Madden-Julian Oscillation: A Factor in Hurricane Season Forecasting You May Not Have Heard of Before

The Madden-Julian Oscillation, or MJO, plays an important role in the hurricane season, though it's one of the lesser-known global circulations.

The MJO is an atmospheric disturbance of clouds, rain, wind and pressure in the tropics that propagates eastward around the globe over a period of 30 to 60 days. It has a wide range of impacts on the patterns of tropical and non-tropical rainfall, atmospheric circulations and surface temperatures in the tropics and subtropics.

The MJO is characterized by enhanced and suppressed precipitation phases that bisect the planet. Typically, half of the planet will see an upward swing in rainfall while the other half of the planet will see drier conditions. Sometimes, there can be multiple waves of increased rainfall moving around the Earth.

The pattern "begins" in the Indian Ocean and propagates eastward through the Pacific and eventually into the Atlantic.

As the MJO moves east, it can influence tropical-cyclone activity in both the eastern Pacific and Atlantic basins during the Northern Hemisphere summer.

How the MJO Influences the Hurricane Season

When the enhanced phase of the MJO enters the Gulf of Mexico or Atlantic, tropical activity is typically increased for a period of up to two weeks.

The MJO not only increases rainfall during its enhanced phase but also decreases wind shear typically hostile to developing or active tropical cyclones.

Both of these factors allow existing tropical storms and hurricanes to strengthen. The enhanced phase also creates an environment favorable for new tropical waves to fester and organize.

Major hurricanes are five times more likely to develop in a favorable MJO phase, while hurricane formation is four times more likely.

(MORE: 2018 Atlantic Hurricane Season Forecast)

The MJO is only one factor that helps or hurts the development of tropical systems. If other variables such as ocean temperature, air moisture or low-level winds are unfavorable, then the MJO won't help nearly as much.

As the graphic above shows, tropical systems can grow in the suppressed phase of the MJO, but they typically have a more difficult time intensifying.

In the Western Hemisphere, typically either the East Pacific or Atlantic basins will be active and the other basin will be quiet due in part to the MJO.

(MORE: The 15 Most Iconic Hurricane Images of All Time)

More About the MJO

There is evidence that the MJO influences the El Niño-Southern Oscillation (ENSO). It doesn't cause El Niño or La Niña, but it can contribute to the speed of development and intensity of those periods.

(What to Expect in the Tropics in July | in August )

MJO activity varies over the course of the year, with periods of moderate to strong activity followed by periods of little or no activity. Typically, the Northern Hemisphere has the greatest level of MJO activity in late fall, winter and early spring, according to NOAA's Climate Prediction Center.

The MJO tends to be most active during ENSO-neutral years – when neither El Niño nor La Niña conditions are present – such as this year, and is often absent during moderate to strong El Niño and La Niña events, the CPC says.

Precipitation and surface-temperature patterns across the U.S. are influenced at times by the MJO.

The two most significant impacts in the U.S. during winter are an increase in the frequency and intensity of heavy precipitation events along the West Coast, as well as an increase in the frequency and intensity of cold-air outbreaks across the eastern states.