El Nino and La Nina for the Navy

What is a La Niña?

The opposite of the El Niño is a phenomenon called La Niña (also called El Viejo or the cold event).

      La Nina 3D schematic

La Niña is marked by strong trade winds, driven by a strong pressure gradient. Cold water propagates to the west via Rossby Waves, and the warm pool of water that occurs in the western Pacific is shifted even more to the west, and with it, the convection, which alters the global weather pattern. The La Niña weather patterns are not necessarily equal and opposite to the El Niño weather patterns.

Convection is driven by the warming of the surface air over the warm water; as the warm pool is shifted westward during a La Niña, so does the heavy rains caused by the convection. The shift in the weather patterns is what causes drought, floods, storms, and other weather anomalies in many area of the world.

The following graphic shows the temperature distribution in the Pacific Ocean for a La Niña.
La Niña
Dec. 1988
Sea surface temperature Dec 1988

This graphic was taken from the El Niño theme page (NOAA)

A good way to understand the changes from a normal scenario to an El Niño or a La Niña is to see all three illustrations.