Sea surface temperatures in the North Atlantic tend to move through warm periods followed by cold periods, in a 60–70 year cycle known as the AMO. The ocean entered a warm – or positive – phase in the late 1990s.

Yannick Peings and Gudrun Magnusdottir of the University of California Irvine, US, believe that these warmer ocean temperatures can nudge the atmospheric circulation pattern known as the North Atlantic Oscillation into its negative phase. This is likely to cause colder-than-normal temperatures in the northern hemisphere in winter as it moves the North Atlantic jet stream and storm track south.

"Our study contributes to the growing debate about the link between Arctic sea ice loss and mid-latitude weather extremes," Yannick Peings of the University of California Irvine, US, told environmentalresearchweb. "We find that the occurrence of the extreme events that has been attributed to Arctic warming only is actually promoted by the positive polarity of the AMO, especially over Europe and the eastern US. This suggests that the resurgence of cold winters over these regions is consistent with natural climate variability and is not simply a result of global change."

The team reckons that the warmer sea surface temperatures associated with a positive phase of the AMO affect the NAO by shifting the atmospheric baroclinic zone over the North Atlantic basin. (The AMO may also alter atmospheric circulation indirectly by causing sea ice loss.) The warmer ocean takes 10–15 years to boot the North Atlantic Oscillation into its negative phase; it’s not yet clear why.

The findings have important implications for the decadal prediction of climate, according to Peing. "The AMO is an important source of predictability for winter climate of the North Atlantic sector," he said. "For example, provided that the AMO remains in its positive polarity for one to two additional decades, cold and snowy weather conditions in winter should be favoured over Europe and the eastern US. Of course, many other parameters of the climate system may modulate the impact of the AMO on North Atlantic variability, but the AMO remains a non-negligible source of variability that one must consider."

During the AMO’s positive phase, the sea surface off the coast of Newfoundland can be up to 1.5 °C warmer, Peings and Magnusdottir found.

"We show that the AMO significantly modulates the frequency of occurrence of the four typical weather regimes of the wintertime atmospheric circulation over the North Atlantic sector," said Peings. "This result is found over the 1901–2010 period in observations – using the 20th century reanalysis, 20CR – and is supported by modeling experiments with the latest version of the Community Atmospheric Model (CAM5)."

Two of these weather regimes – the negative NAO regime and the blocking regime – are more frequent when the AMO is positive, the team discovered, increasing the likelihood of cold extremes over Europe and the east coast of North America. "It is plausible that the AMO plays a role in the recent resurgence of severe winter weather in these regions and that wintertime cold episodes will be promoted as long as the AMO remains positive," write the scientists in Environmental Research Letters (ERL).

The AMO shows strong variability from year to year. Peings says the winter of 2013/2014, for example, "had rather low values of the AMO index and the pattern of sea surface temperature anomalies was not consistent with the typical positive AMO pattern". What’s more, the NAO was mostly positive, with Europe seeing a relatively mild winter.

Now the team is further investigating the relative roles of the AMO and Arctic sea ice loss on North Atlantic variability. "We are performing a case study of the 2012/2013 winter that was characterized by a strong negative NAO pattern, as well as anomalously low sea ice extent the previous fall in the Arctic and a positive AMO-like SST pattern in the North Atlantic," said Peings. "We also plan to investigate the role of the Siberian snow cover, which was anomalously high in fall 2012 and is known to impact the stratospheric polar vortex and the NAO." Additional experiments using a slab-ocean model in the North Atlantic will assess ocean-atmosphere feedbacks, while use of the WACCM high-top atmospheric model will determine the contribution of the stratosphere to the results.

Related links

• Forcing of the wintertime atmospheric circulation by the multidecadal fluctuations of the North Atlantic ocean Yannick Peings and Gudrun Magnusdottir 2014 Environ. Res. Lett. 9 034018
• Yannick Peings, University of California, Irvine
• Gudrun Magnusdottir, University of California, Irvine

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