SOUTHERN OSCILLATION INDEX (SOI)
The 3-month average of the Southern Oscillation Index for the 2nd Quarter of 2011 was 0.8, with monthly values of 1.9, 0.4, and 0.2 for the months of April, May, and June 2011, respectively. The recent shift from positive SOI values exceeding +1.0 (as present in April) to values below +1.0 indicates a shift from a La Niña event to an ENSO-neutral state.
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.