Behind the long-term means and trends, discussed in the previous chapter, there is a much greater short-term variability of precipitation year by year at all geographical scales. Precipitation can also vary in its extremeness, for example shifts to greater or lesser intensity of individual storms or to changes in the length and frequency of wet and dry periods. We will look at these two aspects of precipitation separately, starting with variability.

It has been shown by many researchers that the variability of precipitation in the tropics (and also further afield), is highly influenced by the cyclic variations of sea surface temperature (SST) across the equatorial Pacific caused by El Niño. We need first to look at these processes and then at the effects they have on precipitation.

El Niño and La Niña

Every year, about Christmas time, a warm southerly-flowing current originating from around the Galapagos Islands replaces the very cold northward current along the coasts of Peru and Ecuador. Every few years the southern current is much warmer. This has been known for centuries by the local inhabitants, but it was only recently recognised that these periodic events extended over the entire tropical Pacific (Bjerknes 1969, Allen et al. 1996, Weather 1998). While originally the term El Niño was used by fishermen to describe the local annual warm current (it means ‘the Christ child’ in Spanish, after its time of onset) it is now used for the large-scale warmer events across the entire Pacific.