SOI (SOUTHERN OSCILLATION INDEX)

During the first 6 months of 2006, the value of the SOI was + 1.8, - 0.2, + 1.4, + 0.9, -0.8, and -0.7 for the months January through June, respectively. Whereas during the latter half of 2005 the SOI trended upward, during the first half of 2006 it reversed itself and trended downward with some large month-to-month variations. The 5-month running mean of the SOI centered on October 2005 was approximately zero, by February 2006 it peaked at + 0.74, and by April it had fallen back to +.12. With the climate expected to be in a state of ENSO-Neutral for the next few months, 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 + 0.5. 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. The SOI is an index representing the normalized sea level pressure difference between Darwin, Australia and Tahiti (or other sites representative of the western and eastern tropical Pacific, respectively).