CANBERRA, Sept 18 (Reuters) - Conditions in the Pacific Ocean have become more La Nina-like in recent weeks, but if the weather pattern does form, it is likely to be weak and short-lived, Australia's weather bureau said.

La Nina and its opposite, El Nino, are caused by the cooling and warming of sea surface temperatures off western South America.

Their development is of huge importance to global agriculture, with La Nina typically bringing more rain to Australia, Southeast Asia and India and less rain to cropping zones in the Americas, and El Nino bringing the opposite.

A U.S. government forecaster this month assigned a 71% chance to a La Nina forming between September to November and said it could persist into March. Several other national weather forecasters also believe a La Nina is likely.

Australia's Bureau of Meteorology has been more cautious. Its in-house climate model suggests La Nina will not develop.

"While some atmospheric indicators such as pressure, cloud and trade wind patterns over the Pacific have been more La Nina-like over the past few weeks, it remains to be seen whether these conditions will be sustained," the bureau said in a report circulated on Wednesday.

"It is possible a La Nina may develop in coming months but if so, it is forecast to be relatively weak (in terms of the strength of the sea surface temperature anomaly) and short-lived," it said.

If a La Nina forms this year it would be the fourth since 2020, which is highly unusual given the weather pattern has historically occurred on average every three to seven years.

Wet weather brought in part by La Nina led to record-breaking harvests in Australia between 2020 and 2023.

The bureau said a sustained period of record and near-record high sea surface temperatures around the world was making forecasting less certain.

Climate patterns like the swing between La Nina and El Nino "may not necessarily behave or evolve as they have in the past," it added.

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https://www.reuters.com/business/environment/la-nina-if-it-appears-will-likely-be-brief-australias-weather-bureau-says-2024-09-18/