Weather claims split
The United States and Australia have presented contrasting outlooks on the likelihood of an El Niño climate pattern emerging this year.
Australia's Bureau of Meteorology (BOM) estimates a 50 per cent chance of the major climate driver, associated with warmer and drier conditions, developing during the winter or spring.
Conversely, the US forecast agency, the National Oceanic and Atmospheric Administration (NOAA), asserts that an El Niño event is almost certain, with a 90 per cent chance of occurrence this year.
The significant disparity in these figures raises questions about the factors influencing the forecasts.
Despite monitoring the same region of the Pacific Ocean, the agencies employ different criteria to determine probabilities, with the BOM adopting a stricter approach.
To understand the variance, it is crucial to comprehend the mechanics of El Niño. This phenomenon is considered “coupled”, as conditions in the ocean interact with the atmosphere in a self-reinforcing manner.
During an El Niño event, the waters in the central and eastern tropical Pacific Ocean become unusually warm, leading to weakened trade winds that typically blow from east to west across the globe.
As a result, the warm water accumulates in the eastern Pacific, further diminishing the trade winds. Consequently, cloud and tropical rainfall increase near South America, while Australia experiences a dearth of precipitation.
Experts say that the difference in criteria between NOAA and the BOM lies in their emphasis on specific aspects of El Niño.
NOAA primarily focuses on sea surface temperatures when determining the probability, while the BOM requires both oceanic and atmospheric indicators to demonstrate change before upgrading the likelihood to an “El Niño alert”, which indicates a 70 per cent chance.
The BOM also sets a higher threshold for sea surface temperatures compared to NOAA.
Specifically, NOAA's outlook considers sea surface temperatures at NINO3.4, with a requirement for them to be more than 0.5 degrees Celsius warmer than average for five consecutive, overlapping three-month periods.
Conversely, the BOM's shift from an El Niño “watch” to an El Niño “alert” necessitates the fulfilment of three out of four criteria relating to sea surface temperature, winds, air pressure and climate models.
When officially declaring the presence of an El Niño, the criteria of both agencies align more closely, with both requiring observed atmospheric changes, although the BOM still mandates warmer ocean temperatures compared to NOAA.
The BOM says that current conditions and forecasts are close to the requirements for transitioning from a “watch” to an “alert” status. However, atmospheric conditions are presently keeping the odds at 50 per cent, according to the agency.
The Bureau's stringent criteria aim to ensure confidence in predicting the impacts of El Niño before sounding the alarm.
The agency wants to avoid prematurely declaring an El Niño if the expected hot and dry conditions are unlikely to materialise.
In contrast, the US and South America are less influenced by conditions on the western side of the Pacific near Australia, making it a lesser consideration for them.
The Bureau says that the metrics employed by the US, Australia, and Japan are tailored to their respective regions' impacts from El Niño and La Niña.