The Relative Role of Indian and Pacific Tropical Heating as Seasonal Predictability Drivers for the North Atlantic Oscillation.

Understanding the predictability drivers for the North Atlantic Oscillation (NAO) during boreal winter at seasonal time scales remains challenging. This study uses large ensembles with the ECMWF seasonal forecasting system to investigate the relative impact of tropical Indian and Pacific heating on NAO predictability by examining the tropical forcing, teleconnection pathways, and sources of uncertainty. We select three case studies ‐ 1997/98, 2015/16 and 2019/20 ‐ with strong Indian Ocean heating anomalies, but with different El Niño conditions. We show that in 2019/20, with neutral ENSO conditions, Indian Ocean SSTs favor a positive NAO response via stratospheric and tropospheric pathways. In the cases with strong El Niño, we find contrasting results: in 1997/98, the Pacific forcing dominates, producing a negative NAO. In 2015/16, despite the strong El Niño, the Indian Ocean forcing dominates, leading to a positive NAO via intensification of the stratospheric polar vortex (SPV). While the stratospheric pathway exhibits varying responses to Indian Ocean forcing ‐ being weaker in 1997/98 and strongest in 2015/16, the Indian Ocean‐related tropospheric pathway remains robust along the Pacific subtropical jet across years. However, there is destructive interference between teleconnections from Indian and Pacific SST anomalies in both the tropospheric and stratospheric pathways. The competing effects of tropical heating in both basins, uncertainties in the Rossby wave response to tropical heating and SPV variability contribute to uncertainty in seasonal NAO predictions. The flow‐dependent nature of the stratospheric pathway underscores the complexity of seasonal forecast predictability, and the existence of windows of opportunity. Plain Language Summary: The NAO is a phenomenon that manifests as a pressure see‐saw in the North Atlantic, affecting the seasonal climate in many parts of the northern hemisphere. In this study, we demonstrate that the predictability of the phases of the NAO can be affected by the remote and opposing influence of surface temperatures in the tropical Indian and Pacific oceans, which propagate via the troposphere and stratosphere. This interplay, in turn, contributes to the uncertainty of seasonal forecasts of the NAO. Key Points: Large ensemble seasonal forecasts reveal a predictable link between tropical heating and the NAO via a flow‐dependent stratospheric pathwayThe impact of Indian Ocean heating on the NAO can be stronger than that of El Niño in the presence of an anomalously strong stratospheric pathwayCompetition between basins, flow‐dependent response, and intrinsic variability contribute to the uncertainty in seasonal forecasts of NAO [ABSTRACT FROM AUTHOR]