Seasonal ITCZ Control on ENSO Responses to Extratropical Volcanic Forcing

The response of El Nino 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 post-eruption ENSO evolution involves the forced inter-hemispheric 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 extra-tropical 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 extra-tropical 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. 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 five 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.