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Two closely related climate patterns have had a significant influence on winter-storm history this winter and last, say specialists.
They point to a phenomenon at the top of the world called the Arctic Oscillation, which appears as a swing between weaker and stronger atmospheric-pressure differentials over the Arctic and at mid-latitudes. When the pressure differences are smaller than normal, a high-altitude wind encircling the Arctic, known as the polar vortex, weakens, allowing cold air to drive south more frequently than when the pressure differences and the vortex are strong.
The Arctic Oscillation – which varies over periods ranging from weeks to decades – has been in a weak, or negative phase, since mid-November, with its weakest stage so far this winter running from mid-December to mid-January.
Meanwhile, over the North Atlantic, another climatological mood swing known as the North Atlantic Oscillation has played an influential role in setting up storm tracks as they migrate over eastern North America across to Europe.
This tends to bring mild, wet winters to the eastern US, a wetter winter to central and northern Europe, and cold, dry winters to northern Canada and Greenland.
In a negative phase, each pressure regime weakens and moves slightly, bringing more storms to the southern and middle Atlantic states and southern Europe, but drying out northern Europe.
Since mid-November, the North Atlantic Oscillation has remained largely in a negative phase, although with February has come a poke into positive territory.
Beyond the tropics, the North Atlantic Oscillation is the dominant driver of weather and climate during Northern Hemisphere winters, says James Hurrell, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo.
Some researchers argue the Arctic and North Atlantic Oscillations are one broad phenomenon.
Other say they are siblings. Either way, these oscillations play such influential roles in winter weather patterns that researchers are trying to identify the factors that tip the swings to one long-term average phase or the other.
For instance, atmospheric scientist Judah Cohen says he is looking at the possibility that the Arctic Oscillation’s two-decade tendency to run negative may be linked to the decline in Arctic sea-ice extent during the spring and summer melt season.
The implication: further expected declines in summer ice extent could shove the Arctic Oscillation into a negative phase more often than not for some time to come, says Dr. Cohen of Atmospheric and Environmental Research, a weather and climate risk-management firm in Lexington, Mass.
For his part, Dr. Hurrell says uncovering some level of predictability – however modest in the North Atlantic Oscillation (NAO) – over time frames of a decade or two “could be extremely important.”
Norway, for instance, derives much of its energy from hydroelectric dams, which rely on precipitation in the mountains. Snowfall there, he says, “is extremely highly correlated to the phase of the NAO.” Even assigning a probability that the NAO is about to undergo a long-term shift, in average phase could help energy planners there, he says.
One key may lie thousands of miles away, over the Indian Ocean. During the past five to 10 years, he says, tantalizing evidence has emerged that rainfall trends over the Indian Ocean may tweak atmospheric circulation patterns in ways that affect conditions over the North Atlantic.
Specifically, as the Indian Ocean warms and rainfall increases, the storms trigger changes in the atmosphere that lead to a generally positive North Atlantic Oscillation.