La Nina is expected to appear for the fourth time in five years, increasing the risk of flooding in Australia in 2025, ABC News Australia said.
La Nina is a cooling phenomenon in the tropical Pacific Ocean that leads to changes in global weather, including increased moist easterly winds blowing towards Australia and subsequently increasing cloudiness and rainfall.
La Nina typically forms during the Australian winter (June to August), peaks in late spring (September to November) and then weakens in the summer. However, the 2025 version of La Nina is expected to break this pattern. It is expected to occur in the middle of Australia's summer for only the second time in 75 years.
La Nina is forecast to appear in 2024, but after an initial development in the fall, cooling in the Pacific stalls during the winter and spring.
With the likelihood of La Nina waning in December, sea temperatures along the equatorial Pacific Ocean have dropped sharply in the last weeks of 2024 and are now approaching La Nina thresholds.
ABC News Australia points out that there are already signs of La Nina in the Pacific Ocean. The key value used to track the state of the Pacific Ocean is called the Nino3.4 Index, and the latest reading reported by the Australian Bureau of Meteorology (BOM) is below 0.9 degrees Celsius, while the BOM's La Nina threshold is below 0.8 degrees Celsius.
In addition to sea temperatures, there are signs that weather across the Pacific is also in a La Nina phase. The trade winds are stronger than usual. The Southern Oscillation Index for December was +10, above the La Nina threshold of +7. Cloud cover has moved from the International Date Line to Indonesia...
The signals, both in the atmosphere and the ocean, need to be maintained for at least three months for La Nina to be officially identified. However, normal weather patterns have changed significantly before an official La Nina declaration is made.
If La Nina conditions persist for another two months, it would be the fifth La Nina since 2020 — a pattern that has only occurred twice in the 125-year history of data, from 2007 to 2011 and in the early 1970s. The only La Nina since 1950 with such a late onset was in 2008-2009.
