ENSO and greenhouse warming

849 The impacts of anthropogenic climate change may be felt through changes in modes of natural climatic vari ability. ENSO is the most important year-to-year fluctuation of the climate system on the planet1, varying between anomalously cold (La Niña) and warm (El Niño) condi tions. Underpinning occurrences of ENSO events is the positive feedback between trade wind intensity and zonal contrasts in sea surface temperature (SST), referred to as the Bjerknes feedback. The trade winds normally pile up warm surface water in the western Pacific while upwelling colder subsurface water in the east along the equator and off the west coast of South America. The resulting east–west surface temperature contrast reinforces an east–west air pressure difference across the basin that in turn drives the trade winds. During La Niña, the system strengthens, but during El Niño, the trade winds weaken as atmospheric pressure rises in the western Pacific and falls in the eastern Pacific. The Bjerknes feedback now operates in reverse, with weakened trade winds and SST warming tendencies along the Equator reinforcing one another. It is still not clear what sets this quasi-oscillatory behaviour, that is, whether ENSO is self-sustaining or triggered by stochastic forcing2. What is clear is that ocean and atmosphere preconditions are required3, as supported by the fundamental characteristics of the mean tropical climate such as thermal gradients and associated circulations that balance radiative heating4. These swings in temperature are accompanied by changes in the structure of the subsurface ocean, the position of atmospheric convection and associated global teleconnection patterns, severely disrupting global weather patterns5–10, and affecting ecosystems11 and agriculture12 worldwide. ENSO and greenhouse warming