Detailed monitoring of recorded El Niño episodes has revealed that once the warmest water reaches the International Date Line, anomalous convection usually appears in that region, accompanied by a weakening of the equatorial easterlies.This pattern typically occurs during the boreal winter (June–August) and may be preceded or followed by a warming that causes the Inter-Tropical Convergence Zone (ITCZ) to move farther south than normal, which contributes to enhanced rainfall across Ecuador and northern Peru, producing the "years of abundance." In determining the atmospheric status of the tropical Pacific, climatologists devised the Southern Oscillation Index (SOI, Figure 3).Most climate anomalies associated with El Niño are reversed during La Niña.In general, a majority of the impacts occur in climates that have significant oceanic influences and border the tropical Pacific.
Coinciding with the extensive warm water were wetness along the Peruvian coast, low surface pressure in the eastern Pacific, and high pressure in the western tropical Pacific.
When the SOI is strongly positive, cooler than normal equatorial water appears throughout the central and eastern equatorial Pacific.
This is called a cold episode or sometimes La Niña, "little girl." Climatologists prefer to use the acronym ENSO (El Niño/Southern Oscillation) to describe the warm (El Niño) and cold (La Niña) episodes that occur periodically across the tropical Pacific.
Additionally, warm water "piles up" in the western Pacific, due to the easterly winds.
Further east, the SE trades and equatorial easterlies in the eastern and central Pacific produce upwelling of cool water along the equator and coast of South America.