Recent dramatic changes in the Arctic climate - melting sea ice, warmer ocean, green fields in place of icy wilderness, etc - might not all be directly related to global warming.
The more clement Arctic climate of recent years could have been triggered by shorter term circulation changes in the oceans and atmosphere.
According to a team of NASA scientists, decade-long variations in ocean circulation, known as the Arctic Oscillation, have an effect on the oceans' salinity. A very salty sea is heavier and circulates differently than a less salty one, the team says. This can affect the temperature of the water in the region and thus the local climate.
The team monitored the Arctic Ocean's circulation between 2002 and 2006 using satellite data and deep sea pressure gauges. They measured the weight of columns of ocean water, and found that between 2002 and 2006, there was a 10-millibar decrease in pressure on the sea floor - equivalent to removing ten centimetres of water from the ocean.
The distribution of the decrease, and its implications for the salinity of the water, suggests that the ocean has switched to a clockwise circulation - the same as was dominant before 1990, the team writes.
In the journal Geophysical Research Letters, the researchers attribute the change to a weakened Arctic Oscillation. This, they explain, reduced the saltiness of the upper ocean near the pole, making it lighter and changing its circulation.
"Our study confirms many changes seen in upper Arctic Ocean circulation in the 1990s were mostly decadal in nature, rather than trends caused by global warming," said James Morison, lead researcher based at the University of Washington's Polar Science Centre Applied Physics Laboratory.
"While some 1990s climate trends, such as declines in Arctic sea ice extent, have continued, these results suggest at least for the 'wet' part of the Arctic - the Arctic Ocean - circulation reverted to conditions like those prevalent before the 1990s," he added.
Since the end of the period covered by the published data, Morison says the measurements of pressure are already swinging back to the higher levels at the beginning of the study. He offers this as evidence of just how short-lived changes in ocean currents can be.
"It is too early to say, but it looks as though the Arctic Ocean is ready to start swinging back to the counter-clockwise circulation pattern of the 1990s again," he said.
The Arctic Oscillation was fairly stable until about 1970, NASA says. From then until the late 1990's it varied considerably, before settling into a strong counter-clockwise circulation. This changed the Arctic environment considerably, and induced changes in the upper ocean that have persisted into the 21st century.
Morison adds that although the recently observed changes are more likely due to the variations in the current than directly to do with global warming, climate models do predict that a warmer world will have a stronger counter-clockwise circulation in the Arctic Ocean. This, he adds, means that the 1990s may have been a glimpse of the region's future. ®