What is SOI?
SOI meaning The Southern Oscillation Index (SOI) has been developed to monitor the Southern Oscillation using the difference between sea level pressures at Darwin, Australia, and Tahiti, although other stations have sometimes been used. Large negative values of the SOI indicate a warm event, and large positive values indicate a cold event (also referred to as La Nina). It is important to note that there is not a one-to-one correspondence between the occurrence of Southern Oscillation events and El Nino events, using the spatially restrictive original definition of El Nino.
reference: National Weather Service Glossary