WMO El Niño/La Niña Update R. Kolli WMO

El Niño and La Niña El Niño (1997) El Niño (La Niña) are terms used to describe extensive warming (cooling) of the surface waters over the central and eastern equatorial Pacific Ocean. These events can cause unusual and sometimes severe weather events and patterns, not only in the immediate area, but also around the world. They have a recurring pattern with an irregular cycle of 2 to 7 years, but no two events are identical. Many groups, research institutes and NMHSs are working to understand El Niño and La Niña, to improve prediction and to clarify variability and impacts. CCl and WCP proactively engage these groups to develop partnerships/ collaboration, and thereby enrich global achievements and support synergy of effort.

ENSO Impacts Once developed, El Niño and La Niña events are known to shift the seasonal temperature and precipitation patterns in many different regions of the world, even ones that are distant from the equatorial Pacific Ocean. Such teleconnections occur because rainfall, which is associated with sea surface temperatures in the tropics, affects wind patterns in the atmosphere. Shifts in tropical convection patterns, rainfall and winds can also affect regions outside of the tropics by altering prevailing wind patterns. In several parts of the tropics, and some areas outside of the tropics, these seasonal shifts are fairly consistent from one El Niño and La Niña event to the next. It is important to remember, however, that no two El Niño or La Niña events are identical and that the seasonal shifts in temperature and precipitation patterns associated with them can vary from one event to the next. Thus, when an El Niño or La Niña develops, it does not guarantee that regions which are typically affected by them will be affected, only that there is enhanced probability that this will be the case.

El Niño and La Niña Impacts

WMO El Niño/La Niña Update The WMO El Niño/La Niña Update is a consensus report prepared in collaboration with the International Research Institute for Climate and Society (IRI) and with contributions from NMHSs, regional and global prediction/research centres and individual experts. Seasonal climate outlooks, as produced by NMHSs, provide detailed information on expected impacts, after considering other factors that influence regional climate. In considering response strategies, it is important to consider regional climate outlooks and not to rely solely on the presence of El Niño or La Niña.

How the message spreads

The Latest WMO El Niño/La Niña Update (30 March 2010) The ongoing El Niño event continues to have significant and widespread impacts. The signature of this event, which started in June 2009, is seen in basin-wide Pacific Ocean conditions, and in many of the climate patterns around the world that are typically impacted by an El Niño event. The most likely outcome by mid-year 2010 is for the El Niño event to have decayed and near-neutral conditions to be re-established across the tropical Pacific. However, this time of year (March-June) is a particularly difficult period for forecasting developments in the tropical Pacific, and forecasters cannot rule out persistence of El Niño or the possible early stages of La Niña by mid-year. Even during the decaying phase of the El Niño expected over the next few months, the conditions associated with it will continue to influence climate patterns at least through the second quarter of the year, and this information will be contained in the available national and regional seasonal climate forecast assessments.

In Summary A basin-wide El Niño event continues. Its strength peaked in November-December 2009 at a moderate level. So far, declines in strength have only been modest; Further decline in strength is expected over the next few months, but considerable uncertainty remains regarding the timing and rate of decay. Nonetheless, the most likely outcome by mid-year is considered to be near-neutral; The period March-June is a particularly difficult time of the year for forecasting tropical Pacific developments, and while near-neutral is considered the most likely outcome by mid-year, it is still considered possible for El Niño to persist or for the early stages of a La Niña to be present by mid-year; It is however important to recognize that impacts of the current El Niño are expected to continue to be felt in many parts of the world through at least the second quarter of 2010. This is because impacts on many climate patterns both close to and remote from the Pacific, can occur even during the decay phase of an El Niño event.