El Niño (warm) and La Niña (cold) episodes are extremes of what is often referred to as the ENSO cycle. The cycle has an average period of about four years, although the period has varied between two and seven years in the historical record. This cycle encompasses changes in ocean surface and subsurface temperatures, tropical rainfall, atmospheric winds and air pressure. During El Niño episodes the equatorial sea surface temperatures (SSTs) are abnormally warm from the date line eastward to the South American coast. However, there is a strong annual cycle in the actual SSTs across the eastern equatorial Pacific, such that SSTs are sufficiently warm [approximately 28°C (82°F)] to support persistent tropical rainfall and convective activity in this region for only part of the year.

This annual cycle in SSTs strongly affects the timing and eastward extent of tropical rainfall during the El Niño. As a typical El Niño develops, above-normal rainfall tends to extend eastward to just east of date line during September-November. The lack of rainfall farther east coincides with the minimum in SSTs across the eastern equatorial Pacific at this time of the year. As the El Niño strengthens during December-January tropical rainfall extends well east of the date line, and by March-April rainfall typically covers the entire eastern Pacific east of the date line. This period coincides with the annual maximum in SSTs across the eastern equatorial Pacific. This shift in the pattern of tropical precipitation weakens the large-scale monsoon circulation systems of Australia/ Southeast Asia, South America/Central America and Africa.

During La Niña episodes the equatorial sea surface temperatures (SSTs) are abnormally cold from the date line eastward to the west coast of South America, and tropical rainfall and convection tends to be focused over the western equatorial Pacific and Indonesia. Little rainfall is typically evident over the eastern equatorial Pacific, as SSTs remain well below 28°C in this region throughout the episode. This pattern represents an amplification of climatological mean conditions, which feature heavy rainfall across Indonesia and little-to-no rainfall over the eastern equatorial Pacific. This persistent pattern of tropical rainfall contributes to stronger than average monsoon systems over Australia/ Southeast Asia, South America/Central America and Africa.