A Coupled Dynamic Index for ENSO Periodicity

The El Niño/Southern Oscillation (ENSO) is the most active interannual climatic mode with great global impacts. The state of art climate models can simulate this dominant mode variability to a large extend. Nevertheless, some of its fundamental time-space characteristics remain to have large spread in the simulations across the array of recent climate models. For example, the large biases of the ENSO periodicity still exist among the Coupled Model Intercomparison Project Phase 5 (CMIP5) model simulations. Based on the Recharge Oscillator framework, a coupled dynamic index for the ENSO periodicity is proposed in this study, which is referred to as Wyrtki Index in parallel to the Bjerknes Index for the ENSO instability. This Wyrtki Index provides an approximate dynamic measure for ENSO linear periodicity. It mainly has two contribution terms from the thermocline and zonal advective feedbacks (Ffactor) multiplied by the efficiency factor (Bfactor) of discharging/recharging of the equatorial heat content driven by ENSO wind stress anomalies. It is demonstrated that the diversity of the simulated ENSO periodicity in CMIP5 models results from the biases in mean state and several key parameters that controls ENSO dynamics. Larger Ffactor would result in shorter ENSO period (e. g. BCC_CSM1.1m); while smaller Bfactor would lead to longer ENSO period (e. g. HadGEM2_ES). The Wyrtki Index serves as a useful tool for a quantitative assessment of the sources for the ENSO periodicity in reanalysis data and its biases in CMIP5 model simulations.