Contents	Introduction	Normal Pacific currents		Changes with El Niño
Contents	Measurement	Effects	Research	Prediction	Conclusions

Introduction

Peruvian fishermen have for centuries been aware that every few years the waters off their coast assume strange properties and cause their fish harvest to dwindle. As it occurs at Christmas, they termed it "el Niño", or 'the boy child'. Peru is now in its sixth consecutive el Niño year, and world interest is growing: the world is wondering if he is to blame for famines, droughts and floods too. The reason the anchovies off Peru and Ecuador disappeared was simple: the coastal waters warmed an d became poor in nutrients. The West first noticed the phenomenon around 1900 when imports of fertiliser (guano) stopped due to the sudden rarity of the fish-eating birds that made it. Since then the phenomeno n has become more and more frequent.

Since it has become clear that the atmosphere is inextricably interlinked, with el Niño years corresponding exactly to those of a "southern oscillation", which was first noticed by Sir Gilbert Walker, Director General of the Observatory in India, w ho in 1904 was charged with predicting the fluctuations in the Asian monsoon following an 1899 famine. He noticed that when the pressure in the East Pacific rises it usually falls in the west, and vice versa. In the 1930s he called this seesaw the Souther n Oscillation (to differentiate from apparently similar oscillations in the North Atlantic). This can be used to quantify the whole of ENSO.

Precise definitions of what the term "el Niño" covers are wide-ranging. Some say that it is simply the phenomenon noticed by the fishermen, while others name ocean currents after it, and some describe it as the whole reversal of the Pacific air and sea current system. The name La Niña (the girl) is gradually coming into use to describe a cooling of the eastern tropical Pacific. Due to this vagueness scientists usually today use the term El Niño Southern Oscillation, abbreviated to ENS O.

The approximate pronounciation of the term is found in a 23K .wav file here.

Modern study of the phenomenon began in 1969 by Jacob Bjerknes. Whilst trying to explain the anomalously cold waters in the east Pacific, he first recognized the normal circulation of the atmosphere over the Pacific and the deviations from it that El Ni&n tilde;o prompted. He coined the normal circulation the Walker cell in honour of Sir Albert, and it is this that we shall examine first.

A current photograph of the Pacific ocean is available here. Other satellite photographs have an index here.

Next: the normal Pacific current system