El Niño and La Niña are phases of a naturally occurring climate cycle called El Niño/Southern Oscillation.ENSO involves large-scale changes in sea-surface temperature across the eastern tropical Pacific. Warmer ocean surface temperatures occur in the eastern tropical Pacific during El Niño. During La Niña, ocean surface temperatures here are cooler. Changes in ocean temperature lead to atmospheric changes, which impact the weather. Changes in the ocean & atmosphere can also have major impacts on the economy.
A look at river discharge, where greater precipitation causes greater discharge, for La Niña of 1988 and El Niño of 1989, shows that global conditions are not always what we would expect - some regions are distinctly wet or dry as we would expect and others are not as easy to predict or distinguish.