat content. It has been used extensively in previous studies to analyse ENSO dynamics (Jin 1997 The idealised ReOsc model is an oscillation between T and h, presenting a 2-dimensional phase space, as known from physics for a harmonic oscillator or complex system analysis (Meinen and McPhaden 2000;Kessler 2002; Dommenget and Al-Ansari 2022; Dommenget et al 2023;Priya et al 2024). Detailed analysis of the characteristics of the ENSO phase can help to understand how the dynamics of ENSO control the amplitude, non-linear behaviour, and time evolution as functions of the ENSO phases.…”