El NiñoEl Niño is a warming of the sea surface in the central and eastern Pacific Ocean along and near the equator to above mean temperatures.
La NiñaLa Niña is generally the opposite conditon to El Niño. The central and eastern Pacific water surface near the equator surface cools to below mean temperatures.
El Niño Southern Oscillation (ENSO)The El Niño Southern Oscillation is a cycle of switching back and forth between El Niño and La Niña conditions with a period of near neutral conditions between the two states.
Arctic Oscillation (AO)
Pacific Decadal Oscillation (PDO)
More on El Niño and La Niña
More on the Arctic Oscillation
ReferencesNOAA National Ocean Service
NOAA NWS Climate Prediction Center
"What is the El Niño–Southern Oscillation (ENSO) in a nutshell?" - Michelle L'Heureux, May 5, 2014, NOAA Climate.gov <--REFERENCES FOR AO AND PDO TO BE ADDED ONCE REVIEWED-->