The unseen forces behind extreme weather events

Have you ever wondered about the origin of extreme weather events like hurricanes, blizzards and droughts? What are the colorful stripes and swirls shown in forecast maps? High above our heads, atmospheric changes are happening every second. Often caused by temperature differences between the ocean and the land, these changes are not really noticeable by lay people until they brew into natural disasters. Fortunately, meteorologists have some reliable friends they can use to make forecasts: the atmospheric circulations that travel across the globe periodically. One of the most influential circulations is the Madden-Julian Oscillation. A 2000 Science study found that it is linked with higher likelihood of hurricane genesis in Gulf of Mexico and western Caribbean.

Scientists didn’t know about the Madden-Julian Oscillation until the early 1970s. While studying winds in the tropical Pacific, Roland Madden and Paul Julian of the National Center for Atmospheric Research found a regular oscillation, or cycle, of winds between Singapore and Kanton Island, a small equatorial island in the middle of the Pacific Ocean. There, the weather patterns at a given spot reoccurred every 40-50 days. It turns out what Madden and Julian observed is part of a worldwide phenomenon, thus the term Madden-Julian Oscillation, or MJO.

No atmospheric circulation influences midlatitude regions as the MJO does. Initiated by convective clouds (those formed as warmer air floats up) above the Indian Ocean, it moves eastward across the tropics. Moving at around 9 to 18 mph, a speed similar to that of recreational cyclists, the MJO sweeps through southern Asia, Australia, Pacific Ocean, the Atlantic Ocean and back to Asia in 30 to 60 days. It also contributes to the winter storms in North America.

This vast scope is not the only thing that makes the MJO so influential. As it traverses the globe, the MJO brings rainfall to one region and simultaneously suppresses rain in another. On a meteorologist’s map, it always has a green blob and a brown blob that are coupled together: the wet phase and the dry phase. In the wet phase, winds near sea level converge to push air up, which increases humidity in the atmosphere and encourages rainfall. In the dry phase, winds at high altitudes converge to push air down. As the air is warmed and dried during the sinking motion, it inhibits rainfall.

Composite evolution of MJO events over 35 days in summer. Photo credit: NOAA/National Weather Service

These two extremes in behavior affect the timing of the Indian Monsoon. In early summer, MJO’s wet phase provides 80 percent of the region’s freshwater. After the summer deluge, Indian farmers can plant crops during the break in rainfall when MJO’s dry phase sweeps through. Active MJOs — which have distinct double phase structures — are also linked with flooding rains on America’s west coast in winter. The winter of 1996-1997 witnessed a prominent MJO. It was the strongest in National Oceanic and Atmospheric Administration’s 30-year records. That winter, an extremely heavy rainfall caused between $2-3 billion damage in flooding. Researchers are exploring whether this year’s record rainfall also has something to do with MJOs.

The MJO’s global impact go beyond rainfall. In winter, air pollution particles from China can hijack a ride on the MJO. As droplets and ice crystals in clouds form around these small bits of air pollution, these “cloud nuclei” can lead to bigger clouds and even storms downstream of MJO’s path in the east. MJOs may also contribute to an El Niño or La Niña, so they might be helpful in predicting the onset and strength of these warming or cooling events.

The MJO, along with other atmospheric changes, belongs to a fascinating field of scientific study, linking the whole world with interconnected weather events, and accordingly, our lives. Knowing about the MJO might not protect you from a sudden pouring rain. But you can drop the term in the middle of a hurricane discussion. Impressive, certainly.