El Niño

This site contains comprehensive information on the past and present impact of El Niño on the climate of Canada and its effects on the global climate. Through animation, this site shows the changes in the Canadian climate and climatic extremes that accompany El Niño. Along with a scientific explanation of the El Niño phenomenon, comparisons between various El Niños and forecast of El Niño are also provided.

• Brief History of El Niño
  
• Why El Niño Occurs
  
• How Information from the Tropical Pacific is Transmitted
  
• How El Niño Affects North America via Changes in Jet Stream
  
• El Niño and Climate Change
  
• Quick Facts on El Niño
  
• Related Section

Brief History, and the Origin of El Niño

For hundreds of years (the first available record dates to 1567), South American fishermen noticed the appearance of warm waters in the eastern Pacific Ocean along the coast of Ecuador and Peru. As the phenomenon typically becomes apparent around Christmas, the name "El Niño", or the Christ Child was eventually bestowed.

At the turn of the 20^th century, a connection between El Niño and other weather patterns had yet to be established. During the 1920s, the head of the Indian Meteorological Service, Sir Gilbert Walker, recognized patterns to the rainfall in South America. His discovery lead him to theorize additional associations with the change in the ocean temperatures, and with atmospheric pressure changes measured at stations on both sides of the Pacific (Tahiti and Darwin, Australia).

Noticing that as the atmospheric pressure rises in the east, there is typically an accompanying decrease in pressure in the west, with the reverse also true, he coined the term Southern Oscillation to categorize his findings.

Further study led to the realization that Asian monsoon seasons under certain barometric conditions were often linked to drought in Australia, Indonesia, India, and parts of Africa and mild winters in western Canada. (see: Global Effects)

Not until the late 1960s did a Norwegian meteorologist, Jacob Bjerknes, a professor at the University of California, establish the connection between the changes in sea surface temperatures, the weak trade winds from the east and heavy rainfall that accompany low atmospheric pressure in eastern tropical pacific ocean.

Ultimately, Bjerknes' discovery led to the recognition that the warm waters of El Niño and the pressure variance of Walker's Southern Oscillation are interrelated, leading to the full naming of the phenomenon as :"El Niño Southern Oscillation".

The term El Niño has now come to refer to a much larger scale phenomenon associated with warmer-than-normal waters that occasionally form across the eastern & central tropical Pacific.

Why El Niño Occurs

El Niño is thought to occur due to changes in the normal patterns of trade wind circulation. Normally, these winds move westward, carrying warm surface water to Indonesia and Australia and allowing cooler water to upwell along the South American coast. For reasons not yet fully understood, these trade winds can sometimes be reduced, or even reversed. This moves warmer waters toward the coast of South America and raises water temperatures. Warmer water causes heat and moisture to rise from the ocean off Ecuador and Peru, resulting in more frequent storms and torrential rainfall over these normally arid countries.

Historically El Niño events occur about every 2 to 7 years and usually alternate with the opposite phase of below-normal water temperatures in the eastern & central tropical Pacific.

Cross section in the tropical Pacific during normal conditions. Winds from the east (indicated by white arrows) push water westward along the equator where solar radiation produces warmer sea surface temperatures. A convection circulation sets up producing abundant rain in the western tropical Pacific and drier conditions in the eastern tropical Pacific.

Cross section in the tropical Pacific during El Niño conditions. Winds from the west (indicated by white arrows) bring warmer water eastward. The convective circulation also shifts eastward producing abundant rains in the eastern and central tropical Pacific and drier conditions in the western tropical Pacific.

How Information from the Tropical Pacific is Transmitted

The appearance of unusually warmer than normal waters in the eastern tropical Pacific Ocean (shown by pink) ushers the arrival of El Niño. As a result of this warm pool of water, the atmosphere is heated and conditions favourable for convection and precipitation occur there. This added heat in the atmosphere interacts with the mean west-to-east flow of air in the middle latitudes, and forms a train of atmospheric waves denoted by high and low pressure centres from the tropics towards the North Pacific. Due to the Coriolis force of the Earth, this wave train of highs and lows curve eastward as they travel northward. The map on the right shows the typical locations of the anomalous highs and lows pressure centres that form in El Niño winters. An anomalous low pressure centre forms in the Gulf of Alaska, and a ridge of high pressure establishes over central and western Canada. These features in the atmosphere alter the course of the jet stream which subsequently changes the patterns of temperatures and precipitation.

How El Niño Affects North America via Changes in Jet Stream

Typical circulation anomaly of air at 500 hectapascals (at an altitude of about 5 km) during past strong El Niños in decametres. Blue areas show below normal pressure, while higher-than-normal pressure occurs in the red regions. Thick black lines indicate position of jet streams. With a ridge of high pressure, most of Canada experiences above normal winter temperatures.

El Niño and Climate Change

Climate scientists are questioning whether climate change, as a result of anthropogenic effects such as the enhanced greenhouse effect, may be affecting the observed increase in strength and frequency of El Niño events since the late 1970s. These changes in El Niño are likely a reflection of warming of the eastern equatorial Pacific. In response to greenhouse gas increases, some model simulations show an increase in ENSO variability while others exhibit no significant increase. Thus, as yet there is no consistent picture of how ENSO variability might be expected to change in response to enhanced greenhouse gases. Further research is needed before scientists can provide confident answers.   

Quick Facts on El Niño

• The phrase "El Niño" refers to the Christ Child and was coined by fishermen along the coasts of Ecuador and Peru to describe the warm ocean current that typically appeared around Christmas time and lasted for several months.
• El Niño is the second largest driver of the world's weather, second only to normal seasonal warming and cooling, which also brings changes in precipitation patterns.
• El Niños appear approximately every two to seven years. They typically last 12 to 18 months. In the early 1990s, a protracted El Niño persisted for four years.
• El Niños have been documented since the early 1700s. More detailed observations from ships led to instrumental record keeping in the earlier half of the twentieth century. It is only since the 1970s, however, that scientists began linking El Niño to massive flooding and severe droughts around the world.
• About every four to five years, a pool of cooler-than-normal water develops off South America. The effects of this cooler water are called La Niña. This usually brings colder winters to the Canadian west and Alaska and drier, warmer weather to the American southeast.

Related Section

• La Nina