Southern Oscillation Index (SOI) is a climate parameter that indicates whether we are in an El Niño or a La Niña phase.
Positive SOI is La Niña; negative SOI is El Niño.
These phases are important, not so much for day-to-day weather, but for the longer term climate.
Sea levels around NZ are also affected, being up to 40 mm lower than average in El Niño and up to 40 mm higher than average in La Niña.
The data for the figure below come from the Australian Bureau of Meteorology, which also has lots of info on SOI.
Only the data from 2000 to the present are shown. In fact, the data go back to 1876.