El Nino is a weather phenomenon that occurs in the Equatorial Pacific Ocean normally lasting between 6 to 18 months. First we should understand the normal conditions in the Equatorial Pacific Ocean to better understand about an El Nino.
Normal Conditions in the Equatorial Pacific Ocean (ENSO neutral) :
The Pacific Ocean stretches half way around the globe at the Equator from South American coast at 80W to Indonesia at 120 E. The Pacific ocean is very large and is divided into three parts viz. West Pacific, Central Pacific and East Pacific ocean. The ocean surface in the West Pacific is normally warm at around 29 C while the ocean surface temperature in the East Pacific is much cooler at around 22 C. to 26 C. depending upon the time of the year as shown in the seasonal changes of average ocean temperature map from CPC.
The ocean surface pressure is lower in the West Pacific compared to Central Pacific and East Pacific and hence Trade winds blow Westwards from the East Pacific towards the West Pacific. The ocean current also moves from East to West. Upwelling of cold water takes place near the coast of Peru/Equador. These deeper waters have nutrients that stimulate the growth of microscopic photosynthetic algae (“Phytoplankton“). Phytoplankton, in turn, serve as food for the zooplankton. Both phytoplankton and zooplankton serve as food for fish thereby increasing the fish production in the East Pacific.
Due to high ocean surface temperature in the West Pacific, there is evaporation and clouding and rain over West Pacific while the East Pacific there is dry condition. Sun heated warm water piles up in the West Pacific ocean and this is the reason why the temperature of the West Pacific ocean is higher than that of the East Pacific ocean.
El Nino is a weather phenomenon that occurs in the Equatorial Pacific Ocean normally of 6 to 18 months time frame when there is an abnormal warming of surface ocean waters in the Eastern & Central Equatorial Pacific Ocean. This occurs when the normal trade winds weaken (or even reverse), which lets the warm water that is usually found in the western Pacific to flow instead towards the east. This warm water displaces the cooler water that is normally found near the surface of the eastern Pacific, setting off atmospheric changes that affect weather patterns in many parts of the world. Upwelling deep water near the coast of Peru/Equador is now warm and do not have nutrients that stimulate the growth of microscopic photosynthetic algae (“Phytoplankton“) and the zooplankton. This in turn decreases the fish production in the East Pacific. This is why these conditions are called El Niño which is Spanish for “the boy”, and the term El Niño refers to the Christ child, Jesus, because this periodic abnormal weather pattern is usually noticed around Christmas.
What happens to the ocean also affects the atmosphere. Tropical thunderstorms are fueled by hot, humid air over the oceans. The hotter the air, the stronger and bigger the thunderstorms. As the Pacific’s warmest water spreads eastward, the biggest thunderstorms move with it. Thus, rains which normally would fall over the tropical rain forests of Indonesia start falling over the central & east Pacific ocean along with deserts of Peru.