Equatorial Kelvin wave

Equatorial Kelvin waves are eastward-propagating, trapped equatorial waves that carry energy along the thermocline in the tropical ocean. Unlike freely propagating surface waves, Kelvin waves are confined to the equator by the Coriolis effect, which acts as a waveguide: any northward or southward displacement is immediately turned back toward the equator by the rotating reference frame. In the Pacific, these waves are the primary oceanic communication mechanism in the Bjerknes feedback, transmitting the signal of wind changes from the western Pacific to the eastern boundary in 2–3 months. When a westerly wind burst in the western Pacific depresses the thermocline, the resulting downwelling Kelvin wave propagates eastward, deepening the thermocline in the eastern Pacific and suppressing the upwelling that maintains the cold tongue — a key trigger for El Niño onset. The wave's finite propagation speed introduces the critical time delay that converts the Bjerknes feedback from a simple positive loop into an oscillator with irregular but bounded periodicity.