On a playground, two swings stand side by side. One child pumps on their own, while the other barely moves until the first gives a push with their foot. Something similar happens in the tropical Pacific and Indian Oceans, except instead of children, there are warm and cold masses of water.
El Niño is the warm phase of an ocean temperature oscillation that occurs every few years and changes weather across the planet. The Indian Ocean Dipole is the temperature difference between the western and eastern parts of that ocean. Both phenomena have long been studied separately, but exactly how they influence each other remained unclear. Scientists built a mathematical model linking these two indices and calculated who is pushing whom.
It turned out that the Indian Ocean Dipole has almost no influence on El Niño: its coupling function hovers near zero all year. But El Niño acts noticeably on the dipole, especially in August, when the dipole is just beginning to mature. At that moment, El Niño stabilizes it, preventing the temperature from straying too far from the average value.
Validation against 149 years of data confirmed that the model reproduces the real fluctuations of both indices and their joint behavior. The coupling peaks in October, when the dipole reaches its maximum variability. This means that warm Pacific waters indeed drive the seasonal rhythm of the Indian Ocean.
This approach can be applied to any pair of climate indices to build a network of interactions. Linking several such models would yield a tool for seasonal forecasting of anomalies in different regions. The swings on the playground turn out to be not random neighbors but a coupled system, where one child sets the rhythm for the other.
The names El Niño and La Niña are linked to Christmas: the warm phase often peaks in December, and Peruvian fishermen named it after the Christ child.