The current MJO started in its usual location around Dec. 25, O'Lenic said. Though it began at about the same time and place as the devastating earthquake and tsunami in that region, he stressed that the events were not related. Ocean waves like tsunamis are driven by gravity, he said, while waves in the atmosphere like those in MJO are driven by temperature differences.

"There were thousands of miles (km) of clouds over the Indian Ocean, and at the same time, a large area of dry air was over Indonesia and the area around the international dateline," O'Lenic said by telephone from a meeting of the American Meteorological Society in San Diego.

These two companion areas of clouds and dry air move eastward, roughly following the equator, over a period of 30 to 60 days, said O'Lenic, who is chief of the operations branch of the Climate Prediction Center for the US National Oceanographic and Atmospheric Administration.

In this case, MJO moved eastward to Hawaii, spawning storms there and generating the so-called Pineapple Express, a weather pattern characterized by heavy moisture that heads for the US West Coast.

MJO does not cause El Nino, the Pacific Ocean weather phenomenon that can cause heavy rains in the US West. But it can intensify El Nino's effects, O'Lenic said.

"Its origins are mysterious," he said of MJO at a briefing at the meteorologists' meeting. "MJO waxes and wanes in strength and is very sloppy in its propagation."

Even though MJO has been observed and monitored for decades, meteorologists are now working with computer models to use their observations to predict weather caused by MJO weeks in advance.