Currents of change: Tracking the El Niño/La Niña cycle
19 June 2014
As emerging El Niño predictions hint at drier and warmer conditions for Australia in the second half of the year, the Bureau has launched an online tool for tracking the evolution of this complex natural phenomenon.
For many individuals and industries in Australia, the period leading up to an El Niño can mean ‘decision time’. With an increase in the likelihood of drier and warmer seasons, and the subsequent impact upon available water resources, it’s critical that people making climate-sensitive decisions have the best information at their fingertips.
El Niño and its opposite phase, La Niña, are the result of periodic changes in the temperature of the tropical Pacific Ocean and the atmosphere above it, as part of a natural cycle known as the El Niño–Southern Oscillation. In the case of El Niño, these changes are associated with a warming of the central and eastern tropical Pacific Ocean, whereas La Niña events are a result of cooling of these same areas.
La Niña events can also have significant outcomes, with the most recent ‘double-dip La Niña’ in 2010–12 causing widespread flooding over much of eastern Australia.
An early indication that an El Niño or La Niña could be developing can give those most susceptible to our variable climate time to prepare—potentially preventing or mitigating financial losses and other adverse consequences.
A continuum of change
An El Niño and its impacts don’t happen overnight; it’s not like flicking a switch. The transition from normal or ‘neutral’ conditions to El Niño should be considered more as a continuum. Changes happen gradually, usually over a period of several months, and can fluctuate back and forth over shorter time scales.
Collectively, this longer-term ‘see-sawing’ between El Niño, La Niña and neutral conditions is what meteorologists refer to as the El Niño–Southern Oscillation, or ENSO.
Climatologists, including those at the Bureau of Meteorology, monitor the oceans and atmosphere in the tropical Pacific regions for signs that the system might be shifting one way or another. At times temperature patterns in the tropical Pacific Ocean may vary away from their normal state. The atmosphere and ocean interact, at times reinforcing each other and creating a ‘feedback loop’, which can strengthen these small changes in the state of the ocean into an ENSO event.
When it is clear that the ocean and atmosphere are fully coupled—that is, they are mutually reinforcing each other—an ENSO event is considered established.
Tracking the ENSO cycle
The ENSO Tracker is based upon a comprehensive analysis by Bureau climatologists, including the survey of eight international climate models which monitor changing ocean temperatures over the central and eastern tropical Pacific Ocean.
The Tracker classifies the current state of the ENSO cycle according to three alert stages (WATCH, ALERT and EL NIÑO/LA NIÑA). An online summary of its findings is updated every two weeks. Each of these alert stages indicate the level of development of an ENSO event—in other words, how far the overall state of the climate seems to be tipping towards either La Niña or El Niño.
If some oceanic or atmospheric indicators are showing values indicative of the early stages of an El Niño event, which in the past have gone on to exceed El Niño thresholds about 50% of the time, an El Niño WATCH is issued. This means there is a 50% chance that an El Niño will develop—usually in the winter to early spring months. There is also a 50% chance, however, that these early signals are just shorter-term fluctuations, and the system will remain in a neutral state.
If these early El Niño signals continue to strengthen, and other indicators start to shift towards El Niño thresholds, an El Niño ALERT will be issued. In this stage, there is a 70% chance that the system will go on to develop into an El Niño—although there remains a 30% chance that conditions will remain neutral.
ENSO Tracker indicating an El Niño WATCH (left) and El Niño ALERT (right)
Finally, when the system is fully coupled (both the atmosphere and ocean are reinforcing the El Niño state), an EL NIÑO, the final stage of the ENSO Tracker, will be declared.
When Bureau of Meteorology climatologists deem that the El Niño has broken down and is no longer likely to impact upon Australian rainfall and temperature patterns, the ENSO Tracker will be returned to NEUTRAL.
ENSO Tracker indicating an El Niño event (left) and no impact with NEUTRAL (right)
The ENSO Tracker provides a simple and convenient snapshot of the current status of ENSO, and the likelihood of an ENSO event developing later in the year. The Tracker will assist people in preparing ahead of time for challenging climatic conditions—and help to protect the livelihoods of many Australians.