ENSO (El Niño Southern Oscillation) is the technical name given to the most prominent source of inter-annual variability in weather and climate around the world. Its ocean temperature signatures, the more widely-known La Niña and El Niño, corresponding to different phases of the oscillation, are officially defined as sustained sea surface temperature anomalies of magnitude greater than 0.5°C across the central tropical Pacific Ocean.
When ENSO is in the phase known as La Niña, the Pacific trade winds blow true and strong causing sun warmed surface water to pile up against Australia and Indonesia. Cool subsurface water rises in the east.
In an El Niño, the trade winds falter and warm water spreads out eastwards across the Pacific Ocean. La Niña and El Niño are the poles of the El Niño Southern Oscillation (ENSO).
ENSO has a influence on global surface temperatures, Australian, American, Indian and African rainfall and Atlantic cyclones. ENSO varies between La Niña and El Niño states over 3 to 7 years but also over periods of decades to centuries. One mode of ENSO variation involves changes in both the frequency and intensity of La Niña and El Niño over at least a few decades.
The Pacific Ocean trade winds set up cloud and rainfall patterns globally with enormous energies transferred between ocean and atmosphere. La Niña conditions see colder water rising in the eastern Pacific and warm, moist air rising over Australia and Indonesia. El Niño conditions see warm water spread across the Pacific.
La Niña (December 2000)
El Niño (December 1997)
Sea surface temperature anomalies (°C)
A simple process of heat transfer from the ocean to the atmosphere occurs. The heat transfer is enhanced in the El Niño state by the enormous area of warm water.
Conversely, heat is gained by the Pacific Ocean during cooler La Niña conditions and global surface temperatures dip. This is reflected in the CRU surface temperature anomalies shown below for the period 1990 to 2006. 1997 and 1998 are hot. By contrast, 2000 was relatively cool.
The recent surface temperature record shows rising temperatures in the 1990's -- a period of sustained and intense El Niño peaking in 1997. The temperature period since certainly reflects less intense El Niño and cooling sea surface temperatures. The planet will remain cooler, certainly through 2007 and into 2008, as cooler La Niña influences prevail.
The colours in the "multivariate ENSO index" represent La Niña or El Niño states - blue for La Niña and red for El Niño. Note the El Niño in 1997 and the 2000 La Niña.
Superimposed on the alternation of La Niña and El Niño are longer term variations in the frequency and intensity of El Niño and La Niña. A period of more frequent and intense La Niña between the mid forties and 1975 followed by more frequent and intense El Niño between 1976 and 1998. The pattern appears in centuries of proxy data - that is in tree and coral rings, sedimentation and rainfall and flood records.
Global surface temperatures have a similar trajectory. Falling from 1946 to 1975, rising between 1976 and 1998 and declining since.
ENSO determines rainfall in Australia, Asia and America, and influences rainfall in Africa and the Indian monsoon. The beginning of the hydrological cycle here appears to be the vast heat sink over the Pacific during an El Niño and warm, moist air rising in the western Pacific in a La Niña.
The longer term variation of ENSO in frequency and intensity has not been explained either as a result of internal feedback or external forcing. Even so, it is difficult to explain how ENSO variations have been neglected by so many for so long. ENSO involves 97% of greenhouse gases. The surface temperature impacts are significant. Note the 0.25 0C difference between 1998 and 2000.
ENSO variation goes in both directions. The indications are that ENSO variation added to global surface temperatures between 1976 and 1998. It has been almost 10 years since temperatures peaked in1998. The planet may continue to be cooler over the next few decades as a cool La Niña phase of ENSO emerges.