The weather over large parts of the world may be influenced by weak La Niña conditions in the next few months for the second consecutive year, according to a new Update from the World Meteorological Organization.
La Niña, also known as a “cold event,”refers to the large-scale cooling of the ocean surface temperatures in the central and eastern equatorial Pacific along with changes in the tropical atmospheric circulation.
El Niño and La Niña are opposite phases of atmosphere-ocean interplay over the tropical Pacific, collectively referred to as the El Niño/Southern Oscillation (ENSO).
They have opposite effects on weather and climate patterns. Areas which receive below average rainfall during an El Niño tend to receive above average rainfall during a La Niña and vice versa.
“Large-scale climate events like La Niña extend their influence over countries which are home to many millions of people. Our ability to predict these events in advance is vitally important to help planning in sensitive sectors like farming, water management, public health, energy and transport and provide early warnings of the associated risks,” said WMO Secretary-General Petteri Taalas.
The WMO Update says that since August sea surface temperatures in the eastern tropical Pacific cooled rapidly, approaching La Niña levels. Atmospheric patterns have however largely remained ENSO-neutral. For a typical La Niña event to occur, it is essential for both ocean and atmosphere to display the associated characteristic features.
Weak La Niña conditions may develop, with about 50-55% probability, in the final quarter of 2017, according to the Update, which is based on climate models from leading prediction centres around the world and expert assessment of these models.
The La Niña event, if it does occur, will likely remain weak, and conditions are expected to return to ENSO-neutral in the first quarter of 2018.
Persistence of the present ENSO-neutral conditions into the rest of the year is also a plausible scenario, with 45-50% likelihood. The emergence of an El Niño can be practically ruled out, according to WMO.
There was a borderline La Niña event late last year. Two consecutive episodes of La Niña following a strong El Niño, such as the one that occurred in 2015-16, is historically not uncommon.
Note to editors:
The outcomes of La Niña are generally opposite to those of El Niño, but depend on the intensity of the event, the time of year it develops and the interaction with other climate patterns.
La Niña episodes feature a wave-like jet stream flow over the USA and Canada in the northern winter, with colder and stormier than average conditions across the North and warmer and less stormy conditions across the South.
Although every La Niña is unique, certain patterns often appear. For example, La Niña is often associated with wet conditions in eastern Australia, Indonesia, the Philippines, Thailand and South Asia. It usually leads to increased rainfall in North Eastern Brazil, Colombia and other northern parts of South America, and drier than normal conditions in Uruguay, parts of Argentina, coastal Ecuador and northwestern Peru.
La Niña events are generally associated with increased rainfall in southern Africa, and rainfall deficiency in equatorial eastern Africa – for instance eastern Kenya and Somalia, as well as deficient northeast monsoon rainfall in the southernmost parts of South Asia.
La Niña tends to suppress tropical cyclone activity in the central and eastern Pacific basins, and enhances it across the Atlantic basin.
It is important to note that El Niño and La Niña are not the only factors that drive global climate patterns. Regionally and locally applicable information is available via regional and national seasonal climate outlooks, such as those produced by WMO Regional Climate Centres (RCCs), Regional Climate Outlook Forums (RCOFs) and National Meteorological and Hydrological Services (NMHSs).