Overview of typical effects of El Niño and La Niña on climate
ENSO events have widespread effects on seasonal weather and climate through their influence on large scale circulation patterns. The maps provided below summarise the main impacts on seasonal precipitation and near-surface temperature over land areas for El Niño and La Niña events. For each region marked, the colour indicates the tendency and the text indicates the seasonality of the impact. The maps are based on analyses of historical datasets extending over several decades and on information in peer-reviewed publications. Details of the analyses and of the regional probabilities are available in Davey et al (2013).
For temperature the regions selected are areas where a substantial impact is likely, in the sense that the seasonal value will be in the top (or bottom) one third of observed values centred on the months indicated, with approximately a greater than 50% chance that the impact will occur if the ENSO event is active, based on historical evidence. For example, for a region marked 'warmer likely' in December in El Niño conditions, this means approximately a greater than 50% chance that the temperature will be in the top third of temperatures observed in the November-December-January season. The criterion for El Niño (La Niña) active is Niño3.4 regional sea surface temperature in the top (bottom) 25% of observed values in the corresponding seasons. For precipitation, for which data are more limited and probability estimates are more uncertain, analyses of composites were also used to determine regions and seasons that tend to be influenced by ENSO events.
Each ENSO event is different, and occurs in conjunction with other climatic events. Not all impacts occur in all events, and impacts may not be confined to the regions indicated. Thus these maps should not be regarded as forecasts for a current event, but rather as an indication of areas where impacts are likely, based on historical evidence.
- The maps are schematic and the areas marked are not geographically precise. There are seasonal shifts in the locations of impacts that are not fully represented.
- Recent research has suggested that there are two main subtypes of ENSO. The terminology varies, but the names Central Pacific (CP) and East Pacific (EP) indicate variations in the location of equatorial sea surface temperature anomalies. The impact maps combine the effects of both types: thus in regions where CP and EP events have contrasting effects the net impact may be reduced, and there may be separate CP and EP impacts that do not appear here.
- Some regions have, for example, an El Niño impact but not a La Niña impact: thus while the El Niño and La Niña maps have many areas in common, usually with opposite impacts, there are several differences.
- Some regions may expect different impacts in differing seasons during an ENSO event, as indicated by overlapping areas of impact. This is a consequence of interactions of ENSO with the regional seasonal cycle.
- Some regions have impacts that depend on the magnitude of events. For example, the North European (Baltic) region included on the El Niño map is marked as a cold impact, but very strong El Niño events (e.g. 1997/98, 1982/83) have an opposite warm impact. A comment is included in the text on the map.
- Observational data coverage varies substantially geographically. There may be some regions with a distinct impact that are not revealed by the analyses.
- These summary maps do not indicate the severity of impacts. The probabilities and magnitudes vary considerably from region to region.
- The impact areas marked depend on the selection criteria, and maps that use other criteria such as composites and correlations will differ. Further, the ENSO event selection includes weak events, and restriction to moderate and strong events would alter the areas of impact.
Our analyses for the temperature impact maps made use of monthly-average near-surface land temperature data from the National Centers for Environmental Prediction atmospheric re-analysis (1948-2011) and the CRUTEM4 gridded observational analysis (1850-2010).
For the precipitation impact maps, our analyses made use of the University of East Anglia Climatic Research Unit 1900-1998 monthly precipitation dataset and the Global Precipitation Climatology Project 1979-2010 monthly gridded precipitation dataset.