This page describes the El Niño-Southern Oscillation (ENSO) phenomenon and provides information on the past and current impact of El Niño on the North American climate.
What is El Niño?
El Niño can be distinguished when the surface waters in the eastern tropical Pacific extending westward from Ecuador become warmer than average. The changing pattern of the Pacific Ocean causes a shift in the atmospheric circulation, which then impacts weather patterns across much of the earth.
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 across the tropical Pacific allowing upwelling of deep, cold water along the South American coast. For reasons not yet fully understood, these trade winds are sometimes reduced, or even reversed. Warmer waters slosh back toward the coast of South America and increase water temperatures in that region. Warmer water causes heat and moisture to rise from the ocean off the coast of Ecuador and Peru, resulting in more frequent storms and torrential rainfall over these normally arid regions.
Historically, El Niño events occur about every two to seven years and usually alternate with a cold phase and a warm phase in the eastern and central tropical Pacific.
Cross section in the tropical Pacific during normal conditions
During normal conditions, winds from the east, also referred to as easterly winds (indicated by white arrows), transport water westward along the equator where solar radiation contributes to the enhancement of warmer sea surface temperatures in the western Pacific, and allows colder water to upwell along the coast of South America. The warm sea surface water in the western Pacific warms the air above it. A circulation cell (indicated by the convective loop) is set up over the tropical Pacific causing cool air to descend over the eastern tropical Pacific and west coast of South America. The resulting high pressure over the region creates drier conditions.
El Niño Conditions
Cross section of the tropical Pacific during El Niño conditions
The arrival of El Niño is characterised by the reversal of the normal flow pattern in the tropical Pacific. Weaker trade winds allow the accumulated warmer water in the western tropical Pacific to traverse back eastward (indicated by the white arrows). This weakens the contrast between the sea surface temperatures in the western and eastern tropical Pacific. In time, the warm surface waters cover most of the tropical Pacific, thus supressing the upwelling of cold waters in the eastern Pacific. The convective circulation also shifts towards the central and eastern tropical Pacific producing abundant rains in those regions and drier conditions in the western tropical Pacific.
How El Niño affects North America via changes in jet stream
Atmospheric circulation departure from normal
El Niño winter
El Niño Winter. Click for more details.
Degrees Celsius (℃)
|Dark Red||6 ℃ and over||The temperatures in these regions are significantly warmer than average temperatures by at least 6℃.||None|
|Red||4 ℃ to 6℃||The temperatures in these regions are much warmer than average temperatures by 4 to 6℃.||Region surrounding Hudson’s Bay - touching North West Territories, Northern Ontario and Northern Quebec|
|Orange||2 ℃ to 4℃||The temperatures in these regions are warmer than average temperatures by 2 to 4℃.||Regions of the Northwest Territories , parts of the northern United States, Southern part of Ontario, Southwestern parts of Québec, the Maritimes and Labrador.|
|Light Orange||1 ℃ to 2℃||The temperatures in these regions are slightly warmer than average temperatures by 1 to 2℃.||Eastern sections of Yukon and British Columbia down into the northern/central sections of the United States.|
|Light Yellow||0.5 ℃ to 1℃||The temperatures in these regions are slightly warmer than average temperatures by 0.5 to 1℃.||Very thin region in eastern sections of Yukon and British Columbia down into the northern/central sections of the United States.|
|White||-0.5 ℃ to 0.5℃||Almost no weather anomalies. The temperatures in these regions are average.||Regions of western Yukon down the Pacific Coast and intersects the central United States.|
|Pale Blue||-1 ℃ to -0.5℃||The temperatures in these regions are slightly colder than average temperatures by -0.5 to -1℃.||Regions of central-southern United States and parts of central Mexico.|
|Light Blue||-2 ℃ to -1℃||The temperatures in these regions are slightly colder than average temperatures by -1 to -2℃.||Regions of central-southern United States and parts of central Mexico.|
|Blue||-4 ℃ to -2℃||The temperatures in these regions are colder than average temperatures by -2 to -4℃.||Over the center of the northern Pacific as well as regions of the south-eastern United States.|
|Dark Blue||-4 ℃ to -6℃||The temperatures in these regions are much colder than average temperatures by -4 to -6℃.||Over the centre of the northern Pacific Ocean|
|Darker Blue||-6 ℃ and under||The temperatures in these regions are significantly colder than average temperatures by at least -6℃.||Over the centre of the northern Pacific Ocean|
Typical air pressure anomaly in decameters (1 decameter = 10 meters) at 500 hectopascals (at an altitude of about 5 km) during past strong El Niños. Blue areas show below normal pressure, while higher-than-normal pressure occurs in the red regions. With a ridge of high pressure, most of Canada experiences above normal winter temperatures. Thick black lines indicate position of jet streams. The subtropical jet stream shifts farther south, and storms associated with this jet stream produce wet weather in California and the southern portions of the United States (represented by the blue areas;) and relatively milder and drier than normal weather in the northern United States and Canada (represented by the red areas;).
El Niño summary: 2015 to 16
In terms of sea surface temperature anomalies in the Equatorial Pacific, the great El Niño of 2015-16 equalled the El Niño episode registered in 1997-1998. These two episodes are the strongest El Niño events observed in the recent decades. However, when measuring additional parameters, such as shifts in atmospheric deep convection and subsurface ocean temperatures, the 2015-16 event is considered to be weaker than the great El Niño of 1997-98. These two parameters are regarded as critical for El Niño’s impact on remote regions such as Canada or the United States. Yet, the 2015-16 boreal winter had most of the characteristics of a typical El Niño winter, based on El Niño historical events.
- The 2015-16 El Niño officially started in March 2015 and ended in May 2016, according to the National Oceanic and Atmospheric Administration (NOAA) El Niño Advisories.
- Over Canada, winter 2015-16 (Dec-Jan-Feb) was one to five degree Celsius warmer than normal across all provinces with especially unseasonal warmth in Quebec, the central Prairies, and Yukon. Subsequently, the 2016 spring remained warmer than normal in the western Canada and the Prairies, while an inflow of cold Arctic air led to colder than normal conditions over Eastern and Northeastern Canada.
- The 2015-16 El Niño brought moderate relief to the severe drought that has persisted in California since 2011, except in the southern part of the state where precipitation remained below normal.
- By shifting the convection centers in the Equatorial Pacific, the 2015-16 El Niño contributed to a weaker Indian monsoon in summer 2015, drought in Central America during summer 2015, drought in Indonesia during the fall 2015 and flooding in northern Peru in fall 2015.
- The year 2015 was the warmest year recorded since sufficiently accurate observations became available in the late 1800s, according to several meteorological agencies. This was due to the combined influences of the 2015-16 El Niño and the overall warming trend of the Earth’s climate. Furthermore, the previous temperature record established in 2014 was broken by the highest margin recorded to date.
The World Meteorological Organization (WMO) has produced an animation that explains the 2015-16 El Niño event.
What are the El Niño impacts in Canada?
Historically, Canada is mostly affected by El Niño during winter and spring. Milder than normal winters and springs occur in Western, Northwestern and Central Canada. Generally, El Nino does not significantly impact Eastern Canada, including the Maritimes, but it may reduce tropical cyclone activity in the Atlantic Ocean.
El Niño and climate change
Climate scientists are studying 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. Currently, a direct link between El Niño and climate change has not been found. The strengthening subtropical jet stream reduces the strength and frequency of hurricanes in the Atlantic Ocean. In response to greenhouse gas increases, some computer model simulations show an increase in El Niño Southern Oscillation (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 effect. 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 climate, second only to normal seasonal warming and cooling, which also brings changes in precipitation patterns.
- El Niño events appear approximately every two to seven years. They typically last 12 to 18 months. In the early 1990s, a prolonged El Niño persisted for four years.
- El Niño events 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.
- Several geographical zones are used in the equatorial Pacific to monitor El Nino development and propagation: Nino 1+2 zone is near the South America coast tends to warm up first, Nino 3 is prominent for large sea surface temperature (SST) variability, Nino 3.4, in the central Pacific, includes the key region for coupled atmospheric-ocean interactions. Nino 4, in the western Pacific, serves as a strong indicator for precipitation.
- 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.
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