SOUTHERN OSCILLATION INDEX (SOI)
The 3-month average of the Southern Oscillation Index for the second quarter of this year was -0.2, with monthly values of -0.3, 0.0, and -0.4 for the months of April, May, and June (AMJ) 2012, respectively. Over the last few months, SOI observations have been trending toward a larger negative value and closer to El Niño. The shift of SOI values from -0.3 to 0.0 to -0.4 over the AMJ months of 2012, indicate a hovering near the threshold of an El Niño.
Normally, positive SOI values in excess of +1.0 are associated with La Niña conditions, and negative SOI values below -1.0 are associated with El Niño conditions. Low SOI values suggest a weak coupling between the ocean and the atmosphere. The SOI is an index representing the normalized sea-level pressure difference between Darwin, Australia, and Tahiti.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: