[...] Tropical sea surface temperatures respond to the change in surface pressure across the globe and in particular to the differential between mid latitudes and the near equatorial zone.
The southern hemisphere and high latitudes in particular experience marked flux in surface pressure. This leads directly to a change in the trade winds and tropical sea surface temperature.
There is an asymmetry between the hemispheres with loss of pressure in the southern hemisphere compensated to some extent by a gain in pressure in the northern hemisphere.
If we wish to understand the ENSO phenomenon we must look beyond the tropics for causal factors. ENSO in the Pacific is just one facet of change in the tropics. Change is driven by air pressure variations at mid and especially high latitudes. This determines the strength of the trade winds and the temperature of the tropical ocean (where solar insolation is greatest and cloud cover is least). There are knock on effects for heat transfer from the tropics to mid and high latitudes, rainfall, flood, drought and tropical cyclone activity worldwide. The tropical oceans are the Earths solar array.
The flux in surface pressure appears to be cyclical. However, the cycle is longer than the sixty years of available data. We cannot say for sure what the cycle length may be or how it varies over time. However, there is good evidence that the warming cycle that began in 1978 peaked in 1998. Cooling is underway.
We must acknowledge that the ENSO cycle is not temperature neutral. There are short ENSO cycles of just a few years and long ENSO cycles that are longer than 60 years.
Is there evidence that the activity of man (adding CO2 to the atmosphere) is tending to produce more severe El Nino events. The answer is no. The flux in surface pressure is responsible for ENSO and for the swing from El Nino to La Nina dominance. In spite the activities of man, the globe is currently entering a La Nina cooling cycle testifying to the strength of natural cycles and the relative unimportance atmospheric composition in determining the issue (if the much touted greenhouse effect exists at all) .
Is there evidence that the ENSO phenomenon is in fact ‘climate change in action’, driven by factors other than the increase of atmospheric CO2? Yes, it appears that whatever drives the flux in surface atmospheric pressure drives ENSO and with it, climate change.
Is recent ‘Climate Change’ driven by greenhouse gas activity? No, it appears that the cause of recent warming and cooling relates to long-term swings in atmospheric pressure that changes the relations between mid and low latitudes thereby affecting the trade winds that in turn determine the temperature of the Earth’s solar array, its tropical ocean, and ultimately the globe as a whole. /full essay here.
Bill Illis on the connection between the Trade winds and ENSO at http://wattsupwiththat.com/2009/02/17/the-trade-winds-drive-the-enso/
And the conventional viewpoint on ENSO can be found here: http://earthsci.org/education/investigations/ies/El%20Nino/El%20Ni%F1o.htm
Natural climate variation
A different view of ENSO and systematic climate change
Sea Level Data Exposes El Nino’s Secret
El Nino. How big? How long?