Thursday, 13 February 2003: 3:30 PM
Will Global warming Induce a Permanent El Nino? (It has happened before!)
The warming of the surface waters of the eastern equatorial Pacific associated with the familiar El Niņo of today involves an adiabatic, horizontal redistribution of the warm surface waters of the tropical Pacific. The same warming is possible by means of a very different mechanism that involves diabatic processes: a net deepening of the tropical thermocline because of an increase in the volume of warm surface waters. What factors determine the depth of the thermocline? Why is it so shallow today? The answers involve the heat budget of the ocean and indicate that changes in that budget, possibly induced by global warming, can result in a deepening of the thermocline and hence permanent El Niņo conditions. A test for this theory is available in the geological records which indicate that, until 3 million years ago approximately, the Earth experienced permanent El Niņo conditions with sea surface temperatures in the eastern equatorial Pacific as high as those in the west, and with an absence of cold surface waters in the coastal upwelling zones off California and elsewhere. The subsequent cooling of our planet was associated with a significant increase in the sensitivity of the climate to modest Milankovich forcing which now induces a response with an enormous amplitude, the recurrent Ice Ages. During this era of enhanced climate sensitivity we are now introducing a disturbance, a rise in the atmospheric concentration of greenhouse gases, that is growing exponentially. Furthermore, we are doing it during one of the brief, warm interglacial periods when, in the geological records, conditions are already close to those of 3 million years ago. This paper explores the factors, including the ocean-atmosphere interactions that can induce a return to the warm world of 3 million years ago.