To understand El Niño, you've got to fathom the planetary "heat engine". Heat engines -- there's one in your car -- use heat to create motion. In your car, heat from burning gasoline creates motion so you can cruise for vegi-burgers.
On Earth, heat from the sun warms the equator much more than the poles. This is because the sun's rays score almost a direct hit on the equator but deliver only a glancing blow to the poles.
A convection cell is driven by heat at the ocean surface. Water from the surface evaporates and the warm, moist air rises, pulling in cool, dry air, and creating a loop that transfers heat and moisture out of the hot zone.
The motion of the atmosphere is powered by heat from the sun which evaporates sea water. The warm, moist air rises from the equator, transferring heat towards the poles. At the same time, dry, cool air drops at the middle latitudes and is sucked below back to the equator. This creates giant atmospheric loops called convection cells that transfer heat around the globe.
While air is exchanging across the latitudes from the tropics to the poles, the rotation of the Earth gives the atmosphere a sideways push. This "force", called the Coriolis Effect, is tiny- far too small to make water spin one way or the other while going down your bathtub's drain. But, it is strong enough to make convection cells curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Time for some mental gymnastics. A hunk of 'still' air at the equator is actually moving eastward with the earth's surface at about 1000 miles per hour! If that packet of air starts to move straight north it will keep its eastward speed but the earth below it is moving more and more slowly the further north you go. This causes the air to curve to the right when observed from the earth's surface. (For those of you with more math and physics background, think of all this in terms of vectors and frames of reference.)
Winds in the Nothern Hemisphere are "deflected" to the right due to the rotation of the Earth.
The Coreolis Effect is commonly referred to as a "force", although it is not truly a "force". Forces can be measured and are related to mass and acceleration. The Coreolis Effect is simply something we observe due to the Earth's rotation. Winds, like any object in motion, want to move in straight lines. But, since the Earth is curved and rotating, winds look like they are being deflected.
Where convection cells converge, bands of prevailing winds and jet streams are created. The "easterlies" (west-blowing prevailing winds) at the equator are also known as the Trade Winds.
A sketch of surface winds created by convection cells and curved by the rotation of the Earth.
The prevailing winds at the surface of our planet are named after the direction from which they are blowing. The famous Trade Winds powered the ships of merchants, explorers, and pirates many years ago. Notice the Doldrums where winds are very mild and many early sailing ships were trapped and perished.
Clouds being circulated by prevailing winds can be seen clearly in satellite images on your nightly news. Jupiter also has strong prevailing winds and jet streams which are visible in images captured by Voyager. Since Jupiter screams along at one rotation every ten hours, it is terribly windy with many different jet streams.
But here on Earth, our relatively gentle prevailing winds and jet streams along with evaporation and precipitation in convection cells determines weather all around the globe.
Convection cells and prevailing winds, eh? I thought we were talking about El Niño?
