El Niño affects global weather by changing the distribution of heat and atmospheric convection across the tropical Pacific, which in turn alters large scale atmospheric circulation and changes the probability of particular weather patterns in distant regions. During El Niño, unusually warm central and eastern equatorial Pacific waters encourage convection and rainfall farther east than under normal conditions.
This changes upper level atmospheric divergence and the propagation of planetary scale waves, which can modify jet streams, storm tracks and pressure patterns. The resulting teleconnections can influence temperature and precipitation thousands of kilometres away. El Niño does not impose one identical global weather pattern, however. The effects vary according to event strength, spatial structure, season, background climate and interactions with other climate modes.
In the tropical Pacific itself, El Niño typically weakens trade winds, reduces eastern Pacific upwelling and shifts warm water eastward. Rainfall that normally concentrates over the western tropical Pacific can shift toward the central and eastern Pacific. Areas such as Indonesia and parts of Australia can experience increased drought risk during many El Niño events, while portions of Ecuador and Peru can experience enhanced rainfall and flooding. The exact distribution depends on the structure of the event. These changes can strongly affect agriculture, water supplies, fisheries and ecosystems. The eastern Pacific warming also changes marine nutrient availability because reduced upwelling limits the supply of nutrient rich deep water to the surface.
North America often experiences substantial El Niño related winter effects, although the response differs between regions and individual events. The altered tropical heating can influence the Pacific jet stream and the position of storm tracks. In many El Niño winters, the southern United States experiences an increased tendency toward wetter conditions, while parts of the northern United States and western Canada can experience relatively warmer conditions.
California and the southwestern United States can receive increased precipitation during some events, which can be beneficial for water supplies but can also increase flood, landslide and erosion risks. These are statistical tendencies rather than guarantees, and other atmospheric patterns can override or modify the ENSO signal.
South America experiences some of the most direct impacts because the tropical Pacific anomaly is geographically close to the continent. Coastal Ecuador and northern Peru can experience unusually heavy rainfall during strong eastern Pacific El Niño events, sometimes producing flooding and landslides. Farther south and east, rainfall responses can differ by season and location. Fisheries along the Pacific coast can be strongly affected because warming suppresses the upwelling that normally brings nutrients to surface waters. This can reduce the productivity of marine ecosystems and alter populations of fish and seabirds. El Niño therefore affects both atmospheric weather and ocean ecosystems.
Australia and Southeast Asia commonly experience increased drought and heat risk during El Niño because the main tropical convection zone shifts eastward away from the western Pacific. Reduced rainfall can affect agriculture, reservoirs, soil moisture and wildfire conditions. Some parts of Southeast Asia can experience particularly dry conditions, although the severity varies among events. Drought can also increase the risk of vegetation fires and smoke pollution. The relationship is strongest during certain seasons and can be modified by the Indian Ocean Dipole, monsoon behaviour and other atmospheric processes. Consequently, El Niño should be treated as a factor that changes probabilities rather than a deterministic cause of every regional drought.
El Niño can influence tropical cyclone activity in different ocean basins. In the Atlantic, El Niño often increases vertical wind shear, which tends to suppress the development and intensification of tropical cyclones. At the same time, El Niño can favour tropical cyclone activity in parts of the central and eastern Pacific by altering wind shear and atmospheric conditions. These effects are not identical in every season because storm development depends on sea surface temperatures, moisture, atmospheric stability, shear and other conditions. The global effect is therefore a redistribution of tropical cyclone likelihood rather than a simple worldwide increase or decrease.
El Niño also affects temperature patterns beyond precipitation. The redistribution of ocean heat can contribute to higher global mean surface temperatures during some El Niño events because heat stored in the tropical Pacific is transferred more strongly to the atmosphere. Individual years with El Niño can therefore become unusually warm at the global scale, although long term global warming is distinct from ENSO variability. Regional temperature anomalies can include winter warmth in some higher latitude regions, increased heat in drought affected areas and altered seasonal conditions elsewhere. These temperature responses depend strongly on atmospheric circulation and local feedbacks.
El Niño is therefore a major source of global weather variability because a relatively concentrated tropical Pacific ocean anomaly can alter atmospheric circulation across the planet. Its impacts commonly include shifts in tropical rainfall, drought and flood risk, changes in jet streams, regional temperature anomalies and altered tropical cyclone probabilities. However, the phrase affects global weather should not be interpreted as meaning that every region experiences the same effect or that an El Niño event determines a specific weather outcome. The most accurate interpretation is that El Niño changes the statistical background conditions under which regional weather develops, increasing or decreasing the likelihood of particular patterns depending on location and season.
