Yes it has. At the Spring American Geophysical Union annual convention in Boston on MAy 27-29, 1998 John Gipson at NVI Inc based at the NASA, Goddard Space FLight Center showed that El Nino had increased the length of the day by 600-700 microseconds ( 0.0006 to 0.0007 seconds). The length of day or 'LOD' is monitored by using trans-atlantic Very Long Base Line Interferometry; a technique pioneered and still used by astronomers to study distant quasars. Older methods relied on the accurate timing of the transits of stars through specially designed astronomical 'transit telescopes'. The bottom line is that since the 1930's astronomers have known that the rotation of the earth is modified by seasonal changes in weather systems. During the 1982-1983 El Nino, an even stronger LOD increase of 800 microseconds was detected, with a milder LOD increase detected a year later of about 400 microseconds. If the current El Nino follows the same pattern, we should see a second LOD increase in early 1999.
Astronomers follow these LOD changes meticulously because they have serious affects upon precision navigation, especially interplanetary travel. The MArs Pathfinder mission would have entered the martian atmosphere too high or too low if these LOD effects had not been accounted for, and that would have had disasterous impact on the success of the mission.
To understand how air currents can affect earth rotation, you have to consider the ice skater on the ice doing a spin. If she changes how far our she holds her hands by just a little, it affects how rapidly she spins. Air currents change their location on the earth, and their distance from the earth's center by a few miles, and they also carry thousands or even millions of tons of air in clouds. It is easy to understand from this how, with conservation of angular momentum, the earth's spin is constantly changing.
All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY project.