Don't sweat the small stuff
ENSO this, IOD that, SAM what? Long-range weather forecasting is complicated. Factoring in all of
the influences on the months and season ahead can get confusing. But it doesn't have to be with the
Bureau of Meteorology's Climate Outlooks.
In the past, the ability to forecast monthly and seasonal weather depended on an understanding of
the major influences, or drivers, of our climate. But since 2013 the Bureau has been using a
dynamical computer model that accounts for all of the major influences and much, much
more—even rare events like the recent Sudden Stratospheric Warming. The state-of-the-art physics-
based model uses ocean, atmosphere, ice and land measurements, to simulate future scenarios on
When the computer forecasts align with conventional wisdom, decision makers can have extra
confidence in the outlook. It's one of the reasons why the Bureau provides a commentary on the
state of Australia's climate drivers, rather than just churning out raw forecast data.
In Victoria, three of the climate drivers that influence our weather are the El Niño Southern
Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM).
ENSO is a natural cycle in tropical Pacific Ocean sea surface temperatures, wind and cloud. ENSO
swings between three phases: El Niño, neutral and La Niña. Typically, an El Niño or La Niña will start
in the first half of the year and last until the following autumn. When ENSO is neutral, it has little
effect on rainfall and temperature patterns. But during El Niño, Victoria usually has a drier than
normal winter–spring, while La Niña usually brings a wetter winter–spring for many areas.
Similar to ENSO in the Pacific Ocean, in the Indian Ocean we have the IOD. Again, the IOD naturally
cycles through three phases: positive, neutral and negative. Positive or negative IOD events typically
start in autumn and finish in spring. For Victoria, a negative IOD normally means above average
rainfall and cooler than average days. In contrast, a positive IOD normally means below average
rainfall and warmer than average days.
SAM is the irregular, north-south movement of the strong westerly winds that bring storms and cold
fronts to Victoria. And, you guessed it, SAM has three phases: neutral, positive and negative. But
SAM's influence changes during the year; a positive SAM in summer usually means above average
rainfall in central and eastern Victoria but in winter it's often drier than average in central and
western Victoria. Broadly, a negative SAM has the opposite effect.
Victoria's climate drivers interact with one another—they don't work in isolation. So, don't look to
just one indicator or driver for guidance on the months and season ahead. Instead, look at the
Bureau's Climate Outlooks for the complete picture.