Under "normal" conditions in the Pacific Ocean, the westerly winds tend to push water to the west along the Equator. This causes water to "pile up" in the west, and results in a circulation pattern that "pulls" deep water up off the coast of N. and S. America. This "upwelling" of cold, nutrient-rich water (dark blue) feeds a very productive fishery in this area. In the atmosphere, the heated surface waters of the western Pacific (red) generate convection cells (rain storms) that move back in an eastern direction (the "convection loop" in the diagaram).

Under El Nino conditions (bottom right), the prevailing westerly winds weaken and ocean currents may actually reverse to an eastern course. This eliminates the "pull" on the deep ocean water along the coast, and the upwelling does not occur, resulting in massive fish kills, "red tide" algal blooms, and generally poor fishing. In the atmosphere the convection cells (thunderstorms) form much closer to the America's coastlines, resulting in much heavier rains on the land surface.