Arctic winter warming events - winter days when temperatures peak above minus 10 degrees Celsius - are a normal part of the Arctic climate over the ice-covered Arctic Ocean, but new research finds they are becoming more frequent and lasting longer than they did three decades ago.
A new study analyzing winter air temperatures over the Arctic Ocean from 1893 to 2017 shows that since 1980, an additional six Arctic winter warming events are occurring each winter at the North Pole and these events are lasting about 12 hours longer, on average. In December 2015, scientists recorded a temperature of 2.2 degrees Celsius (36 degrees Fahrenheit) in the Central Arctic, the warmest temperature ever recorded in this region from December through March.
Winter warming events have been observed by scientific excursions in the Arctic as early as the Fram expedition in 1896, when Norwegian explorers froze their ship into the ice in an attempt to reach the North Pole. But the prevalence and length of these events has broken records in recent years, according to authors of the new study in Geophysical Research Letters, a journal of the American Geophysical Union.
The new findings build on other evidence of Arctic warming. The average surface air temperature for the year ending September 2016 was the highest since 1900, and new monthly record highs were recorded for January, February, October and November 2016. Minimum sea ice extent at the end of summer 2016 tied with 2007 for the second lowest in the satellite record, which started in 1979, according to the National Oceanic and Atmospheric Administration.
Because fall and winter is when Arctic sea ice grows and thickens, warmer winter air temperatures may further impede ice growth and expansion, accelerating the effects of global warming in the Arctic, according to the study's authors.
"These (winter warming) events are not unusual, but they are happening more frequently and with longer durations," said Robert Graham, a climate scientist at the Norwegian Polar Institute in Tromsø, Norway, and lead author of the new study.
The study attributes the increase in warming events to an increase in major storms in the Arctic. In the most recent years of the study, each warming event was associated with a major storm entering the region. During these storms, strong winds from the south blow warm, moist air from the Atlantic into the Arctic, Graham said.
"The warming events and storms are in effect one and the same," he said. "The more storms we have, the more warming events, the more days with temperatures greater than minus 10 degrees Celsius rather than below minus 30 degrees Celsius, and the warmer the mean winter temperature is."
The new study does not address what is causing the increase in major storms in the Arctic, but recent research shows that reduced ice cover and shifting weather patterns due to climate change may increase storms' frequency and impact, Graham said.
Natural variability can cause regional trends in Arctic warming, but the new study suggests human-caused climate change could be driving the increase in warm temperatures, according to Julienne Stroeve, Professor of Polar Observation and Modelling at University College London, who was an editor of the new paper.
Measuring Arctic temperatures
After the United Kingdom's winter storm Frank caused the North Pole to reach record temperatures in late December 2015 - passing above freezing for several hours - Graham and his colleagues realized they had observed similar events the previous winter during the N-ICE2015 field campaign. During N-ICE2015, scientists aboard a Norwegian research vessel froze their boat into the Arctic sea ice and gathered data from January to June of 2015. Graham wondered how common it was to observe these warm temperatures in the Arctic during the winter.
"In particular, we wanted to look as far back in time as possible, and preferably using field data rather than climate models," Graham said.
Graham and his colleagues gathered data from field campaigns, drifting weather stations and buoys across the Arctic Ocean from 1893-2017 and analyzed the ERA-Interim record, a global atmospheric reanalysis provided by The European Centre for Medium-Range Weather Forecasts (ECMWF), from 1979-2016.
The researchers found Arctic winter warming events have been documented as far back as 1896, but these events have become more numerous and reached higher peak temperatures in recent decades. Since 1980, the number of winter warming events in the North Pole each year has more than doubled, from fewer than five events to more than 10 events, on average, and the average length of each event has grown from fewer than two days to nearly two and a half days. The total duration of winter warming events has increased from around 7 days per year to 21 days per year, on average, according to Graham.
The Atlantic side of the North Pole now has 10 warming events each winter, on average, while the Pacific Central Arctic has five such events, on average, according to the study. More storms come in to the Arctic from the Atlantic Ocean during winter, which results in more warming events on the Atlantic side of the North Pole.
Two of the study's authors, Alek Petty and Linette Boisvert of NASA's Goddard Space Flight Center in Greenbelt, Maryland, previously researched one such storm that took place in the Arctic during the winter of 2015-2016.
"That particular cyclone, which lasted several days and raised temperatures in the region close to the melting point, hindered sea ice growth while its associated strong winds pushed the sea ice edge back, leading to a record low spring sea ice pack in 2016," said Petty and Boisvert. "This new study provides the long-term context we were missing, using direct observations going back the end of the 19th century. It shows that these warm events have occurred in the past, but they were maybe not as long-lasting or frequent as we're seeing now. That, combined with the weakened sea ice pack, means that winter storms in the Arctic are having a larger impact on the Arctic climate system."
The recent increase in the frequency and duration of Arctic winter warming events could negatively affect sea ice coverage and development, according to the study's authors. The storms that bring warm air to the Arctic not only prevent new ice from forming, but can also break up ice cover that is already present, according to Graham. Snowfall from the storms also insulates ice from the cold atmosphere that returns after the storms, which can further reduce ice growth, Graham said.
The next step for Graham and his colleagues is to understand what is fueling the increase of these storms and how they might change.
"It is difficult to say how much this pattern will amplify in the future," he said.
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Robert M. Graham et al, Increasing frequency and duration of Arctic winter warming events, Geophysical Research Letters (2017). DOI: 10.1002/2017GL073395