The winter of 2013 to 2014 was memorable for the extreme chill it produced across North America, with record low temperatures extending from January right through to March. In early January temperatures plummeted as far as –38°C. The cold air even penetrated to the far south, cooling the usually scorching Houston to –6°C. Some have suggested that the unusual atmospheric circulation patterns associated with extreme cold winters like this could be related to sea surface temperatures in the North Pacific and sea ice coverage in the Bering Sea.

Takuya Nakanowatari from the National Institute of Polar Research in Japan and his colleagues investigated this possible link using atmosphere and ocean data collected between 1980 and 2014. Using canonical correlation analysis, they compared the height of the 500 hPa contour in the atmosphere in late summer (August to September) with sea ice area over the Bering Sea in early winter (November to December).

In this case the height of the 500 hPa contour is part of a wider atmospheric phenomenon known as the Pacific Transition Pattern. When atmospheric convection is weaker over the western subtropical North Pacific, the height of this contour is higher over the Gulf of Alaska. And this set up often leads to less ice over the Bering Sea.

Nakanowatari and colleagues found that when the 500 hPa contour was higher than average over the Gulf of Alaska, there was a greater chance of a diminished sea ice area over the Bering Sea later in the year. The height of this contour could explain 29% of the sea ice variability.

"The analysis of ocean temperature and current data demonstrates that the late summer 500 hPa contour influences the sea surface temperature in the Gulf of Alaska, and the resultant thermal signal is advected to the Bering Sea," explained Nakanowatari. The delayed reaction occurs because it takes two to four months for the Alaska Coastal Current to travel from the Gulf of Alaska to the Bering Sea.

The scientists also found a strong correlation between Bering Sea ice area in early winter and surface temperatures in eastern North America one month later (December to January), with nearly 40% of the air temperature change explained by the sea ice area change. They published their findings in Environmental Research Letters (ERL).

Ultimately understanding these longer term linkages in climate could help improve long-term weather forecasts. "If the sea ice area in the Bering Sea is skilfully predicted at the lead-in time of three months, then the forecast skills of predicting surface air temperature in North America in December to January is estimated to be improved by 38%," said Nakanowatari.

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