Most of us are probably familiar with the pattern of Arctic sea ice decline between 1979 and 2007, followed by a period of relative stability.
Most of the decline took place after the mid-1990s.
The decline is nearly always explained away as the result of global warming, but a couple of old studies show this not to be the case.
In 2011, Robson & Sutton found that the subpolar gyre underwent remarkable and rapid warming in the mid-1990s and that this was linked to changes in the North Atlantic Oscillation:
A gyre is simply a system of rotating ocean currents:
This is the key chart in their paper:
Graphs a) and b) compare sea surface temperatures, and show just how radically seas warmed between the two decades in the N Atlantic.
Perhaps even more remarkable are the temperatures for the oceans down to 500m, shown in c) and d).
There is no physical basis for such enormous changes to have been caused by greenhouse gases because the heat capacity of the oceans is far too massive. The explanation must lie elsewhere.
The Robson study fits in with an earlier one from NASA in 2007, which linked climatic changes in the Arctic to the Arctic Oscillation:
PASADENA, Calif. – A team of NASA and university scientists has detected an ongoing reversal in Arctic Ocean circulation triggered by atmospheric circulation changes that vary on decade-long time scales. The results suggest not all the large changes seen in Arctic climate in recent years are a result of long-term trends associated with global warming.
The team, led by James Morison of the University of Washington’s Polar Science Center Applied Physics Laboratory, Seattle, used data from an Earth-observing satellite and from deep-sea pressure gauges to monitor Arctic Ocean circulation from 2002 to 2006.
They measured changes in the weight of columns of Arctic Ocean water, from the surface to the ocean bottom. That weight is influenced by factors such as the height of the ocean’s surface, and its salinity. A saltier ocean is heavier and circulates differently than one with less salt.
The very precise deep-sea gauges were developed with help from the National Oceanic and Atmospheric Administration; the satellite is NASA’s Gravity Recovery and Climate Experiment (Grace). The team of scientists found a 10-millibar decrease in water pressure at the bottom of the ocean at the North Pole between 2002 and 2006, equal to removing the weight of 10 centimeters (four inches) of water from the ocean. The distribution and size of the decrease suggest that Arctic Ocean circulation changed from the counterclockwise pattern it exhibited in the 1990s to the clockwise pattern that was dominant prior to 1990.
Reporting in Geophysical Research Letters, the authors attribute the reversal to a weakened Arctic Oscillation, a major atmospheric circulation pattern in the northern hemisphere. The weakening reduced the salinity of the upper ocean near the North Pole, decreasing its weight 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 Morison.
“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.
The Arctic Oscillation was fairly stable until about 1970, but then varied on more or less decadal time scales, with signs of an underlying upward trend, until the late 1990s, when it again stabilized. During its strong counterclockwise phase in the 1990s, the Arctic environment changed markedly, with the upper Arctic Ocean undergoing major changes that persisted into this century.
Many scientists viewed the changes as evidence of an ongoing climate shift, raising concerns about the effects of global warming on the Arctic.
Morison said data gathered by Grace and the bottom pressure gauges since publication of the paper earlier this year highlight how short-lived the ocean circulation changes can be. The newer data indicate the bottom pressure has increased back toward its 2002 level. “The winter of 2006-2007 was another high Arctic Oscillation year and summer sea ice extent reached a new minimum,” he said. “It is too early to say, but it looks as though the Arctic Ocean is ready to start swinging back to the counterclockwise circulation pattern of the 1990s again.”
The Arctic Oscillation is closely linked to the NAO, and is said to be “negative” when high pressure dominates the pole:
According to NSIDC:
The Arctic Oscillation primarily affects sea ice through winds that cause changes in where the sea ice drifts.” When the Arctic Oscillation is in its negative mode, he said, the winds and ice tend to flow in a clockwise direction, generally keeping more of the older, thicker ice in the middle of the Arctic. In the positive phase, that old ice tends to get pushed out of the Arctic along the Greenland coast. Meier said, “This means that the sea ice tends to be younger and thinner and more prone to melt after a winter with a strong positive Arctic Oscillation
The unusually positive AO is evident in the late 1990s, since when it has been more stable.
Note also positive AO during the 1920s, when the Arctic underwent similar warming.
The exact mechanisms are complex and still not well understood by scientists. But both of these studies point to natural, decadal atmospheric changes as being responsible for Arctic sea ice trends since the 1990s, which include the self-evident stabilization of sea ice extent since 2007.
Arctic image by Free-Photos from Pixabay
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