How are atmosphere and oceanic
conditions related during an El Niño?
Under “normal” conditions, often referred to
by the term La Niña, the easterly trade winds
blow across the Pacific, generating
upwelling along the equator across most the
Pacific, and piling up warm water in the
west.  The east-west contrast in sea surface
temperature sets up low pressure and rising
motion in the east, and high pressure and
sinking motion in the west.
When an El Niño occurs, the trade
winds collapse, upwelling of cold
water ceases along the equator, and
sea surface temperatures rise in the
central and eastern equatorial
Pacific.  Pressure decreases in these
regions, and rising motion leads to
precipitation.