The SOI indicates whether we are in an El Niño or a La Niña pattern.

• Sustained negative SOI indicates El Niño
• Sustained positive SOI indicates La Niña

SOI for the last two years is plotted below. The data come from the Bureau of Meteorology, Australia SOI website, which also contains more details of what SOI means and how it is calculated.
The plot below is updated on the 5th of each month when new SOI data become available.