El Nino Watch
Australia's version of the SOI, on a different scale from NOAA's: about 10 times larger. Sustained values below -7 often mean El Nino, above +7 La Nina.
Indices
The Bureau of Meteorology (BoM) computes the SOI with the Troup method: the standardised anomaly of the Tahiti minus Darwin pressure difference, multiplied by 10, with a 1933-1992 base period.
So a BoM SOI of -12 is roughly comparable to a NOAA CPC SOI of about -1.2. Do not compare the two numbers directly.
BoM's rule of thumb: sustained values below -7 often indicate El Nino; sustained values above +7 are typical of La Nina.
Negative = El Nino-like. Look at several months: single months are noisy.