Article in Nature 461 (24 September 2009) on el Nino of interest

http://www.nature.com/nature/journal/v461/n7263/full/461481a.html

http://www.nature.com/nature/journal/v461/n7263/abs/nature08316.html

El Niño events, characterized by anomalous warming in the eastern equatorial Pacific Ocean, have global climatic teleconnections and are the most dominant feature of cyclic climate variability on subdecadal timescales. Understanding changes in the frequency or characteristics of El Niño events in a changing climate is therefore of broad scientific and socioeconomic interest. Recent studies^1,2,3,4,5 show that the canonical El Niño has become less frequent and that a different kind of El Niño has become more common during the late twentieth century, in which warm sea surface temperatures (SSTs) in the central Pacific are flanked on the east and west by cooler SSTs. This type of El Niño, termed the central Pacific El Niño (CP-El Niño; also termed the dateline El Niño², El Niño Modoki³ or warm pool El Niño⁵), differs from the canonical eastern Pacific El Niño (EP-El Niño) in both the location of maximum SST anomalies and tropical–midlatitude teleconnections. Here we show changes in the ratio of CP-El Niño to EP-El Niño under projected global warming scenarios from the Coupled Model Intercomparison Project phase 3 multi-model data set⁶. Using calculations based on historical El Niño indices, we find that projections of anthropogenic climate change are associated with an increased frequency of the CP-El Niño compared to the EP-El Niño. When restricted to the six climate models with the best representation of the twentieth-century ratio of CP-El Niño to EP-El Niño, the occurrence ratio of CP-El Niño/EP-El Niño is projected to increase as much as five times under global warming. The change is related to a flattening of the thermocline in the equatorial Pacific.

Hmm that's interesting.  In short, global warming is moving the hottest SSTs during an El Nino event to the central Pacific rather than eastern eh?

Would that perhaps reduce the impact of El Nino events on US weather, since higher SSTs are further away?

Reduction in impact is not the right way to think about it.  The change in type of el nino changes the impact.  Here are a couple of lines from the Ashok et al. JGR reference:

"... The ENSO Modoki events significantly influence the temperature and precipitation over many parts of the globe. Depending on the season, the impacts over regions such as the Far East including Japan, New Zealand, western coast of United States, etc., are opposite to those of the conventional ENSO. ..."

I am not taking these out of context.