Scientists zero in on Arctic, hemisphere-wide climate swings

In the late 1990s, as scientists were reaching consensus that the Arctic had gone through 30 years of significant climate change, they began reading the first published papers about the Arctic Oscillation, a phenomenon reported to have hemisphere-wide effects.

In short order the arctic-science and the global-change communities were galvanized, says Richard Moritz, polar oceanographer with the UW’s Applied Physics Laboratory and lead author of a review of recent Arctic climate change in the Aug. 30 special polar-science issue of Science.

“We’ve learned more about the dynamics of post-glacial arctic climate change in the last five years than in the 50 years previous,” Moritz says. “For example, the recent trend in the Arctic Oscillation explains the warming observed in the Arctic better than anything else.”

Advances in understanding arctic climate change are particularly timely, with some studies indicating that the recent trend in the Arctic Oscillation results partly from human activities that generate greenhouse gases and sulfate particles, and deplete stratospheric ozone. Scientists, planners and policymakers need to know what the changes of the last 30 years portend.

Thus climate modelers have redoubled their efforts to determine the physics behind the patterns of change. Although their models portray realistic day-to-day and month-to-month variations in the Arctic Oscillation, they fail to capture the magnitude of the longer term trend in the Arctic Oscillation that was observed from 1970 to 2000. While paleoclimatologists studying the climate record of the past 1,000 years have not reached a consensus on the importance of the Arctic Oscillation pattern over this longer period, some surprising findings indicate that past Arctic warmings tended to coincide with low-frequency El Nino-Southern Oscillation events in the tropical Pacific.

The review by Moritz and co-authors Cecilia Bitz, a sea-ice expert with the UW’s Applied Physics Laboratory, and Eric Steig, assistant professor with the UW’s Quaternary Research Center, refers to more than 80 published papers, most appearing in just the last two years. The co-authors say that warming of the surface from 1970 to 2000 in the Northern Hemisphere was greatest in the Arctic, causing changes in precipitation, snow cover and the extent of sea ice.

The Arctic Oscillation is a seesaw pattern in which atmospheric pressure at the polar and middle latitudes fluctuates between positive and negative phases. The wind patterns associated with the Arctic Oscillation affect the surface temperature over North America and Eurasia, as well as the Arctic. The Arctic Oscillation was first described in a 1998 article by David Thompson, then a graduate student at the UW and now an assistant professor at Colorado State University, and John M. Wallace, a UW professor.

“The Arctic Oscillation provides a very fruitful framework and the result is that a tremendous amount of work has been done in a relatively short period of time,” Moritz says. “Attempts to model the pattern of recent Arctic and global warming have to come to grips with the problem of the Arctic Oscillation.” Climate modelers have benefited from a growing understanding of sea-ice physics and the best-ever measurements of how heat from the sun and the atmosphere affects the pack ice that covers the Arctic Ocean. Moritz, for example, is director of the SHEBA (Surface Heat Budget of the Arctic Ocean) Project Office funded by the National Science foundation and Office of Naval Research. Now in its analysis phase, SHEBA locked an icebreaker into the pack ice for a full year in the late ’90s to measure the interactions of the ice, atmosphere and the ocean during all four seasons.

Because so many climate modelers worldwide are working on the Arctic Oscillation, Moritz says it’s conceivable that in a year or two we will understand the fundamental physics of the Arctic Oscillation, and be able to account for its recent trend. “If we can’t, it won’t be for lack of trying.”