SOUTHERN OSCILLATION INDEX (SOI)
The 3-month average of the Southern Oscillation Index for the 1st Quarter of 2011 was 2.5, with monthly values of 2.3, 2.7 and 2.5 for the months of January, February, and March 2011, respectively. These recent positive SOI values coincide with abnormally cold ocean waters across the eastern tropical Pacific typical of a La Niña episode.
*Note: 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, 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.