El Nino and La Nina

The following graphics are from The National Weather Service Climate Prediction Center.

The graphic above shows surface temperature in the equatorial Pacific Ocean. El Nino is characterized by unusually warm temperatures and La Nina by unusually cool temperatures in the equatorial Pacific. Anomalies (below) represent deviations from normal temperature values, with unusually warm temperatures shown in red and unusually cold anomalies shown in blue.

El Niño translates from Spanish as 'the boy-child'. Peruvian fisherman originally used the term - a reference to the Christ child - to describe the appearance, around Christmas, of a warm ocean current off the South American coast. Nowadays, the term El Niño refers to the extensive warming of the central and eastern Pacific that leads to a major shift in weather patterns across the Pacific. In Australia (particularly eastern Australia), El Niño events are associated with an increased probability of drier conditions. Changes to the atmosphere and ocean circulation during El Niño events include: Warmer than normal ocean temperatures across the central and eastern tropical Pacific Ocean. Increased convection or cloudiness in the central tropical Pacific Ocean - the focus of convection migrates from the Australian/Indonesian region eastward towards the central tropical Pacific Ocean. Weaker than normal (easterly) trade winds. Low (negative) values of the SOI (Southern Oscillation Index). Monitoring these changes help to detect an El Niño event and forecast its lifetime.