A La Niña event emerged during the second half of 2020 and has since grown to be of moderate strength. This event is predicted to last through to at least April 2021. Find out more about La Niña and its impact in this blog.
What is ENSO?
The El Niño/Southern Oscillation (ENSO) is a naturally occurring phenomenon involving fluctuating ocean surface temperatures in the central and eastern equatorial Pacific, accompanied by changes in the overlying atmosphere. ENSO is the most prominent inter-annual climate variation on the Earth, with widespread effects on the global climate system and large ecological and societal impacts.
Opposing oceanic components El Niño and La Niña, are respectively the unusual warming and cooling of oceanic surface water temperatures over the central and eastern equatorial Pacific Ocean, which affect the atmospheric circulation, throughout the world; while the Southern Oscillation is the atmospheric counterpart, based on the pressure difference between two tropical South Pacific locations; Darwin, Australia, and the South Pacific island of Tahiti. Over periods of a month or longer, higher than normal pressure at one site is almost always concurrent with lower pressure at the other, and vice versa. This pattern which reverses every few years represents a “see-saw”, of air masses oscillating back and forth across the International Date Line in the tropics and subtropics. Together these features give rise to the phenomenon El Niño/Southern Oscillation.
The El Niño/Southern Oscillation comprises three phases: El Niño (warm phase), La Niña (cold phase) and neutral phase.
What is La Niña?
La Niña — translated from Spanish as “little girl” – is a recurring naturally occurring phenomenon involving unusual large-scale cooling of the ocean surface temperatures across the central and eastern equatorial Pacific that affects global weather patterns. During La Niña events, the trade winds in the central and eastern equatorial Pacific strengthen, and the pattern is a more intense version of the “normal conditions”, with a much colder block of sea surface temperatures in the central and eastern equatorial Pacific. Compared to El Niño events, where the trade winds weaken, leading to a rise in sea surface temperature in the central and eastern equatorial Pacific. La Niña is therefore the cold extreme of sea surface temperature changes in the central and eastern Pacific Ocean, away from the average state.
When is La Niña Declared?
In order for La Niña conditions to be declared, sea-surface temperatures (SSTs) across a region in the eastern and central tropical Pacific Ocean defined as Niño3.4 (120-170W, 5S-5N) must be at least 0.5°C below the seasonal average, with accompanying changes to the atmospheric conditions. To be deemed as a La Niña episode in the historical record, these conditions must be sustained for at least five overlapping three-month periods.
Why are we concerned?
La Niña is associated with extremely wet conditions, below or above average temperature and rainfall extremes across different regions which can lead to hazards such as floods, landslides and slips. However, this is the statistical picture, since the impacts of La Niña vary from one La Niña to the next and no two La Niña’s have proven to be the same.
When was the last La Niña?
The last strong La Niña event occurred late 2010 to early 2011, followed by a moderate event from late 2011 into 2012 and a weak one in from late 2017 into 2018.
How often does La Niña typically occur?
La Nina events occur every 3 to 5 years or so, but on occasion can occur over successive years.
How long does La Niña typically last?
La Niña events typically last approximately 9-12 months but some episodes can persist for as long as 2 years.
Impacts of La Niña
La Niña impact on Trinidad and Tobago’s rainfall and temperature is not uniform but varies not only from location to location and from season to season, but also from one La Niña event to another. The outcomes of each La Niña event depend on the intensity of the event, the time of year when it develops and the interaction with other climate influencers.
Why are La Niña conditions important for Trinidad and Tobago?
La Niña during the period November to February is usually at its strongest, and historically, the most commonly experienced impacts in Trinidad and Tobago during this same period when La Niña is present are wetter than usual conditions. This is because La Niña favours upward moving air in the Caribbean region and tends to be associated with reduced wind shear; thus making it easier for rain producing clouds to form. The local atmospheric readjustments most often triggered by La Niña are an increase in the frequency of visits to our region by migrating shear lines or zones of low level convergence. These are due to fragments of, or decaying elongated cold fronts reaching as far as the southern Caribbean region. This often results in cooler conditions, especially colder nights. La Niña typically peaks during December to February. It is likely that at its peak, the current La Niña will be moderate to strong.
The Trinidad and Tobago Meteorological Service provides an El Niño/La Niña Watch/ENSO Monitor, which indicates whether the El Niño/Southern Oscillation (ENSO) is in El Niño (warm phase), La Niña (cold phase) or neutral phase. It provides an assessment of the current and future state of ENSO and likely effects on the local climate. The latest update can be accessed at: https://www.metoffice.gov.tt/enso.