SOUTHERN OSCILLATION INDEX (SOI)
The 3-month average of the Southern Oscillation Index for the 4th Quarter of 2011 was +1.5, with monthly values of +0.8, +1.1, and +2.5 for the months of October, November and December 2011, respectively. A relatively strong phase of La Niña opened the year, then dissipated in the spring. It then re-emerged in October and lasted through the end of the year.
*Note: The SOI is defined as the normalized pressure difference between Tahiti and Darwin, Australia. Normally, positive values of the SOI in excess of +1.0 are associated with La Niña conditions, and negative values of the SOI below –1.0 are associated with El Niño conditions. There are several slight variations in the SOI values calculated at or other sites representative of the western and eastern tropical Pacific, respectively. 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.