Atmosphere, Not Oceans, Carries Most Heat To Poles

This is far more heat than previously estimated and much more than the oceans carry poleward. Until now scientists have been unable to reconcile observations of the atmosphere and ocean with results from global climate models. The new study establishes the role of each in total heat transport poleward.

"This new analysis makes the observations more consistent with the most stable global climate models and gives us confidence that the models are on target," says Kevin Trenberth of the National Center for Atmospheric Research.

Trenberth and NCAR colleague Julie Caron performed the analysis, which was published in a recent issue of the Journal of Climate, a publication of the American Meteorological Society. It was selected this month by the journal Science as an Editor's Choice of important new findings.

The atmosphere and oceans help to even out the planet's temperatures by moving vast amounts of solar heat from the equator toward both poles, primarily during winter in each hemisphere. Without this leveling effect, all the high latitudes would be frozen solid while the tropics would be much warmer and wetter.

Based on a reanalysis of data gathered between February 1985 and April 1989, the study shows that the atmosphere handles 78% of the total heat transport in the Northern Hemisphere and 92% in the Southern Hemisphere at 35 degrees latitude--where the total poleward transport in each hemisphere peaks. The ocean carries more heat than the atmosphere only in the tropics between 0 and 17 degrees north, according to the study.

In the past, computer models attempting to mimic the Earth's climate have required artificial fixes to match real-world observations. Only recently have NCAR and the United Kingdom's Hadley Center developed climate models stable enough to simulate centuries of climate without these fixes.

Their results now nearly match the observations. To complete the picture, recent results from ocean measurements fit well with those deduced by Trenberth and Caron from the atmospheric component and both now add up to the alreadh known total heat transport.

In the late 1970s the ocean and atmosphere were thought to be conveying about the same amount of heat globally. Scientists estimated that the atmosphere was hauling 57% of the heat load, with oceans bearing a hefty 43% at the 35-degree latitude. As analyses have improved, estimates have steadily increased the magnitude of poleward heat transport occurring in the atmospheres of both hemispheres.

The atmosphere's role may have been slighted in the past because of a lack of data over the oceans, where substantial atmospheric heat transport occurs. Satellites have helped fill that gap. Trenberth and Caron focused on the 1985-1989 period because it offers reliable top- of-the-atmosphere radiation data from satellite measurements taken during the Earth Radiation Budget Experiment.

The new study was based on two data reanalyses, one by the National Centers for Environmental Prediction and NCAR, the other by the European Centre for Medium-Range Weather Forecasts. The study was funded by the National Oceanic and Atmospheric Administration and NASA. NCAR is managed by the University Corporation for Atmospheric Research with primary sponsorship by the National Science Foundation.