What exactly is La Nina?

Question

I've heard about it before, but what exactly is it? How ofter does it happen? What happens when it's here?

Answer 1

La Nina is the exact opposite of El Nino; La Nina is when the sea surface temperature of the eastern Pacific Ocean is below normal.

El Nino, on the other hand, is when the sea surface temperature is above normal, which means more evaporation (and condensation), and is the main cause why every four or five years there is above-normal rainfall in the Western states, particularly California.

Answer 2

La Niña is the name for the cold phase of ENSO, during which the cold pool in the eastern Pacific intensifies and the trade winds strengthen. The name La Niña originates from Spanish, meaning "the little girl", analogous to El Niño meaning "the little boy". It has also in the past been called anti-El Niño.

La Niña causes mostly the opposite effects of El Niño, for example, El Niño would cause a wet period in the Midwestern U.S., while La Niña would typically cause a dry period in this area.