12/15/25 Monday 7 am
I was able to knock-out the first post more quickly than I anticipated. I will be working on Part 2 tomorrow.
What is the Madden-Julian Oscillation (MJO)?
The MJO is the largest and most important pattern of weather variability in the tropical atmosphere on a weekly to monthly timescale (what meteorologists call “intraseasonal” variability). And yes, it was discovered by a guy named Madden and a guy named Julian in the early 70s.
Imagine a giant, slow-moving “wave” of clouds, rainfall, and wind that circles the entire globe along the equator.
- Period: This wave typically takes between 30 to 60 days to complete one full trip around the world.
- Direction: It moves from west to east in the Tropics, starting over the Indian Ocean.
The MJO has significant impacts on temperature and precipitation across all seven continents. Its signal, which is stronger in the Northern Hemisphere winter, offers untapped predictability for long-range global weather pattern forecasting.
The 200mb (below) is approximately 40,000 feet: The 850mb is approximately 5000 feet.
(NOAA)
The Two Key States
The MJO is often described as having two main components that travel together:
- Enhanced Convection (Wet): This is a massive, eastward-moving area where air is rising, leading to intense thunderstorms, heavy cloud cover, and anomalously high rainfall in the tropics. This phase is often favorable for tropical cyclone development.
- Suppressed Convection (Dry): Trailing behind the wet phase is an area where air is sinking. This leads to clear skies, dry conditions, and anomalously low rainfall in the tropics .
The wet phase and the dry phase form an opposing pair that slowly moves across the entire world, causing shifts in global weather.
Global Impact and Significance
The MJO’s influence extends far beyond the tropics. It is a critical component for extended-range forecasting (several weeks in advance).
- Tropical Cyclones: The enhanced convection phase can provide favorable conditions for the formation and intensification of tropical cyclones (hurricanes/typhoons) in ocean basins like the Indian Ocean and Western Pacific.
- Monsoons: It strongly controls the timing and intensity of monsoon systems around the world. The wet phase can bring the onset of a monsoon, while the dry phase can cause breaks in the rainfall.
- Mid-Latitude Weather: Through teleconnections—atmospheric ripples that connect the tropics to the rest of the globe—the MJO can influence weather patterns in North America, Europe, and elsewhere:
- It can affect the position and strength of the jet streams.
- It is linked to extreme precipitation events, like the “Atmospheric Rivers” on the U.S. West Coast.
- ENSO Connection: The MJO interacts with and can help determine the timing and intensity of an El Niño or La Niña event (collectively known as ENSO), though it does not cause them directly.
Because it consistently cycles through different areas of the globe (there are 8 defined phases, or locations, of the MJO), monitoring its position is key for sub-seasonal (14 to 60 day) weather prediction.
🌧️ Global Weathermakers: Monsoons, Floods, and Droughts
The MJO not only determines temperatures but also governs global rainfall patterns. Its powerful wave of wet and dry air determines when and where extreme rainfall events occur.
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Classic Monsoon: The MJO strongly controls the timing and intensity of the big monsoon systems around the world, like those in India, Australia, and South America.
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Think of the Wet Phase (Enhanced Convection) as the starting pistol, often kicking off the entire monsoon season.
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When the Dry Phase (Suppressed Convection) rolls through, it causes “breaks” in the rainfall, which is where those concerning dry spells and potential drought conditions come from.
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Feast or Famine: This system is all about extremes.
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On one hand, a slow-moving Wet Phase can bring prolonged, heavy rainfall, drastically increasing the risk of major flooding in places like Southeast Asia or even right here in the U.S. Western States.
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On the other hand, the Dry Phase creates huge areas of sinking air, which warms everything up and shuts down cloud formation, leaving those areas exposed to drought-like conditions.
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Distant Connections: The MJO’s tropical changes even have a surprising reach, linking the convection over the Indian Ocean to rainfall changes in West Africa a couple of weeks later.
🌊 MJO’s Influence on the Pacific Ocean
The Pacific Ocean is where the MJO truly becomes critical, acting like a global thermostat and a hurricane starter kit for both the U.S. and the Western Pacific Basin.
1. Hurricane and Typhoon Fuel
The MJO is often the deciding factor in whether a tropical storm forms and strengthens:
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The Power-Up Phase: When the MJO’s Wet Phase (the area of intense storms) is hovering over the Western Pacific, it creates the perfect large-scale environment for tropical cyclones (typhoons) to pop up and gain strength.
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East Coast Connection: The same influence applies in the Eastern Pacific, affecting the formation and intensity of hurricanes off the coast of Mexico and Central America.
2. The “Atmospheric River” (formerly known as The Pineapple Express) on the U.S. West Coast
During the winter months (November–April), the MJO can act like a giant water hose pointed at the United States:
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Jet Stream Steering: Through atmospheric ripples known as teleconnections, the MJO shifts the Pacific jet stream.
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The Moisture Bridge: When its convection is over the central or western Pacific, the MJO helps build a “bridge” of tropical moisture. This often forms an Atmospheric River, once famously dubbed the “Pineapple Express,” which slams the U.S. West Coast and brings extreme rain and flooding to California and the Pacific Northwest, and if we are lucky, a nice winter storm or two to our area.
3. The Relationship with ENSO (El Niño/La Niña)
The MJO is a separate phenomenon, but it’s constantly chatting with the much slower El Niño–Southern Oscillation (ENSO):
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The Final Push: A strong MJO can provide (but not always) the crucial “nudge” that either speeds up the development of a full-blown El Niño or triggers the switch into a La Niña event.
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Altered Path: During an El Niño, the MJO’s storms tend to travel much farther east across the central Pacific than usual. During La Niña, the storms are often held back closer to the western Pacific.
Tracking the MJO is essential because it gives us the best glimpse into the weather trends for that important 2- to 4-week window.
Now that you know more about the MJO and its impacts, we will next discuss how to track the MJO and its eight phases.