SOI (SOUTHERN OSCILLATION INDEX)

In October, the SOI was negative for the sixth consecutive month. During the past 6 months, the value of the SOI was -0.8, -0.7, -0.8, -1.6, -0.7 and -1.7 for the months May through October, respectively. The persistently negative values of the SOI that occurred since May 2006 are consistent with weak El Niño in the Pacific basin. With the climate expected to be in a state of weak to moderate El Niño for the next few months, the SOI should average near –1.0 for the next three to six months, with month-to-month fluctuations within the range of –0.25 to - 1.75. 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.

*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.