La Niña is a massive ocean-atmosphere phenomenon that has dramatic effects on global weather. It is part of a broader climate pattern known as the El Niño Southern Oscillation (ENSO).Climate variability explains short phases in climate that can occur over months, seasons, and years. This variability is caused by natural, large scale features such as La Niña. It is a part of what climate scientists have to model in order to understand climate variability.
El Niño Southern Oscillation is a singular climate occurrence which is comprised of three phases. The first two phases are El Niño and La Niña which are two opposite phases. Both of these phases change ocean and atmosphere patterns. Since ENSO events are considered to be a coupled climate occurrence, the third phase is the neutral phase, referring to a middle range.
Effects and Duration
La Niña originates in the central and eastern tropical Pacific Ocean, where below average sea surface temperatures occur. These changes from normal surface temperatures can have large-scale impacts ocean processes, but also on global weather and climate. During a La Niña event, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest. In regions such as Indonesia, Malaysia, and northern Australia, heavy rainfall become more common while in contrast rainfall decreases in the central tropical Pacific Ocean.
In addition, normal easterly trade winds along the equator become quite strong. The trade winds are enhanced by a pressure gradient between the eastern and western Pacific. This causes a surge of strong wind along the South American coast which contributes to colder surface waters over the eastern tropical Pacific Ocean and warmer surface waters in the western tropical Pacific Ocean.
La Niña episodes typically last 9 to 12 months, however some prolonged events may last for years. La Niña events occur on average every 2 to 7 years. Typically, El Niño occurs more frequently than La Niña.
The global effects of La Niña are characterized by above average wetter conditions west of the equatorial central Pacific Ocean over Northern Australia and Indonesia during the northern hemisphere winter season, and over the Philippines during the northern hemisphere summer season. Southeastern Africa and northern Brazil are also regions that experience wetter conditions during the northern hemisphere winter season. During the northern hemisphere summer season, northwestern India receives heavier rainfall during their monsoon period. In contrast to wetter conditions, El Niña brings above normal drier conditions to other regions located along the west coast of South America, Gulf Coast, and in regions from southern Brazil to central Argentina during their winter seasons.
Below is a video to help understand what La Niña is with visual aid:
- "Variability and impacts from El Niño and the Southern Oscillation", Intergovernmental Panel on Climate Change, 2016. [Online]. Available: http://www.ipcc.ch/ipccreports/tar/wg2/index.php?idp=522. [Accessed: 15- May- 2016].
- "What are El Niño and La Niña?", National Oceanic and Atmospheric Administration, 2016. [Online]. Available: http://oceanservice.noaa.gov/facts/ninonina.html. [Accessed: 10- May- 2016].
- "La Niña Fact Sheet : Feature Articles", NASA, 2016. [Online]. Available: http://earthobservatory.nasa.gov/Features/LaNina/. [Accessed: 13- May- 2016].
- Image from wikimedia (and uncited, but it's on the La Nina page on August 1st, 2016), data from NOAA. (Accessed May 20, 2016). Cold and Warm Episodes by Season [Online], Available: http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml
- Australian Bureau of Meteorology. (Accessed May 20, 2016). La Niña - Detailed Australian Analysis [Online], Available: http://www.bom.gov.au/climate/enso/lnlist/index.shtml
- "Understanding La Niña", NOAA Climate.gov, 2016. [Online]. Available: hhttps://www.youtube.com/watch?v=fAvk4RXrW_E. [Accessed: 15- May- 2016].