The recurrence of ENSO has a periodicity of approximately 2 to 7 years. By some estimates, ENSO is responsible for a large number of weather-related disasters, dramatic changes in marine ecology over tropical islands and coastal regions, and severe economical losses due to floods, famine and crop failures in many countries along the tropical and subtropical belt. During the early calendar months of a warm event year, the SST and rainfall in the east attain exceptionally high values at the time of their seasonal maxima. While warm, wet conditions prevail in the eastern equatorial Pacific, the convective zone over the western Pacific moves eastward starting around in April of the year so that rainfall decreases and sea level pressure increases over Indonesia. The large anomalies attain their maximum areal extent towards the end of the year and early in the next year.
Torrential rains occur in the central Pacific islands and regions along the South American coast.
The low SSTs spread westward and inaugurate La Nina. The SOI is a measure of the phase and amplitude of the SO which describes the seesaw in surface patterns between the Eastern and Western Pacific regions. The El Nino events show large negative SOI deviations, while the La Nina events show positive SOI deviations. SST changes associated with ENSO are Pacific-wide, and possibly global in extent. The appearance of warm water over the equatorial central and eastern Pacific is associated with anomalous weather fluctuations elsewhere in the tropics and the extratropics, due in part to the influence of SST on the large-scale atmospheric circulation.