Assessing the chance of unprecedented dry conditions over North Brazil during El Nino events

The strongest El Nino events of the past four decades were associated with large rainfall deficits in North Brazil during the December to February mature phase, leading to substantial societal and ecological impacts and influencing the global carbon cycle. While the teleconnection between El Nino and northern South America is well studied, the small number of El Nino events-and especially high magnitude 'major' El Ninos-in the recent observational record make a robust characterisation of the response over North Brazil in today's climate difficult. Here we use a large, initialised ensemble of global climate simulations to provide a much greater sample of North Brazil rainfall responses to recent El Nino events than is available from observations, and use this to form an assessment of the chance of unprecedented dry conditions during El Nino. We find that record low rainfall totals are possible during El Nino events in the current climate, and that as the magnitude of El Nino increases, so too does the chance of unprecedented low rainfall, reaching close to 60% for major El Ninos. However, during even the largest El Ninos, when the observed North Brazil response has been similar and very dry, we find rainfall rates close to normal are still possible due to internal atmospheric variability. In addition to the predictable influence of the tropical Pacific, an unpredictable influence from the extratropics appears to play a role in modulating the North Brazil rainfall response via an equatorward wave-train that propagates down the western coast of North America and across to the Caribbean. Combining forecasts of El Nino with this improved information on the underlying chance of extremely low rainfall could feed into improved assessments of risk and preparedness for upcoming droughts in Brazil.