El Nino Watch
The large area of the warmest ocean water on Earth, normally around Indonesia and the western Pacific. The biggest rain clouds sit above it.
El Nino basics
The warm pool is a region of ocean, around Indonesia and the western Pacific, where the surface water is usually above about 28 °C. Warm water heats the air above it, the air rises, and huge rain clouds form. That is why Indonesia and the western Pacific are among the rainiest places on Earth.
In an El Nino, the warm pool stretches east, towards and past the date line. The rain clouds follow it.