The Coriolis force, as mentioned before, effects the traversing of the Kelvin and Rossby waves in the Pacific Ocean. Because of the Earth's rotation, it appears to an observer on Earth that a force is always pushing the wind to the right of the direction of motion in the Northern hemisphere and to the left in the Southern Hemisphere [Laws, E. A., 1992]. To better understand this force, imagine traveling north or south from the equator. As your distance from the equator increases, the circumference of the earth decreases. This causes an exaggeration of the Earth's rotation under an object which moves independently of the Earth itself. The direction of the Earth's rotation thus causes the shift to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. In the case of the internal waves traveling over the pacific in the ENSO cycle, the Kelvin waves move in the relative direction of the Coriolis force (which effectively moves to the right, or towards South America). The Rossby waves, however, must move against this prevailing west-to-east motion and thus travel much more slowly.

The trade winds are caused by a cycle called Walker cell circulation. Waters in the western Pacific are much warmer than those in the east. The warmer air over waters in the west is consequently less dense than the air over waters in the east. As a result, by convection, the western air moves across the Pacific, losing altitude as it slowly cools. Lower air moving from the east to the west, on the other hand, rises when it is heated by the warmer eastern shores. Thus,these disparate temperatures cause a circulation of air which produce the trade winds. El Niño, however,changes that.

Another factor that must first be explained concerning the wind patterns in the Pacific (and elsewhere on the planet) is Meridional cell circulation. Although air tends to rise near the equator, as it moves poleward it radiates heat into outer space and eventually cools and sinks at about 30 degrees latitude. Similarly, cold air that sinks at the poles tends to be warmed as it flows along the surface of the Earth toward the equator and begins to rise near 60 degrees latitude.[Laws, E. A., 1992]

Given this knowledge, we can discuss the trade winds. First, understand that winds are named after the direction from which they are coming, not that in which they are going. The 'westerlies' in the Northern and Southern Hemispheres, therefore, flow from the west because of deflection from the rotation of the earth (which is rotating eastward). The tricky ones are the Northeast and Southeast trade winds. As the Earth's circumference decreases, the eastward rotation becomes slower. Moving north or south will give apparent eastward movement because of this slower rotation.

When El Niño brings its warmer waters to the South American coast, it raises the air temperature. In so doing, it brings the temperature of the east about even with that of the west. Consequently, there is nothing to fuel Walker cell circulation, and the trade winds decline dramatically. The absence of a greater pressure difference also means that warmer waters can stay on the coast. As a result, this part of the ENSO cycle serves as positive feedback by encouraging the prevailing El Niño (or lack there of) conditions.

Return to the Course: Phase 1.