Weather forecasting: El Niño dons winter disguise as La Niña.

Seasonal weather forecasting relies almost exclusively on El Niño, the climate phenomenon associated with warming in the equatorial Pacific Ocean. When the strongest El Niño on record developed last autumn, it offered an opportunity to showcase decades of investment and advances in long-range forecasting (see Nature 529, 267–268; 2016). Surprisingly, however, actual winter weather events were the opposite of those predicted. For example, southern California's winter was more about heatwaves and wildfires than deluges; Seattle in Washington endured the wettest winter on record rather than a worsening drought; and the upper Mississippi valley experienced flooding to an extent that had previously occurred only in summer. El Niño should strengthen the side of the jet stream that is nearer to the Equator, bringing wet weather to the southwest United States and cool temperatures to the southeast. Instead, the side nearest to the pole strengthened. This brought weather that would be more expected of La Niña, the opposite phase of the Southern Oscillation that results from cooling waters. I suggest considering this inaccurate El Niño forecast in the wider context of the Arctic's influence. Low Arctic sea ice and high Eurasian snow cover this autumn increased a Siberian high-pressure system and heat transport towards the North Pole, weakening the polar vortex this winter (see J. Cohen et al. Nature Geosci. 7, 627–637; 2014). The atmospheric responses to Arctic 'amplification' were better predicted than were those to El Niño outside the tropics (see www.aer.com/winter2016).