Major fires in Indonesian Borneo are possible under all ENSO phases.

Major fires in Indonesian Borneo over the past few decades caused devastating impacts on human health, livelihoods, economy, the natural habitat and the carbon sink. Despite the proven link between the positive phase of the El Niño Southern Oscillation (ENSO) and rainfall deficit, a primary driver of fires, the rare occurrence of historical major fires makes it difficult to robustly assess their climatic cause using purely the observational record. We analysed a large initialised ensemble of global climate simulations to demonstrate that although the risk of severe fires that level or surpass those observed in 1997 and 2015 is strongly boosted by an El Niño by a factor of 2.7, such risk exists regardless of the ENSO phase. Other factors modulating the risk include the Indian Ocean Dipole (IOD) and a Rossby wave train over the southern extratropics which acts to decouple the rainfall response from the ENSO state. Our study highlights that even in places characterised by high seasonal predictability, there is still a possibility of high impact weather events to occur against the expected response to a given teleconnection mode, which implies the necessity to build long-term resilience against the associated hazards on top of improving our sub-seasonal forecasting capability.