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.