The Role of Ocean Phenomena
The sea's influence on climate is periodically highlighted by two ocean phenomena that exert dramatic influences on weather patterns across the United States and many other countries. These phenomena are called El Nino and La Nina.
During an El Nino event, which usually lasts about a year and recurs every two to seven years, east-to-west trade winds in the tropical Pacific weaken or reverse direction. The change in winds causes ocean currents to flow eastward, transferring warm water from the western Pacific to the central and eastern Pacific. A low-pressure air mass—the type of air mass in which stormy weather develops—builds over the warm waters of the central and eastern Pacific. This air system carries heavy rainfall to the Pacific coast of South America. At the same time, a high-pressure system forms over the cool western Pacific and may lead to drought conditions in Southeast Asia.
Because the changes in air pressure associated with El Nino disrupt the normal circulation of the atmosphere, weather patterns in other parts of the world are also altered. In the United States, for example, El Nino events usually result in milder winters in the Midwest, heavy rains in the South, and dry conditions in the Pacific Northwest. Meteorologists said the El Nino of 1997-1998 led to severe flooding, landslides, and several deaths in North Carolina, Tennessee, and California.
A La Nina event often develops after an El Nino. La Nina is the climatic opposite of El Nino and occurs when strong trade winds push warm surface water westward, exposing lower cool waters in the east. As a result, a La Nina episode is characterized by cooler-than-normal water in the central and eastern Pacific and warmer-than-normal water in the western Pacific. This situation can lead to severe storms in Southeast Asia and drought in South America. Meteorologists said a La Nina that occurred in 1998-1999 also brought heavy rain and snow to the upper Midwest and the Pacific Northwest.
Oceanographers noted that conditions typical of La Nina continued into 2001, well beyond the one-to-two year length of a typical La Nina episode. They said this was probably due to the development of a long-term ocean condition called the Pacific Decadal Oscillation. Many scientists believe that this condition, which is characterized by cold waters off the Pacific coasts of North and South America, can last 20 to 30 years and may recur every few decades. They said this cold-water phase might cause harsh winter weather across the Midwest and Northeast for years to come.