El Nino Watch
The standardised air pressure difference between Tahiti and Darwin. Negative values go with El Nino; positive with La Nina.
Indices
The SOI measures the atmosphere half of ENSO. It compares sea-level air pressure at Tahiti (central Pacific) with Darwin (northern Australia).
The sign convention is the opposite of the ocean indices: a warm Pacific (positive ONI) goes with a negative SOI. One month of SOI is noisy (a single storm near Darwin or Tahiti can swing it). Look for values that stay negative for several months.
The NOAA CPC version is standardised (in units of standard deviations), so typical values are between about -3 and +3. The Australian version uses a different scale: see SOI (Australian Bureau of Meteorology, Troup).
Below zero for months = consistent with El Nino. In September 2026 the CPC reported both the traditional and equatorial SOI as negative.