SOI (SOUTHERN OSCILLATION INDEX)

During the second half of 2004, the SOI* was persistently negative with an average value of approximately -1.0. This was consistent with weak El Niño. For the past few months, the SOI has undergone some wild fluctuations with individual monthly index values of -1.1, + 0.3, -4.1, and -0.2 during December 2005, January, February and March 2005, respectively. February's SOI value of -4.1 was the second lowest value of the SOI recorded in over 100 years. The lowest value of the SOI ever recorded was -4.34 in June, 1877. This is just another indicator of the extreme departure from normal of the tropical Pacific climate during February 2005 - it was an extraordinary month by many measures. With the climate expected to continue its slide towards El Niño Neutral conditions, the SOI should average near zero for the next three to six months, with month-to-month fluctuations within the range of -1.0 to +1.0.

*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.