The tropical Pacific Ocean and atmosphere swings, or oscillates, between warm, cool and neutral phases on a timescale of a few years.
A typical El Niño or La Niña event may show its first signs of development during the southern hemisphere autumn and strengthen over winter and spring. It will normally start to decay in the mid to late southern summer, and finally dissipate in the subsequent autumn. ENSO events typically decay during autumn, as this is the time of year when the tropical Pacific Ocean naturally evens out the temperature difference between the east and west. This annual weakening of the temperature gradient across the Pacific also means the weather patterns which help reinforce a La Niña or El Niño ease, allowing ENSO to return to neutral.
In general, El Niño events tend to only last for a single cycle (i.e. one year from autumn to autumn), but it is not uncommon for multi-year La Niña events to occur. For example, the 1998–2001 La Niña affected three consecutive years from autumn 1998 to autumn 2001.
Watching out for events
The first signs of an emerging El Niño or La Niña event are often observed in the ocean. The Bureau of Meteorology monitors and reports on a range of ENSO indicators, including:
- short-term bursts of tropical rainfall activity
- water temperatures at the sea surface and at depth
- ocean heat content – measuring the amount of energy stored in the ocean
- the Southern Oscillation Index
- atmospheric air pressure
- cloudiness – measuring the amount of cloud in tropical regions
- the strength of the trade winds and winds higher in the atmosphere
- ocean currents.
These climate indicators provide information about current ENSO conditions, and are inputs into climate models that are used to predict conditions for the months ahead.
Climate models come in two forms:
- Statistical climate models are based on what has happened in the past – that is, they use historical patterns to estimate what is likely to happen in the future.
- Dynamical climate models are based on physics – that is, they model the physical processes driving the current climate situation forward in time to predict what is likely to happen.
Data are collected from a wide variety of sources – including satellites, buoys (moored, drifting and expendable), sea level analysis and meteorological surface observations – all drawn from a network of national and international observing systems.