SOI (SOUTHERN OSCILLATION INDEX)

During the fourth quarter of 2004, the SOI was moderately negative. Individual monthly index values were -0.3, -0.9, -1.1 and .3 during October, November, December, and January, respectively. The most recent 5-month running mean value of the SOI was -0.5. This weakly negative 5-month running mean is consistent with weak El Niño conditions. Nearly all El Niño events are associated with a persistently negative SOI near -1.0 or lower. During La Niña, the SOI is persistently positive, near +1.0 or higher. With a gradual weakening of El Niño conditions in the Pacific basin, the SOI should be near zero for the next three to six months.