I am finally giving in and starting what I'm sure will become a series of updates on the emerging warm-phase of the ENSO phenomenon, namely El Nino. It's becoming pretty apparent that the tropical eastern Pacific is anomalously warm, and is likely to stay that way for the next year or so [LINK]. At this point, what we can say is that the indices that are used operationally to define and track El Nino point toward a moderate to strong El Nino, but their nature is difficult to predict. You can see the development of warm anomalies of 1-2 degrees Celsius along the equator and stretching from South America into the central Pacific Ocean at the CPC page. This is the major symptom of El Nino. The impacts are not completely understood, but the slackening of the trade-winds and shift in northern hemisphere jetstream are normal. These changes tend to increase wind shear over the tropical Atlantic, which reduces the number of Atlantic hurricanes (note there haven't been any yet this year). It's also expected to make the winter of 2009/2010 mild across much of the northern hemisphere, which might lead to 2010 being a record warm year in the global average.
I'm sure we'll revisit the topic frequently in the coming months, including some review of important aspects of ENSO, and maybe some debunking of ENSO myths (e.g., increased rainfall in Southern California?).