Normal conditions:
As the trade winds blow westward over the Pacific, they push the warm top layer of the ocean with them, causing the hottest water to pile up around Indonesia, where, because of both wind action and thermal expansion, the sea level is usually about 18 inches [46 centimeters] higher than it is off the west coast of Mexico. All along the eastern Pacific, and especially off Ecuador and Peru, colder subsurface water wells up to replace the sheared-off top layer, bringing up a bevy of nutrients from the deep ocean. That chemical bounty sustains an enormous food web and makes the coastal waters off Peru one of the world’s most prolific fisheries
el Nino:
For reasons that scientists still do not comprehend, every few years the trade winds subside or even disappear. The usual air-pressure pattern reverses itself in a phenomenon called the southern oscillation, making barometer readings higher in Australia than they are in the central Pacific. The resulting pattern—known as ENSO, for El Niño/Southern Oscillation—involves only one-fifth of the circumference of the planet. But it transforms weather around the globe.
Without the trade winds the top layer of the eastern Pacific does not move west. It stays in place, getting hotter and hotter, swelling as it warms. Eventually it hits the threshold for what meteorologists call deep convection—the point at which the steamy surface air blasts into the upper atmosphere.
la Nina:
In La Niña years the easterly winds from the Americas are stronger than usual. That drives more than the normal amount of warm sea-surface water westward, in turn causing larger than normal volumes of deep, chilly water to rise to the surface and producing a “cold tongue” that extends 3,000 miles [4,800 kilometers] along the Equator from Ecuador to Samoa.
With so much warm water flowing toward Asia, the Pacific’s mighty heat engine remains firmly anchored in the west, causing heavier monsoon rains in India, higher than average precipitation in Australia, and wetter than normal conditions as far west as southern Africa. The huge air masses and cloud banks associated with the hot zone also change the path of the jet streams, which move high-altitude air from west to east across the ocean.
