Explanation:

Normally the northward flowing Humboldt Current brings relatively cold water from the Southern Ocean northwards along South America's west coast to the tropics, where it is enhanced by up-welling taking place along the coast of Peru.  Along the equator trade winds cause the ocean currents in the eastern Pacific to draw water from the deeper ocean to the surface, thus cooling the ocean surface. Under the influence of the equatorial trade winds, this cold water flows westwards along the equator where it is slowly heated by the sun.  As a direct result sea surface temperatures in the western Pacific are generally warmer than those in the Eastern Pacific. This warmer area of ocean is a source for convection and is associated with cloudiness and rainfall. During El Nino years the cold water weakens or disappears completely as the water in the Central and Eastern Pacific becomes as warm as the Western Pacific.