The SOI measures the difference in surface air pressure between Tahiti and Darwin. Sustained positive SOI values above about +8 indicate a La Niña event while sustained negative values below about –8 indicate an El Niño.
Positive values indicate enhanced easterly winds over the tropical Pacific which tends to cool sea surface temperatures (La Nina). Prolonged negative values indicate reduced easterlies which allow warming or El Nino conditions to develop.
Since October the SOI has reversed from a strong negative signal to mostly positive which suggests that the current El Nino is set to weaken rapidly.
Data source https://www.longpaddock.qld.gov.au/