SOI (SOUTHERN OSCILLATION INDEX)

The 3-month average of the Southern Oscillation Index for the 2nd Quarter of 2008 was +0.3, with monthly values of +0.9, -0.5 and +0.5 for the months of April, May and June 2008, respectively. After reaching a high value of +4.4 in March, the SOI weakened considerably. May marked the first time in 11 months that the SOI value has been negative. The SOI is expected to hover between +0.5 and –0.5 as ENSO-neutral conditions continue through the end of the Northern Hemisphere Fall season.

*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.

Ropelewski, C.F. and Jones, P.D., 1987: An extension of the Tahiti-Darwin Southern Oscillation Index. Monthly Weather Review 115, 2161-2165.