Seasonal ITCZ Control on ENSO Responses to Extratropical Volcanic Forcing

Abstract The response of El Niño–Southern Oscillation (ENSO) to a strong volcanic eruption is controlled by several mechanisms, including the spatial structure of the volcanic forcing. In the case of extratropical eruptions, a mechanism contributing to posteruption ENSO evolution involves the forced interhemispheric energy imbalance and the associated displacement of the intertropical convergence zone (ITCZ). In this study, using 100 members of diverse (in locations and magnitude) idealized volcanic forcing ensembles from the Max Planck Institute Earth System Model (MPI-ESM1-2-LR), we find that large (≥10-Tg sulfur) north and south extratropical eruptions lead to strong El Niño–like and weak La Niña–like responses in the second winter following the eruption, respectively. We demonstrate that the shifts of the ITCZ are the main drivers of the ENSO response. The weaker ENSO response for the south extratropical eruptions is due to contrasting westerly anomalies in the eastern Pacific from multiple local responses and easterly anomalies in the central Pacific directly from ITCZ displacement along the equator, thereby reducing the overall response. Furthermore, when separating the 100 members based on initial ENSO states, distinct ENSO responses are found, and significant differences in ENSO tendencies can be observed even 5 years after eruptions. This study stresses the importance of seasonal differences in ITCZ displacement when studying volcanic forcing and contributes to explain the inconsistency of ENSO responses to past volcanic events. Significance Statement In this study, we investigate whether different latitudinal locations of volcanic eruptions can result in distinct El Niño–Southern Oscillation (ENSO) responses by using large ensemble simulations. We find that the locations of the eruption and its emission strength can all lead to distinct ENSO-like responses, highlighting the importance of the changes in the position of the intertropical convergence zone in driving such responses. Our study enhances the dynamical understanding of ENSO-like responses to volcanic forcing, which may lead to better future predictions on volcanic impacts, better past volcanic reconstructions through climate records, and a new perspective on studying ENSO dynamics through interhemispheric energy transport.