SOI (SOUTHERN OSCILLATION INDEX)

During 2005, the SOI trended upward with some large month-to-month variations. During the first months of 2006 the overall positive trend continued with January, February March, and April monthly values of + 1.8, -0.2, + 1.4, and +0.9 With the climate transitioning from weak La Niña to ENSO neutral conditions, the SOI should average near zero (or slightly positive) for the next three to four months, with month-to-month fluctuations within the range of -0.5 to +1.5. 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). During El Niño the value of the SOI tends to be negative and during La Niña it tends to be positive