Days after Australia's Bureau of Meteorology (BOM) issued an El Niño warning, its US equivalent has raised the stakes and declared the weather event is already underway.

The National Oceanic and Atmospheric Administration (NOAA) — the authority on weather updates in the US —  issued this statement in its monthly Pacific analysis:

"El Niño conditions are present and are expected to gradually strengthen into the Northern Hemisphere winter 2023-24."

The evolution to El Niño has been predicted by models since late 2022 and the event is almost guaranteed to become the first fully established Pacific warm episode in eight years. When this is combined with a dry signal also emerging from the Indian Ocean, the dice are now heavily weighted in favour of a hot and dry second half of 2023 for Australia.

The transition to El Niño has been rapid following three years of the Pacific's opposite cool phase, La Niña. This was responsible for a record wet summer across parts of northern Australia and record flooding over south-east Australia through 2021 - 2022.

Pacific showing clear signs of El Niño

There are several key indicators used to assess the state of the Pacific and the majority are now firmly in El Niño territory.

• Sea surface temperatures along the equatorial Pacific: The water temperature is currently 0.81C above average in the key monitoring zone, exceeding the El Niño threshold of 0.5C to 0.8C depending on which criteria is used
• Sub-surface temperatures along the equatorial Pacific: Water temperatures are well above average below the surface, which often promotes further surface warming

Critically, the atmosphere is now responding to the change in the ocean and the two are beginning to re-enforce one another.

• Tropical winds: Easterly trade winds weakened during May indicating a weakening of the Walker circulation
• Southern Oscillation Index (SOI): The latest 30-day value of -20 is well beyond the El Niño threshold of -7. This indicates pressure is increasing over Australia's latitudes and decreasing over the central Pacific
• Tropical cloud cover: Convection during May was enhanced along the equator and was suppressed over Indonesia

The atmospheric indicators above all suggest weather patterns, which suppress rain, are now active over Australian longitudes, and patterns which enhance rain have developed over the Pacific.

Why the BOM has not declared El Niño

The BOM abide by a stricter definition of El Niño compared to NOAA to suit Australia's complex relationship with the Pacific.

"Our own criteria for assessing El Niño places an emphasis on atmospheric indicators as well as oceanic ones, since these are an important driver of the impacts of El Niño on Australian rainfall," a bureau spokesperson said.

"While the bureau and NOAA scientists routinely discuss current climate conditions and collaborate on research, it is important that the bureau follows a process that is consistent and objective from one year to the next."

Despite the BOM's cautious approach, their seasonal rain outlooks have already picked up on the Pacific transition, displaying very little hope of good rains for the remainder of the year.

"We have an El Niño event developing in the Pacific that is at ALERT status, and now requires just one additional criteria for the declaration of an event," the spokesperson said.

The July to September outlook shows little chance of above median rain for Australia (Bureau of Meteorology)

What's impossible to determine, though, is how much the dry signal is due to El Niño and how much is due to the Indian Ocean, which often has a greater impact on Australia's weather during the cooler months.

This was no more apparent than through a record hot and dry 2019 when the Pacific was neutral, but the Indian Ocean was in a positive (dry) phase.

El Niño brings heat, drought, bushfires and less snow

El Niño is a warming of the central and eastern equatorial Pacific which alters weather patterns around the globe.

For Australia, the greatest impact is due to a weakening of the Walker Circulation, a broadscale circulation that normally drives moist, easterly winds across the tropical Pacific Ocean towards Australia.

Global temperatures will differ according to location (Bureau of Meteorology)

The absent or weaker easterlies during El Niño, along with higher pressure and cooler sea surface temperatures over northern Australia, reduces convection and therefore cloud and rain.

Impacts of El Niño on Australia 

•  Reduced winter and spring rain across the eastern two thirds 
•  Higher maximum temperatures for the southern two thirds of Australia
•  Increased risk of extreme heat
•  Increased bushfire danger
•  Less alpine snow
•  Late start to the northern wet season
•  Reduced tropical cyclone numbers
•  Potential for more frost due to clear skies

El Niño's impact on Australian weather diminishes during summer and the episode typically decays by the following autumn.

// TOM SAUNDERS