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.