La Nina is caused by cooler sea-surface temperatures in the central and eastern tropical Pacific Ocean. The atmosphere is affected by the ocean, and sea surface temperatures create these weather changes. The patterns oscillate between El Nino, neutral, and La Nina every few years.
The ocean temperature changes on the surface as cooler sub-surface waters are pushed into the regioun by waves and currents, though the full phenomenon in all its complexity is still being discovered and studied.
According to NOAA, La Nina is caused by an aggregation of “cooler than normal subsurface waters in the tropical Pacific” (see link below). Though still uncertain as to the exact mechanisms, what is known is that colder water is brought up to the surface by oceanic and atmospheric wave as they move eastward. This leads to the intensifying of the easterly trade winds and cold upwelling off Ecuador and Peru, which in turn causes sea surface temperatures to drop (during the 1988-89 La Nina, temperatures dropped by as much as 7 degrees farenheit below normal).
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