Evaluation of a Southern Hemispheric ENSO Transition Mode in CMIP6 Models: Insights and Implications

Abstract

The ENSO Transition Mode (ETM) is a distinct driver of ENSO multidecadal climate variability. The ETM plays an important role in influencing ENSO's seasonal transition from boreal winter to the following summer by affecting zonal winds in the central-eastern equatorial Pacific Ocean during boreal spring. In this study, we show that ETM's spatial pattern and seasonality are robustly captured by the CMIP6 models' historical simulations. However, they struggle to capture its multidecadal variability. Nonetheless, these models effectively depict ETM's influence on equatorial winds, thereby affecting the seasonal transition of ENSO from boreal winter to the following summer (ΔT). We further demonstrate that the models that more accurately represent the relationship between ETM and ΔT (referred to as “Good ETM models”) are those in which ΔT is less influenced by the preceding winter Niño3.4 sea-surface temperature anomalies (N34DJF) and largely influenced by phases of ETM. Additionally, these models better capture the 2–7-year spectral peak observed in N34DJF. On the contrary, Bad ETM models are characterized by a dominant higher-frequency, 2–3-year quasi-biennial peak. Consequently, Good ETM models render ENSO states that align more closely with observations compared with Bad ETM models. This study underscores, through CMIP6 models, the significance of boreal winter ENSO amplitude in influencing how extratropical climate affects the seasonal transitions of ENSO.