In the atmosphere above the North Atlantic, a large zone of high pressure near the Azores called the Azores High and a zone of low pressure near Iceland and southern Greenland called the Icelandic Low help to funnel winds and fuel a storm track. This storm track has profound effects on weather patterns across the Northern Hemisphere.
These effects intensify and weaken as the storm track gains and loses strength every 25–30 years or so, a wobbling called the North Atlantic Oscillation (NAO). A strong NAO may dole out harsher winters to Canada and milder ones to northern Europe, for example, as well as make the Middle East even drier.
Understanding what drives such oscillations—not just in the NAO but in similar systems across the globe—is crucial if scientists are to improve predictions of future climate. Does an atmospheric jet weaken because of changes in ocean current or temperature, for example, or does it work the other way around, with the ocean responding to changes in the atmosphere? Or is it some of both? In a new study, Vannitsem and Ghil present a new mathematical tool for determining what drives the system: ocean, atmosphere, or the two combined.
The team used the NAO as a test case. They compared data sets from several previous studies with earlier theoretical models of the NAO to tease out the phenomenon’s most important drivers.
They found that the oscillations were driven by the confluence of ocean and atmospheric factors: the ocean temperatures, atmospheric pressures, and ocean currents. The effects were particularly strong during winter, implying that the effects of this genuine coupling between oceanic and atmospheric processes vary with the season, the authors say. (Geophysical Research Letters, https://doi.org/10.1002/2016GL072229, 2017)
—Emily Underwood, Freelance Writer