ENSO & IOD Bureau of Meteorology climate driver indices

About these indices

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Source: Australian Bureau of Meteorology climate driver data (bom.gov.au/climate/influences). Weekly Relative Niño1, Niño2, Niño3, Niño3.4 and Niño4 and the IOD (Dipole Mode Index) are from the Bureau's GAMSSA sea surface temperature analysis, as anomalies from the 1991–2020 average, from July 2008. The 30‑day SOI is updated daily (from December 2023 in this file); the monthly SOI runs from 1876. Data © Commonwealth of Australia (Bureau of Meteorology).

Thresholds: the Bureau treats sustained values beyond ±0.8 °C (Relative Niño3/Niño3.4), ±7 (SOI) and ±0.4 °C (IOD) as typical of El Niño / La Niña or positive / negative IOD. A value past a threshold is not by itself an event: the Bureau's official status also uses sub-surface temperatures, winds, cloud and model outlooks (Climate Driver Update). “Periods beyond the thresholds” are a simple screen of each index on its own.

Method: “same time of year” compares a week with the week ending nearest the same calendar date in every other year (within 3 days), a 30‑day SOI value with the same date, and a month with the same calendar month. Ranks are among those values; percentiles place the value within them. The 30‑day SOI is also set against every monthly SOI for the calendar month its window mostly covers. Smoothed lines are trailing means.

Created by: Nick Silkstone. Contact: nick.silkstone@gmail.com · About these indices