El Nino Watch
A slow, huge underwater wave that travels east along the equator in 2-3 months. A "downwelling" one carries warm water east and can start or boost an El Nino.
El Nino basics
When the trade winds weaken or a westerly wind burst blows, the thick warm layer in the western Pacific starts to slide east along the equator. It moves as a wave that you cannot see at the surface (the sea surface rises only a few centimetres) but that pushes the Thermocline down by tens of metres.
A Kelvin wave takes about 2 to 3 months to cross the Pacific. That delay is useful: forecasters see it coming.