When the Southern Annular Mode is positive – storms spinning off the polar front push further in higher latitudes. The circumpolar flow through Drakes Passage is reduced. Cold southern ocean water piles up off the coast of South America and upwelling in the region of the Humboldt Current intensifies – initiating La Nina. When SAM goes negative – the cold tongue slows, temps in the ocean surface increase and the trade winds falter – initiating an El Nino in which warm water piled up against Australia and Indonesia flow eastward across the Pacific.
Solar UV heats ozone in the middle atmosphere and is significant source of warmth and variability above the Antarctic (and Arctic).
http://www.osdpd.noaa.gov/data/sst/anomaly/2011/anomnight.7.28.2011.gif
You can see in the map cold upwelling still in the south east Pacific. The conditions for a La Nina are still in progress. Once the ITCZ moves to the north in the SH spring – La Nina is odds on of reforming stronger than last year. ENSO is a SH summer phenomenon.
There is a similar process in the NH – with a different oceanic morphology. I would link it physically to the PDO.
ENSO is a southern hemisphere summer phenomenon. SAM responds to temperature in the middle and upper atmosphere. Colder and air densities increase. So it tends to be more positive in winter – which is when the polar fronts most commonly have landfall in southern Australia and Africa, New Zealand and South America.
Complex system I believe – where solar UV pushes systems past La Nina threshold and then feedbacks of wind, cloud, currents and waves kick in. Upwelling in the Humboldt Current is the ENSO switch.
In a La NIna – clouds form over cool seas and are carried on the trade winds to create rain in Australia, Asia and Africa. In an El Nino cloud dissipates over warm water – but the sub-tropical jet streams carry high cloud and moisture over north and south America.
La Nina result as a global average in increased reflected shortwave – strongly in the tropical Pacific – and vice versa for El Nino.