What is La Niña?

Satellite images shows that the tropical Pacific has switched from warm, or higher-than-normal sea surface heights (shown in red) to cold, or lower-than-normal sea surface heights. NASA file photos.

Chris Scott, meteorologist

La Niña vs. El Niño

La Niña and El Niño are phases of the climate cycle called the El Niño/Southern Oscillation which describes the ocean and atmospheric patterns occurring in and over the equatorial Pacific Ocean.

El Niño is characterized by warmer than normal sea-surface temperatures west of South America that endure for several seasons, where as La Niña represents the opposite cooler phase.

Changes in water temperature across vast stretches of the Pacific Ocean cause disruptions in the trade winds and weather conditions. Both phenomena have a large impact on the jet stream and weather patterns around the world.

Stronger trade winds in the central Pacific bring cold waters up to the surface.

El Niño and La Niña events vary in intensity and recur on a semi-regular basis between two and seven years apart. La Niña and El Niño can be thought of as opposite ends of a seesaw or teeter-totter. The water temperatures in the east-central equatorial Pacific go up and down over a period of many months and years as part of a natural cycle that involves motions in the ocean and atmosphere. The last La Niña episode occurred from the fall of 2007 through spring of 2008.

La Niña and Canadian winter

La Niña winters tend to have large month-to-month variations in temperature, precipitation and storminess across Canada. A more highly variable jet stream pattern over the east Pacific Ocean can often bring wetter and snowier than normal conditions to British Columbia.

Additionally, the jet stream pattern tends to allow more cold-air outbreaks to occur from Western Canada through the Great Lakes. During a La Niña winter, the orientation of the storm track across eastern North America favours increased precipitation across Ontario and Quebec.

La Niña conditions in the Pacific Ocean can impact the hurricane season in the Atlantic Ocean. Generally, La Niña conditions are associated with an increased number of hurricanes in the Atlantic, particularly those that originate from waves of low pressure that move westward from Africa and cross the tropical Atlantic Ocean.

Hurricanes require very uniform winds from the ground to the jet stream level, and La Niña conditions tend to support this requirement. The opposite occurs during El Niño when increased wind shear across the tropical Atlantic Ocean tends to disrupt hurricane development.