Nino El Ni o is a phenomenon that occurs when the ocean and air currents change. The trade winds slow and cause a depression, which leads to an increase in ocean temperature. These changes cause increased rainfall across the southern United States and drier conditions in the west. In normal conditions, the trade winds blow in a westerly direction across the Pacific Ocean, carrying warm water to the west. Colder water rises from below the surface near South America.
Rainfall occurs when air rises above the warm water in the west and the east Pacific area is dry.
When the easterly tradewinds become weakened, the colder water doesn t rise as much, so the water surface stays warmer. The rain builds up over the warmer water and therefore drenches the eastern areas, and the western areas stay dry. El Ni o events can be tracked by taking measurements of the ocean temperatures and testing the strength of the tradewinds. Since the atmosphere and the ocean together create the wind, any changes in either or both of these can be important. There are various theories about what causes El Ni o, such as thunderstorm activity that creates strong winds blowing to the east and starting the process of preventing the cooler water to rise.