Cold Winter, Mild Winter

A new study recently published in “Environmental Research Letters” shows that the extremely cold winter just experienced across the central and eastern U.S. is likely due to a natural variation is Atlantic sea surface temperature known as the Atlantic Multidecadal Oscillation (AMO). The positive phase of the AMO (a phase of warmer Atlantic temperature) when combined with the negative phase of another circulation pattern, the North Atlantic Oscillation (NAO), a stronger than usual semi-permanent low pressure system near Iceland, results in a much greater frequency of arctic air intrusions into the central and Europe. The AMO changes over decades whereas the NAO changes over periods of weeks or months. The study found that the present positive phase of the AMO when combined with the positive phase of the NAO is likelier than average to yield a mild winter, which could explain our recent run of winters being either extremely cold or extremely mild. This suggests we should expect the winter variability to continue for another decade or two.         Meteorologist John Wheeler

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