An El Niño is a warming of the Pacific Ocean between South America and Papua New Guinea. This occurs when the trade winds in that region weaken causing warm water to move closer to the west coast of South America and the thermocline to move deeper. This is shown in the image below.
Image Courtesy of Wikimedia
The thermocline is the boundary between warmer surface water and colder deep water.
These conditions result in more precipitation in the western United States but allow for a cooler winter for the southeastern US. Typically, with an el niño, you will see warmer conditions.
When a La Niña occurs, it is a cooling of the Pacific Ocean between Papua New Guinea and South America. This is the opposite of an el niño. The formation of a la niña begins when the trade winds strengthen causing warm surface water to be pushed further away from the west coast of South America. This allows for cool deeper water to rise through upwelling.
This causes the thermocline to move up.
The results of a la niña are almost the opposite of an el niño. Instead of generally rising temperatures, we see cooler temperatures as well as wet conditions. However, in the southeastern US, we see the opposite, warmer and drier conditions.
Greater Environmental Impacts
An el niño can have impacts that are shown around the world. First, when the ocean warms, different marine species may have to leave because they are unable to live in warm conditions. Furthermore, these temperatures changes may impact the migration of marine birds and species overall.
On a more global scale, a warmer ocean won’t be able to absorb as much CO2 leaving it in the atmosphere. As for the weather, flooding and drought can result from precipitation changes caused by these temperature changes in the Pacific.