Jim Hurrell and the North Atlantic Oscillation
By the 1990s, El Nino was a well-studied phenomenon, but less was known about its Atlantic cousin. Jim Hurrell, then a junior NCAR scientist, analyzed decadal trends in the North Atlantic Oscillation or NAO, a fluctuation in atmospheric pressure between Iceland and the Azores. He discovered that when the NAO is positive (a large difference in pressure between the two points), a persistent westerly flow tends to bring winter warmth to Europe and the northeast United States, while northern Canada and Greenland cool to below normal. A negative NAO brings the reverse weather pattern, with cold winters more likely in Europe and the U.S. Northeast and warm ones in the Canadian Arctic. While the dynamics of the NAO are well understood, it has proved fiendishly difficult to predict.