El Nino shifts Earth's momentum.

The current El Nino warming has grown so strong, it has added a noticeable zip to atmospheric winds and slowed Earth's spin, suggest scientists who track the planet's rotation.

El Nino exerts these profound effects by speeding up the eastward movement of the atmosphere, relative to the solid body of the planet, says David A. Salstein of Atmospheric and Environmental Research in Cambridge, Mass. The change shows up in analyses of the atmosphere's angular momentum--a property comparable to the momentum of a spinning tire. From mid-March through late November 1997, the atmosphere's angular momentum remained significantly above average, reports Salstein.

During non-El Nino years, winds in the tropics blow from east to west, whereas winds over the rest of the globe travel from west to east. Combined, they give the atmosphere a net eastward momentum.

The atmosphere routinely trades some of this momentum back and forth with the solid Earth as winds drag across the surface of the planet and push against mountain ranges. In the Northern Hemisphere's winter, the atmosphere speeds up and Earth slows. In summer, the reverse happens.

El Nino boosts the atmosphere's angular momentum by slowing down the tropical easterlies and speeding the westerlies outside the tropics, says Salstein.

As the atmosphere speeds up during El Nino, Earth itself slows down to conserve the combined angular momentum. John M. Gipson of NASA's Goddard Space Flight Center in Greenbelt, Md., has tracked the planet's spin by monitoring changes in the length of the day. Over a typical year, the day shortens and lengthens by roughly 1 millisecond, mostly because of shifts in atmospheric angular momentum. During the current El Nino, the day has grown longer by four-tenths of a millisecond, he says.