After three years of dominating weather patterns around the globe, La Niña has now ended.
La Niña — the cool phase of the eastern Pacific Ocean, which typically brings rain and floods to Australia on the ocean's warmer western edge — has been weakening for months, as evidenced by sea surface temperatures returning to normal along equatorial areas of the globe.
Critically, the atmosphere has now responded to the oceanic change and is returning to a near-average state.
In other words, the weather is finally getting back to normal.
La Niña's demise was confirmed overnight by the National Oceanic and Atmospheric Administration (NOAA), whose monthly analysis said: "La Niña has ended and ENSO-neutral conditions are expected to continue through the Northern Hemisphere spring and early summer 2023."
The third edition of a triple La Niña was a relatively short and weak phase, but still helped to produce a record wet summer for parts of northern Australia.
Stronger and longer La Niñas, from late 2020 to mid-2022, brought record rain to parts of south-east Australia and widespread major flooding.
It was only the third triple La Niña since 1900 — following ones from 1998 to 2000 and from 1973 to 1975 — although borderline triples also occurred in the 1950s and 1910s.
The Pacific's opposite phase, El Niño, which can bring drought to eastern Australia has not occurred since 2015.
La Niña changes weather patterns
La Niña refers to a cooling of the central equatorial Pacific and the subsequent change to weather patterns.
For Australia, the greatest impact is due to an enhancement of the Walker Circulation, a broadscale circulation that drives moist, easterly winds across the tropical Pacific Ocean towards Australia.
This leads to a piling up of warm water and humid air off our northern coastline — along with lower air pressure, promoting increased convection — and, as a result, increased cloud and rain.
How we know La Niña is over
There are several key indicators used to assess the state of the Pacific and the majority are now back in a non-La Niña or neutral state:
- Sea surface temperatures along the equatorial Pacific: The water temperature is currently only 0.2 below average, well above the La Niña threshold used by the Bureau of 0.8C below average.
- Sub-surface temperatures along the equatorial Pacific: Water temperatures are now above average below the surface. During La Niña episodes the Pacific sub surface is cooler than normal.
- Trade winds across the tropics: Easterly wind anomalies are weakening. During La Niña, easterly winds are enhanced by a strengthening Walker circulation.
- Upper-level westerly winds above the tropics: The return leg of the Walker circulation is also easing, but remains near La Niña levels over the central Pacific. During La Niña, the enhanced Walker circulation increases upper-level westerly winds.
- Southern Oscillation Index (SOI): Pressure patterns are now returning to normal and the SOI has fallen close to neutral values. During La Niña, the pressure increases over the eastern Pacific and decreases around Australian longitudes.
- Tropical cloud cover: The amount of convection along the equator is returning to normal after a near three-year uninterrupted spell of suppressed activity.
What La Niña's demise means
The rains don't simply turn on and off like a tap when the Pacific moves between phases, and often there is a lag of a few weeks until the weather fully recovers from its previous state.
However, the change is similar to a lowering of water pressure and speed in a rotating sprinkler: the rains still come but they won't be as heavy or as frequent.
What follows through the rest of 2023 is a question that won't be answered until late autumn or winter.
One scenario — which is universally agreed to be nearly impossible — is a fourth La Niña, meaning whichever way the Pacific rolls, drier and hotter times are ahead for Australia.
