From Environment Canada:

El Niño

For hundreds of years (the first available record dates 1567), South American fishermen have 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 this 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 pressure rises in the east, there is typically an accompanying decrease in the west, with the reverse also true, he coined the term Southern Oscillation to categorize his find.

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 & Local 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 and the weak winds from the east and heavy rainfall that accompany low pressure conditions.

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 (ENSO)”.

El Nino 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.

During El Niño winters As the jet stream travels over the North Pacific, it is likely to split on its approach to North America. A weaker branch would be diverted northward into the Northwest Territories while the lower subtropical branch (whose mean position is over the Pacific northwest/southwestern Canada) would be shifted several degrees of latitude southward. The southern Canadian region lies in between the two jets and receives a milder and drier-than-normal winter.

La Niña

La Niña, meaning the little girl, names the appearance of cooler than normal waters in the eastern and central Pacific Ocean. Sometimes called El Viejo, anti-El Niño, or simply “a cold event”, it is the antithesis of El Niño.

La Niña is thought to occur due to increases in the strength of the normal patterns of trade wind circulation. Under normal conditions, 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, periodically these trade winds are strengthened, increasing the amount of cooler water toward the coast of South America and reducing water temperatures.

 The increased amount of cooler water toward the coast of South America, causes increases in the deep cloud buildup towards southeast Asia, resulting in wetter than normal conditions over Indonesia during the northern hemisphere winter.

The changes in the tropical Pacific are accompanied by large modulations of the jet stream within the middle latitudes, shifting the point at which the stream normally crosses North America. The shifted jet stream contributes to large departures from the normal location and strength of storm paths. The overall changes in the atmosphere result in temperature and precipitation anomalies over North America which can persist for several months.

During La Niña winters, the airflow pattern would include a stronger jet stream moving across the eastern Pacific Ocean and into southern Canada. This position of the jet stream usually means colder-than-normal and wetter-than-normal winters for southern Canada from the B.C. west coast to the Great Lakes.