SOUTHERN OSCILLATION INDEX (SOI)
The 3-month average of the Southern Oscillation Index for the 3rd Quarter of 2011 was +0.8, with monthly values of +1.0, +0.4 and +1.0 for the months of July, August, and September 2011, respectively. These positive SOI values, in addition to negative sea surface temperature readings, indicate the presence of a La Niña event in the tropical Pacific Ocean. Currently, this La Niña event is not as strong as it was at the end of September 2010.
Normally, positive SOI values in excess of +1.0 are associated with La Niña conditions, and negative SOI values below -1.0 are associated with El Niño conditions. Low SOI values suggest a weak coupling between the ocean and the atmosphere. The SOI is an index representing the normalized sea-level pressure difference between Darwin, Australia, and Tahiti. The historical record of the SOI from 1866 to present was calculated based on the method given by Ropelewski and Jones (1987). The complete table can be found at http://www.cru.uea.ac.uk/cru/data/soi.htm.