Just What Is La Nina?

La Niña is characterized by unusually cold ocean temperatures in the Equatorial Pacific, compared to El Niño, which is characterized by unusually warm ocean temperatures in the Equatorial Pacific. The graphic below shows the sea surface temperature in the equatorial Pacific  (20ºN-20ºS, 100ºE-60ºW) from Indonesia on the left to central America on the right. 

Strong La Niña conditions during December 1998 are shown in the top panel. The Eastern Pacific is cooler than usual, and unusually cool water extends farther westward than is usual (see the blue color extending further off-shore from South America along the equator).

Normal Equatorial Pacific Ocean surface temperatures (December 1993) are shown in the middle panel, including the usual cool water, called the 'cold tongue', in the Eastern Pacific (in blue, on the right of the plot) and the usual warm water, called the 'warm pool' in the Western Pacific (in red, on the left).

https://www.pmel.noaa.gov/elnino/what-is-la-nina