El Niño intensifies and shifts winter and spring storms

The frequency and intensity of winter and spring storms in North America are increasing. Climate change is the likely culprit, but the interannual climate disturbance known as El Niño might also be contributing to the upward trend. To see if that’s the case,Xiangdong Zhang of the University of Alaska Fairbanks and his colleagues conducted a series of simulations using the National Center for Atmospheric Research’s community atmosphere model . Because Earth’s weather is so sensitive to initial conditions, Zhang and his team ran 120 simulations in all, each beginning a little differently. Half included the elevated sea-surface temperatures off Ecuador that characterize El Niño. The other half didn’t. After seven months of number crunching, the results are in. El Niño intensifies winter–spring storms and boosts their frequency in the northwestern, southeastern, and southwestern quadrants of North America. But in the northeastern quadrant, El Niño’s main effect was to weaken winter storms. From the various parameters generated by the model, Zhang and his team identified the cause of that regional pattern. The largest nursery of storms was a concentration of angular momentum that lay above the Aleutian Islands in the non–El Niño runs but moved south in the Niño ones. El Niños are not caused by climate change. But if, as observations and simulations suggest, climate change is making El Niños stronger and more frequent, most of Canada and the US could be in for worse winter weather. (S. Basu et al., Geophys. Res. Lett., in press, doi:10.1002/grl.50990. )—Charles Day