ENSO events are the combination of ocean and atmosphere feedback loops. Normally, the conditions in California and Peru are a result of upwelling — cold water, cold air, low moisture, low precipitation, arid, dry conditions in a high pressure system. On the other side of the Pacific Ocean in East and Southeast Asia, the normal conditions are warm air, high precipitation in a low pressure system. The trade winds push water west so the Coriolis effect is zero at the equator. In an El Nino, the conditions flip flop. It starts with the trade winds weakening (nobody knows why) which causes the pressure to decrease and less upwelling. With less wind and upwelling, there is nothing keeping the cold water near the Americas. The warm pile of water near Asia relaxes and spreads across the Pacific to California and Peru. A mature El Nino is characterized by lots of rain, high humidity, and low pressure in California/Peru and arid temperatures and high pressure in Asia. The water off the coast of Peru can be up to 10 degrees warmer, and since there is less upwelling there is less phytoplankton in the water. …show more content…
The cooler weather in the west Pacific triggers a rise in the thermocline. The trade winds pick back up a lot, and the resulting powerful trade winds cause amplified normal conditions. A La Nina is characterized by these amplified normal conditions, the opposite of an El Nino. There are stronger trade winds, more upwelling, more productivity, and the sea temperature can be up to 5 degrees colder than average in eastern Pacific. Eventually the trade winds weaken to their normal state and conditions return to