SOI (SOUTHERN OSCILLATION INDEX)

The 3-month average of the Southern Oscillation Index was 1.1 for the 4th Quarter of 2007, with monthly values of +0.6, +0.9 and +1.8 for the months of October, November and December 2007, respectively. The SOI has had a positive sign since August 2007. Monthly SOI values are expected to remain positive for the next several months, as moderate La Niña conditions are expected to continue through the northern hemisphere’s spring months.

*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.

 

Oceanic Nino Index graphic from NOAA's U.S. Climate Prediction Center