Normally, sea surface temperature is about 14°F higher in the Western Pacific than the waters off South America.
This is due to the trade winds blowing from east to west along the equator allowing the upwelling of cold, nutrient rich water from deeper levels off the northwest coast of South America.
Also, these same trade winds push water west which piles higher in the Western Pacific (downdwelling).
When the air pressure patterns in the South Pacific reverse (the air pressure in the west is higher than in the east), the trade winds decrease in strength (and can reverse direction).
This pushes the thermocline deeper and a decrease in the upwelling.
With a deeper thermocline and decreased westward transport of water, the sea surface temperature increases to greater than nornmal in the Eastern Pacific. This is the warm phase of ENSO, called El Niño.
The net result is a shift of the prevailing rain pattern from the normal Western Pacific to the Central Pacific.
Results:
There are occasions when the trade winds that blow west across the tropical Pacific are stronger than normal leading to increased upwelling off South America and hence the lower than normal sea surface temperatures.
The prevailing rain pattern also shifts farther west than normal. These winds pile up warm surface water in the West Pacific. This is the cool phase of ENSO called La Niña.
What is suprising is these changes in sea surface temperatures are not large, plus or minus 6°F (3°C) and generally much less.
Results: the opposite of El Nino