The trade-wind are controlled by the interaction between the atmosphere and the ocean, with sea-surface temperature being an especially important factor.
In a non-El Niño year, trade-winds blow westward along the equator and push warm surface water against Asia on the Western side of the Pacific.
During an El Niño year, the trade-winds weaken and reverse direction along the equator in the west. These are called westerly wind bursts. This change in winds allows a large mass of warm water that is normally located near Asia to move eastward along the equator towards the Americas.
Scientists are still working on an explanation for what causes the trade-winds to weaken.
Equatorial Kelvin Waves
Equatorial Kelvin waves always move eastward, along the equator, and cross the Pacific Ocean in about two months. During this time, the Kelvin waves travel across about one-third of the Earth's circumference.
As the trade-winds relax, Kelvin waves are produced by westerly wind bursts. The waves move internally along the deepened thermocline, the transition zone between the warm upper ocean layer and the colder water below.
Kelvin waves travel eastward along the equator at relatively high speeds, which can reach up to 200 kilometers per day.
After the Kelvin wave reaches the South American shores, it travels poleward as coastal Kelvin waves along the Americas' shores.
Rossby waves always move westward.
After the Kelvin wave reaches the South American shores, it reflects off the coast as long-lived Rossby waves.
Rossby waves can take 10 to 30 years to cross the Pacific Ocean, depending on their latitude. Rossby waves in the upper latitudes take longer to cross.
Rossby waves can alter sea-surface temperatures and affect weather patterns for long periods of time.
The Intertropical Convergence Zone (ITCV) is the zone near the equator where trade-winds converge, causing air there to rise. The zone is very cloudy and is marked by heavy rainfall.
The rain drives the atmospheric circulation and the trade-winds.