The Arctic Multidecadal Oscillation (AMO), also known as the Arctic Multidecadal Variability (AMV), is a reasonably periodic variation in the sea surface temperature (SST) of the North Atlantic Ocean. Year-to-year variability is great, but a rolling average of SSTs shows a clear, if not smooth, pattern. The two names, ending in oscillation and variation, highlight the debate over its nature: is the change from cold to warm modes regular enough to be called an oscillation, or is it too erratic to merit comparison to a wave?
The AMO index is calculated from long-term surface temperature records, once any linear trend (overall ocean warming) has been removed. The goal is to identify shorter-term variation while minimizing long-term forcing. Temperature records from the last 150 years show a roughly 70-year period, but the historical record is too short to extrapolate far. Nor have any mechanisms been clearly identified.
The AMO appears to be correlated to air temperatures and weather patterns across the northern hemisphere. The severe droughts and Dust Bowl of the American midwest in the 1930's, and the droughts of the 1950's, occurred during one of the AMO's warm phases. The AMO also seems to contribute to the development of severe hurricanes from tropical cyclones. It is also correlated with increased rain in the Sahel (the arid land between the Sahara to the north and the Sudanese savanna to the south) and India. Even so, the link between the AMO and North Atlantic hurricane activity is disputed by some climate scientists. There is as yet no consensus.
A 2021 study of the last millennium by climate science magnate Michael Mann identifies volcanoes as the main driver of the AMO. In other words, the apparently regular variation over the last 150 years is in fact “red noise”, that is, mostly random noise which favors lower frequencies (such as red in the visible spectrum), as opposed to “white noise”, where frequencies are spread evenly across the full spectrum. But suffice it to say that the Atlantic Multidecadal Oscillation is an area of active research.
In that way, it's an excellent example of the scientific process. A scientist sees a record—perhaps by reinterpreting existing data—and finds a new pattern. She or he lights on a hypothesis, tests it, and publishes results and conclusions. Colleagues read the article, or listen to the talk. Some might agree. Others think, “No way, you're wrong, and I'll prove it,” and set out to test and destroy it. Though it lacks the obvious bravado of a football field or ice rink, scientific research can be a pretty vicious arena. (One reason why I'm in applied science!)
Tomorrow: the Indian Ocean Dipole (IOD).
Be brave, and be well.
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