El Niño declared by BOM, and it could become the strongest on record

The Bureau of Meteorology has declared an El Niño. (ABC News) Link copiedShareShare articleThe Bureau of Meteorology (BOM) has officially declared El Niño active, and modelling tips it could become the strongest ever event in the modern era. El Niño refers to an extended period of warmer than normal waters in the central tropical Pacific. It is a state that can linger for up to 12 months and disrupt weather patterns across the globe. Australia is especially prone to the impacts of these changes and past events have been responsible for some of our hottest and driest weather on record. It is somewhat ironic that El Niño's arrival coincides with what is fast becoming a wet month across much of Australia. This week alone a major north-west cloudband is dropping rain over the southern two-thirds of the nation, adding to multiple previous soakings since early May. The key to understanding this apparent contradiction is that El Niño is an observation and not a forecast. When El Niño is declared, it is solely on the basis of what is happening in the atmosphere and oceans across the tropical Pacific. The prevailing conditions over Australia, or any individual country for that matter, are irrelevant in determining whether or not El Niño is present. And for Australia, surrounded by three oceans, El Niño is not the only driver of our weather patterns. Both the Indian Ocean and Southern Ocean, along with local water temperatures, also play a major role in influencing our weather, and currently they are swinging the pendulum to favour rainfall. But the odds are, as El Niño develops further, a drier second half to the year is likely, and the BOM's latest three-month outlook still favours below median rainfall for much of southern and eastern Australia. Below median rainfall is still tipped from July to September as El Niño continues to develop in the Pacific Ocean. (ABC News) El Niño also typically reduces mid- and late-spring rainfall, however even though El Niño might peak in summer, the impacts on Australia's rainfall diminishes, as was the case during the last El Niño in 2023. But El Niño also influences other components of our weather, including a likelihood of: However, perhaps the most important point is that every event is different, and while generally trends can be derived from past events, when it comes to weather, there is no such thing as a guarantee. Meteorologists and climatologists monitor several features of the Pacific Ocean to gauge its status. The most commonly used indicator is a simple measure of the water temperature in a box along the equator called the Niño3.4 region. The temperature in the Niño3.4 region is now above El Niño thresholds. (Supplied: NOAA) And the latest weekly values of Niño3.4 now exceed the El Niño threshold of 0.8C for the first time since the last event ended in early 2024. But it is one of the atmospheric indicators that is currently showing the strongest signal — the 30-day Southern Oscillation Index (SOI) has plummeted to -23.3, well beyond the El Niño threshold of -7. A deeply negative SOI denotes a change in the pressure pattern across the tropical Pacific where high pressure is dominating Australian longitudes and lower pressure resides near Tahiti. Other signals that confirm El Niño are weaker trade winds, changes in tropical cloud cover, and a slab of very warm water sitting just below the surface. Since most El Niño's do not peak until November to January, an event emerging as early as June will have around six months to intensify. The rate of warming this year in the Niño3.4 zone is already the fastest since 1943, and the vast majority of modelling forecasts the temperature of the equatorial Pacific will continue to rapidly climb during the coming months. The BOM's seasonal model called ACCESS-S, is tipping an all-time record later in the year with a peak warming in excess of 3C above normal, comfortably above the previous post 1900 high of 2.65C from November 1902. Thankfully though for Australia, there is only a very weak relationship between the strength of the Pacific warm signal and the local impacts, so a record event does not mean a record drought. Another noteworthy development from a statistical standpoint is 2026 is now on track to be the 7th consecutive year with either El Niño or La Niña, when historically about 50 per cent of years are neither, what is called a neutral Pacific. The last time this many non-neutral years were tallied in a row was the period from 1969 to 1976.

El Niño declared by BOM, and it could become the strongest on record

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