A Two-Year Forecast for a 60–80% Chance of La Niña in 2017–18

Historical observations show that one in two La Niña events last for two consecutive years. Despite their outsized impacts on drought, these 2-year La Niña are not predicted on a routine basis. Here, we assess the predictability of 2-year La Niña using retrospective forecasts performed with a climate model that simulates realistic multi-year events, as well as with a statistical model based on observations. The skill of the retrospective forecasts allows us to make predictions for the upcoming 2017-2018 boreal winter starting from conditions in November 2015. These two-year forecasts indicate that the return of La Niña is more likely than not, with a 60% probability based on the climate model and an 80% probability based on the statistical model; the likelihood of El Niño is less than 8% in both cases. Retrospective verification of forecasts for historical 2-year events demonstrate the feasibility of long-range predictions of the duration of La Niña. These forecasts will be verified by the time of the conference.