Why Rains in Brazil Follow the Sun's Schedule ✛

Original: "Multidecadal Cycles Study in the Climate Indexes Series Using Wavelet Analysis in North/Northeast Brazil"

· Cleber Souza Corrêa, Roberto Lage Guedes, Karlmer Abel Bueno Corrêa, Felipe Gustavo Pilau

arXiv:2604.03359 · 2026-04-03 · CC BY 4.0 · 2 min

Atmospheric and Oceanic Physics Applied Statistics

Sunspots, the ocean, and rains in northeastern Brazil pulse in the same rhythm, with periods of 11 and 21 years.

The Sun's face breaks out in spots, then clears again. This cycle lasts about 11 years, and it turns out that rainfall in northeastern Brazil changes in step with it. If you think of the climate as an orchestra, the Sun is the drum setting a slow beat, while the ocean and air pick it up on other instruments.

To see this, scientists took records of sunspots, pressure and temperature in the Pacific Ocean, and rainfall in five Brazilian cities, spanning 80 years. They applied wavelet analysis, a way to break a long series of numbers into waves of different lengths, much like splitting a musical recording into individual notes. It turned out that all these series share the same periods: 5.33 years, 10.66 years, and 21.33 years. The strongest signal is the 11-year one, matching the sunspot cycle.

Wolf numbers are simply the count of dark spots on the Sun. They have been counted for 400 years, and they show that solar activity rises and falls every 11 years, though sometimes the cycle stretches to 17 years.

Previously, no one knew whether these slow solar and oceanic rhythms were linked to rainfall in Brazil specifically. Now it is clear that they are: the rainfall records from five stations repeat the same periods as the sunspot numbers. This means that rains in this region are not random. They follow a rhythm set by the Sun and transmitted by the ocean. For farmers and reservoir builders, this is useful: knowing that the odds of drought rise every 11 or 21 years allows better planning of planting seasons and water reserves.

Whole graph →

The sunspot cycle lasts on average 11 years, but over the past 80 years its length has varied from 8 to 17 years. Wavelet analysis allows you to see this period instability directly on the spectrogram.

\psi(t) = \pi^{-1/4} e^{i\omega t} e^{-t^2/2}

Morlet wavelet with angular frequency ω=6, used for time series analysis.

W_x(\tau, s) = \int x(t) \frac{1}{\sqrt{s}} \psi^*\left(\frac{t-\tau}{s}\right) dt

Convolution of the series x(t) with a scaled and shifted version of the mother wavelet.

Scientists

Ludwig Boltzmann John von Neumann Josiah Willard Gibbs Henri Poincaré

Concepts

decadal variability solar cycle seasonal cycle Pacific decadal oscillation Atlantic multidecadal oscillation interannual variability Multivariate ENSO Index seasonal forcing climate OISST v2.1 NOAA CPC tropical cyclone Indian Ocean Dipole ENSO Niño 4 mixed mode oscillations GPCP IRI zebiak cane model

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Why Rains in Brazil Follow the Sun's Schedule ✛