Here is an Arctic Bottom Pressure Recorder
deployed to the Arctic Ocean floor to monitor changes
in Arctic Ocean circulation.
Round and round it goes, and where it stops, the ice still floes.
Or at least that’s what a team of scientists — equipped with a gravity-detecting satellite and ocean-bottom sensors — have discovered about a dramatic reversal in the power and direction of air and ocean currents circulating around the North Pole.
This Arctic Oscillation — one of the main drivers of weather in the Northern Hemisphere — has stopped moving as it did during the 1990s, suggesting that the causes of the climate change may be even more complicated than we thought.
“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 lead scientist James Morison, with University of Washington’s Polar Science Center Applied Physics Laboratory, in an online news story from the Jet Propulsion Lab.
“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 ingenious study (it focused on the weight of water on the ocean floor) was all about fathoming the Arctic Oscillation — one of the most important (and mysterious) drivers of weather and climate in the Northern Hemisphere — and its relationship to global warming now underway.
The AO may be the yin and yang of atmospheric pressure patterns over the Far North. When the AO becomes “negative,” high pressure dominates the Arctic. When the AO goes positive, an opposite patter occurs.
Here’s a good explainer from the National Snow and Ice Data Center:
Positive Phase on left, Negative phase on right
Source: J. Wallace, University of Washington and NSIDCThe oscillation exhibits a “negative phase” with relatively high pressure over the polar region and low pressure at midlatitudes (about 45 degrees North), and a “positive phase” in which the pattern is reversed.
In the positive phase, higher pressure at midlatitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska, Scotland and Scandinavia, as well as drier conditions to the western United States and the Mediterranean. In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leaves Greenland and Newfoundland colder than usual.
Weather patterns in the negative phase are in general “opposite” to those of the positive phase, as illustrated below.
Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s, however, the oscillation has tended to stay in the positive phase, causing lower than normal arctic air pressure and higher than normal temperatures in much of the United States and northern Eurasia.
The JPL story explains what the scientists have found out and how it complicates the study of climate change:
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 Morison, 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.
This chart shows how Arctic circulation has changed —
the contours of the trend in ocean bottom pressure from
2002 to 2006 as measured by GRACE. It also shows the
hypothetical trends that would apply at the circles if ocean
salinity reverted from 1990s values to climatological
conditions over the same period.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.
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.”
Morison cautioned that while the recent decadal-scale changes in the circulation of the Arctic Ocean may not appear to be directly tied to global warming, most climate models predict the Arctic Oscillation will become even more strongly counterclockwise in the future.
“The events of the 1990s may well be a preview of how the Arctic will respond over longer periods of time in a warming world,” he said.
