Tuesday, September 27, 2011

La Nina signals strengthening

As seasonal forecasts for the North American heating season will start to filter out, keep in mind the behavior of the Southern Oscillation Index (SOI) which has be edging even further into positive phase, supporting a return to a mild La Nina for winter.  The table and graph below are the recent numbers from the Australia Bureau of Meteorology.

The current monthly Sea Surface Temperature anomaly is shown in the first map below (source: NOAA Earth System Research Laboratory) for the month ending 24 Sep.  Note the significant increase on the cold surface anomaly between 0 and 30S across the Pacific.   Further, most dynamical models are expecting weak La Nina conditions to strengthen at least through the front end of the winter, and some models are showing a continuation of La Nina conditions through spring 2012.  From a probabilistic perspective, there may be a higher likelihood for a snowy eastern US, another cooler year in the western states, and continued drought in the south; see the map at the bottom from the NCDC for general seasonal relationships.

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