The BoM has announced El Nino conditions are underway. Dr Andrew Watkins explains what these climate indicators mean for grain growers. Source: Bureau of Meteorology
The Bureau of Meteorology (BoM) recently announced that El Niño conditions were underway in the Pacific Ocean. Typically this would mean warmer and drier weather for eastern Australia over the next six months. But what do climate terms such as El Niño and ENSO mean, and how can growers use them in decision-making?
The BoM's manager of Climate Prediction services Dr Andrew Watkins, says our weather is affected by a number of different climate drivers, of which the El Niño Southern Oscillation (ENSO) is one of the most influential.
“ENSO is the response of the climate to movements of warm water in the Pacific Ocean. El Nino is when the warm waters move towards the eastern Pacific, the clouds move east, taking rains towards the Americas and away from Australia. The opposite, when the warm water comes west and brings clouds and rain this way, is La Niña,” he said.
El Niño events usually last for one season, developing in autumn and winter and starting to dissipate by summer, though have been known to last between six months and two years. El Niño occurs every three to five years on average.
“In a ten-year period, we would expect to see about two El Niño years, two La Niña years, and six neutral. While La Niña can often follow two years in a row, it’s uncommon for El Niño to do so; it’s usually a one-year situation, affecting winter and spring rainfall and temperature,” he said.
Key climate indicators
ENSO (El Nino Southern Oscillation) – the swing between El Nino and La Nina conditions. During El Nino, seas in the eastern Pacific Ocean warm due to a weakening of the trade winds that bring cool water from the deeper ocean. El Nino reduces rainfall in eastern Australia. La Nina brings about the opposite conditions, resulting in an increase in rainfall.
IOD (Indian Ocean Dipole) – an ocean-atmosphere phenomenon in the Indian Ocean, driven by the difference in ocean surface temperature between the western tropical Indian Ocean, and the east. During a positive IOD event (the sea cools in the eastern part of the Indian Ocean) there is decreased rainfall in south-east Australian. In a negative IOD event (sea warms in the east Indian Ocean) there in increased rainfall.
SAM (Southern Annular Mode) – the north/south movement of strong westerly winds. A positive SAM event results in contraction of strong winds towards the South Pole, resulting in high pressure over Australia and decreases the chance of winter rainfall in south-eastern Australia. A negative SAM event will increase the likelihood of winter rainfall. SAM has a shorter life than IOD or ENSO, varying on a fortnightly basis.
The Indian Ocean Dipole (IOD) is a similar phenomenon to ENSO, only in the Indian Ocean. It is currently neutral, but about half the models surveyed by the BoM suggest it could move into a positive mode in spring. A positive IOD is generally associated with lower rainfall in south-east Australia.
Since 1958 there have been 11 years where the IOD is positive, with the average rainfall in these years in the South Australian wheat belt at decile 3. While the relationship between ENSO and IOD is complex, about half of all IOD events occur in El Niño years, such as this year.
“The effect of a positive IOD is similar to El Niño, though it is shorter lived as it usually starts in winter and collapses in late spring-early summer, when the monsoon season starts. The combination of El Niño and a positive IOD are likely to result in particularly dry conditions for early spring,” Dr Watkins said.
The easiest way for a grower to keep ‘on-top’ of these climate influences is by following the Bureau's ENSO Wrap Up, released every second Tuesday, and by keeping an eye on outlooks from POAMA, the Bureau of Meteorology's climate outlook model.
“Unlike older statistical forecasts, POAMA uses mathematical equations representing the laws of physics to model the ocean and the atmosphere. It can effectively take into account the complex relationships between each of these indicators to provide the more probabilistic outcomes,” Dr Watkins said.
Once a grower has the best available climate information, decisions can then be considered to make the most of the forecast conditions.
It is essential to always keep in mind that weather forecasts can only ever be probabilities; there will always be a level of risk involved with making agricultural decisions based on climate outlooks.
Diversifying a cropping program with a range of crops, varieties and sowing dates, is a solid strategy for minimising risk exposure.
At this stage in the season, the key decision growers need to make is to choose whether to apply or hold off on nitrogen applications. The GRDC-funded app ‘CliMate’ can help make this decision by analysing seasonal and agricultural factors.
Dr Andrew Watkins, 03 9669 4360, firstname.lastname@example.org
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