Pacific ENSO Update

3rd Quarter, 2007 Vol. 13 No. 3

SOI (SOUTHERN OSCILLATION INDEX)

The 6-month average of the Southern Oscillation Index was -0.4 for the first half of 2007, with monthly values of -1.1, - 0.5, -0.4, -0.4, -0.4 and +0.2 for the months of January – June 2007, respectively. This shift in sign from a negative to positive SOI value in June came after 6 consecutive months with negative indices (December 2006 - May 2007). Monthly SOI values will likely remain near neutral or slightly positive over the next several months, as La Niña conditions may begin to develop during July-September 2007.

*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.