Waves grew bigger and spaced farther apart as ice cover diminished in the Arctic and sub-Arctic waters off Alaska and western Canada, new research shows.
Since the 1970s, the biggest waves in the Beaufort, Chukchi and Bering Seas have grown at a rate of 0.3 to 0.8 percent per year, according to a comprehensive study led by Environment Canada. The time it takes waves to cycle, a measurement known as period, has grown even more, by 3 to 4 percent per year, more than tripling since 1970, according to the study, published by the American Meteorological Society's Journal of Climate.
The study tracks significant wave height, which is the height of the biggest third of the waves, and mean wave period, which is the average of time for wave crests and troughs to complete their cycles.
It uses a wide variety of weather and climate observations collected over the years, including recorded sea-surface temperatures, atmospheric conditions, wind speeds and other parameters, to calculate the wave patterns going back over the past decades.
While wind patterns have changed over the period of time covered in the study, wind changes are not responsible for most of the increase in wave action, the study found. Instead, swells -- the rolling mechanical waves that travel long distances over the ocean -- account for most of the wave changes researchers measured, said lead author Xiaolan Wang, a research scientist with Environment Canada.
The study does not conclude that the expanding open water caused the bigger waves. "It's not my area of research," Wang said.
Still, it points out the correlation, and includes maps showing the expansion of open-water area in the three seas over the decades.
"Waves cannot be generated in ice-covered conditions," she said.
That means that waves are present only seasonally, as ice covers the Beaufort, Chukchi and Bering Seas during part of the year.
The biggest increases in wave activity over the past few decades are in the farther-north areas where seasonal ice-free conditions are the newest, the study found, and the most dramatic findings are generally for the month of September, when ice extent is lowest.
If reduced Arctic sea ice enables bigger waves, the relationship can go the other way as well. Bigger and stronger waves can weaken the ice, the study said.
"The trend of lengthening wave period and increasing wave height imply a trend of increasing wave energy flux, providing a mechanism to break up sea ice and accelerate ice retreat," researchers wrote.
The study lines up with other recent findings about increased wave activity.
Between 1993 and 2011, significant wave height increased by 0.02 meters (0.787 inches) per year in the southeast Chukchi Sea and 0.025 meters (0.98 inches) per year for the Pacific Arctic, correlating with sea-ice loss there over the two decades, according to a study by scientists with the International Arctic Research Center at the University of Alaska Fairbanks.
That study, published at the end of 2011 in the journal Geophysical Research Letters, used a combination of satellite records and readings from bottom-mounted recording instruments that were placed in the Chukchi Sea in 2007 and 2009.
In a particularly ice-sparse season, scientists saw big waves in the central Beaufort Sea. Scientists from the University of Washington's Applied Physics Laboratory found waves reaching 16 feet in the central Beaufort Sea in September of 2012, the time when Arctic ice extent hit the lowest point since satellite records began in 1979.
The observations were described in a study published in Geophysical Research Letters in 2014; the study said the big waves seen in 2012 suggest that "further reductions in seasonal ice cover in the future will result in larger waves, which in turn provide a mechanism to break up sea ice and accelerate ice retreat."
The University of Washington scientists are continuing to do field work to collect information to understand the physics of Arctic ice breakup, including the contribution of waves; their work is shown in a video made in in 2014 in the Beaufort Sea.
Big waves have been noticed elsewhere in the Arctic.
Svalbard-based scientists working in the Barents Sea in 2010, another sparse ice year, encountered waves up to 6 meters (19.685 feet) in height, believed to be a record for the Arctic, according to a report in Scientific American.
Alaska Dispatch Publishing