Mimicking Mother Nature   Page 2

Though studying the short-term variations of the NAO is key to understanding the phenomenon, climatologists at present are much more excited about what has occurred to the NAO index since the late 1960s. For a little over 100 years, since people started collecting reliable pressure readings of the Atlantic, the yearly average NAO values remained within a set range—the peak negative and positive NAO values never went far above or below what they had been in years past. Then about 30 to 40 years ago the entire NAO index started to become more positive. The peak NAO years began growing more positive and the negative NAO years began growing less negative.
 

   
Graph of Winter NAO Values from 1864 through 2002

“It’s to the point to where over the past 20 years we’ve seen a predominantly positive North Atlantic Oscillation,” says Hurrell. The overall increase in the index has corresponded to a noticeably longer growing season in Europe and milder winters in the mid-Atlantic region of the United States. This past winter has been one of the few exceptions.

Lately, Hurrell’s research into the NAO has focused on uncovering the cause of the upward trend. Such a trend would suggest that something bigger than ocean temperatures or currents in the Atlantic, something acting on a global scale, was pushing at the NAO. For now, the prime suspect is global warming. Hurrell explains that over the past 30 years a correlation has existed between the rise in the North Atlantic Oscillation and an increase in temperatures in the Indian Ocean. Within the climatology community, it’s fairly well established that the increased temperatures and rainfall in the Indian Ocean is due directly to global warming produced by greenhouse gases.

In an attempt to verify that warming temperatures in the Indian Ocean actually caused a change in pressure differences 7,000 miles (11,000 kilometers) away over the Atlantic, Hurrell turned to a computer simulation of the Earth’s atmosphere and ocean. Using such models, researchers can explore connections between the ocean and the atmosphere that span half the globe. “When we ran our model forward [in time] and forced an increase in sea surface temperatures and precipitation over the Pacific Ocean and Indian Ocean, we saw a strong positive NAO response. The Indian Ocean appears to have a significant influence over the North Atlantic region,” says Hurrell.

Though much work still has to be done to reach the point where scientists understand this overall shift in the NAO index, Hurrell believes that in the end, long-term predictions of the NAO’s sign will likely be more feasible than year to year predictions. He explains there are simply too many random variables that influence the NAO on a yearly basis. He does, however, envision a day when scientists will be able to say with a degree of certainty how positive or negative the NAO will be in the coming decade. As to the benefits of such a prediction, he points to the Norwegians reliance on hydropower. A positive NAO brings an abundance of water and power to Norway come spring, and a negative NAO sends Norway searching for imported power. “If we can tell the Norwegians that the NAO is generally positive for the next 10 years, then they have at least some assurance they can rely on hydropower for the next decade. If not, then they can begin to make plans in advance to import electricity,” says Hurrell. “I feel some really useful things can come of this.”

    References
  • Mehta, V.M., M.J. Suarez, J. Manganello, and T.L. Delworth, 2000: Oceanic influence on the North Atlantic Oscillation and associated Northern hemisphere climate variations. Geophysical Research Letters, 27, 121-124.
  • National Oceanic and Atmospheric Administration, 1999: North Atlantic Oscillation, Silver Spring, Maryland.
  • World Climate Research Program, 1997: CLIVAR International Implementation Plan, Southampton, United Kingdom, D1.
  • Rodwell et al., 1999: Oceanic forcing of the wintertime North Atlantic Oscillation and European climate Nature, 398, 320-323.
  • Rodwell and Folland, 2002: Quarterly Journal of the Royal Meteorological Society, 128, 1413-1443.

back Relying on the Ocean’s Long Term Memory

  From year to year, the winter NAO index fluctuates dramatically, but there are some long-term trends. Since a low point in 1960, winter NAO Indices have been rising. This is associated with mild winters in the eastern U.S. and longer growing seasons in western Europe. (Graph by Jim Hurrell, University Corporation for Atmospheric Research)
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