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Rain Waves: Why Some Are Strong and Others Aren't ✛

Original: "Evolution of Extreme Madden-Julian Oscillation Events and their Impacts on South America"
· Mónica Minjares, Álvaro Corral, Marcelo Barreiro
arXiv:2507.20289 · 2025-07-27 · CC BY 4.0 · 1 min
Atmospheric and Oceanic Physics
In the tropics, waves of downpours circle the globe. Rare strong waves affect South American rainfall more than frequent weak ones.

In the tropics, an invisible rain wave travels around the Earth from east to west. Most such waves are weak and quickly dissipate, but occasionally a strong wave emerges and crosses the entire Pacific Ocean.

A rain wave is a region where air rises and forms clouds. It moves and carries rain to new areas.

Scientists identified 132 strong waves over 43 years. They tend to start over the Indian Ocean, where the water is warm. When the Pacific is cooler than average (a condition called La Niña), strong waves live longer, on average 40 days.

These waves affect rainfall in South America: if the north of the continent gets drenched, the south dries out, and vice versa. Strong waves make this seesaw more pronounced. Previously, forecasters couldn't tell a strong wave from a weak one at the outset. Now they look at where the wave was born and how quickly it gains strength. This helps warn of drought or flood in advance.

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An extreme MJO event starting in phases 2–3 can pass through five consecutive phases before decaying, whereas a weak event often dies after just one or two phases. The longest extreme event in the sample lasted 32 days, comparable to a full MJO circuit around the globe.

f(x) = \frac{\alpha}{a} \left( \frac{a}{x} \right)^{\alpha+1} \quad \text{для } x \ge a
The probability that the size or duration of an MJO event takes the value x decreases according to a power law with exponent α+1, starting from a threshold a.
Concepts
Madden–Julian Oscillation El Niño-Southern Oscillation ENSO El Niño monsoon onset La Niña monsoon Westerly wind burst Multivariate ENSO Index mixed mode oscillations spring predictability barrier recharge oscillator model Jin-Timmermann model interannual variability seasonal cycle TAO/TRITON moorings equatorial wave propagation marine heatwave North Equatorial Countercurrent tropical cyclone