SOI (SOUTHERN OSCILLATION INDEX)

Throughout 2005, the SOI has trended upward with some large month-to-month variations. From January 2005 through December 2005, the monthly values of the SOI were: + 0.3, -4.1, -0.2, -1.0, -1.2, + 0.1, 0.0, -0.8, + 0.4, + 1.1, -0.3, -0.2. The 5-month running mean centered on October was approximately zero. With the weak La Niña forecast, the SOI should average slightly positive for the next three to six months, with month-to-month fluctuations within the range of -0.5 to +1.5. During La Niña the value of the SOI tends to be positive and during El Niño it tends to be negative. (See the special discussion of the SOI ).

*Note: The SOI is defined as the normalized pressure difference between Tahiti and Darwin. There are several slight variations in the SOI values calculated at various centers. 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.