SOI (SOUTHERN OSCILLATION INDEX)

November marked the first month to have a positive SOI value since April 2006. During the second half of the year, the value of the SOI was -0.8, -1.6, -0.7, -1.7, +0.1 and -0.5 (for the July through December, respectively). These persistently negative values of the SOI are consistent with El Niño in the Pacific basin, although their magnitudes are not that significantly large. Furthermore, the neutral-to-weak negative values of the SOI within the last 2 months are consistent with the weakening ENSO event. Normally, positive values of the SOI in excess of +1.0 are associated with La Niña, and negative values of the SOI below –1.0 are associated with El Niño.

For an historical look at how the SOI relates to Guam rainfall, click here.

*Note: The SOI is defined as the normalized pressure difference between Tahiti and Darwin, Australia. 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.