La Niña refers to persistent colder-than-normal (0.5°C or greater) sea surface temperature (SST) anomalies across the central and eastern equatorial Pacific (170°W to 120°W longitude and 5°N to 5°S latitude). La Niña is part of the phenomena known as the El Niño Southern Oscillation (ENSO). An oscillation is a motion that repeats itself over a period of time. For ENSO, that period is typically between 3 and 7 years. Its opposite counterpart is El Niño, which has warmer-than-normal (0.5°C or greater) sea surface temperature anomalies across the same equatorial waters.
The importance of these SST anomalies lies in the fact that they largely dictate where tropical thunderstorms will develop and be the most persistent. Thunderstorms thrive over warm ocean waters in the same way as tropical storms and hurricanes. During La Niña events, the warmest ocean waters are confined to the western equatorial Pacific region. Therefore, this is the preferred placement for tropical thunderstorms during the Northern Hemisphere cold season. These thunderstorms can be considered as a “bridge” between the ocean and the atmosphere. As these thunderstorms develop, they induce low pressure within the western Pacific region. Meanwhile high pressure sets up across the eastern equatorial Pacific where thunderstorms are less favorable (left hand figure below). This leads to stronger easterly trade winds (flow from high to low pressure). In return, these stronger trade winds help reinforce the SST pattern by pushing the warm water west and enhancing the strength of the cool eastern Pacific water due to upwelling. This coupled tropical circulation, between the atmosphere and ocean, has been linked to other seasonal circulation and weather anomalies throughout the globe.
| La Nada or
Equatorial winds gather
warm water pool toward
west. Cold water upwells
along South American coast.
Typically, La Niñas (average 15.4 months) last longer than El Niños (the average is 9.5 months). Since 1950, the longest La Niña was 37 months, from the spring of 1973 through the spring of 1976. Meanwhile, the longest El Niño was 18 months, from the summer of 1986 through the early spring of 1988.
The strength of La Niña is determined by the extent that the sea surface temperature anomalies fall at or below the -0.5°C threshold. A La Niña is classified as weak when the anomalies range from -0.5°C to -0.9°C, moderate when the anomalies range from -1.0°C to -1.4°C, and strong when the anomalies are -1.5°C or less. The table below lists all thirteen of the La Niña episodes, including their maximum intensities, since the winter of 1949-50.
|La Niña Episodes Since the Winter of 1949-50|
|1950-51||Not Available||FMA||-1.7||DJF 1949-50|
& NDJ 1962-63
& OND 1964
& JFM 1971
& NDJ 1988-89
|* Season is defined as a consecutive 3-month period. For example, SON stands for September, October, and November.|
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