El Niño is the abnormal warming of the surface ocean at the eastern tropical Pacific. No one really knows how El Nino forms. It can cause climate change with parts like Australia and Indonesia getting droughts and parts of South America and North America to get storms and floods. Some signs of an El Nino forming include the trade winds to blow weaker or blow at a different direction. Also there is the Southern Oscillation, which is the air part of El Nino. It causes a reverse in air pressure throughout the Pacific Region.
To try and understand better of an El Nino, scientists have placed buoys around the Pacific ocean to try and get data from them and see if there are any unusual changes that happen. They can also use satellites to get a bigger view of the ocean and try and tell the temperature from what it sees.