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Here's a discussion from Jeff Masters' valuable Wunderblog:

A trend to El Niño at this time of year is unusual; May or June are the typical months that El Niño starts to develop. While the Climate Prediction Center expects that this will be a weak El Niño, the unusual timing of this event puts us in relatively uncharted territory. Since 1950, only one El Niño has started in the Fall, the El Niño of 1968. This event was an average El Niño, with a peak SST warming in the East Pacific of 1.0º C. For comparison, the warming was 2.3-2.5º C in the record El Niño events of 1997-98 and 1982-83. The unusual timing of the 2006 El Niño event comes on the heels of the unusual timing of the La Niña event that ended in May. The 2006 La Niña started very late--no La Niña of similar magnitude had ever formed in the middle of winter, as this one did. One may legitimately ask if these events might be linked to human-caused climate change. I am concerned that this might be the case, but we don't have a long enough record of historical El Niño events to know. Up until 1975, La Niña events and El Niño events used to alternate fairly regularly with a period of 2-7 years. Between 1950 and 1976 there were seven El Niño events and seven La Niña events. Since 1976, El Niño events have been approximately twice as frequent as La Niña events, with ten El Niño events and only six La Niñas. Some researchers have speculated that this is due to the effects of global warming causing a new "resonance" in the climate system. If so, this is one way in which global warming may end up causing a decrease in Atlantic hurricane activity over the coming decades, since the increased wind shear over the Atlantic during El Niño events greatly reduces the number and intensity of these storms.