A La Nina is almost certain to develop in the Pacific this summer, as all eight models used by the Bureau of Meteorology anticipate threshold levels for the key climate pattern will be reached or exceeded by next month.
The bureau's latest fortnightly assessment of conditions in the Pacific and Indian oceans rates the likelihood of a La Nina this summer at about 70 per cent – triple the usual level – prompting it to issue a La Nina alert on Tuesday.
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| La Ninas tend to result in above-average rainfall for eastern Australia and more cyclones than usual crossing the coast. Photo: Glenn Hunt |
While the added cloud cover can moderate temperatures, there is also an increased chance of prolonged warm spells for south-eastern Australia during La Ninas, the bureau says. The cyclone season can also be more active than usual.
But, as reported by Fairfax Media last week, the Indian Ocean's influence is tending to counter some of the typical La Nina characteristics, prompting the bureau to predict much of the country may yet get a warmer and drier than average summer.
All eight of the main models used by the bureau point to La Nina thresholds being reached or exceeded next month, while most suggest the event will last until at least February. (See bureau chart below.)
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| Much of southern Australia has been in a grip of early season heat so far this November. Photo: Daniel Munoz |
Sea-surface temperatures in the eastern Indian Ocean have been cooler than average, which "may be limiting the feed of moisture over Australia, and opposing more typical La Nina influences", the bureau says.
'Blocking high'
In the near term, conditions have contrived to generate exceptionally warm conditions over much of southern Australia.
A large blocking high has set up in the Tasman, allowing cities such as Melbourne and Hobart to experience extended early season heat.
"It's unusual to get such a persistent blocking in November," Blair Trewin, senior climatologist at the bureau, said.
Record tumble
Melbourne, for instance, has posted its eighth day of 30 degrees this spring, with as many as three more to come. That compares with an average of just two pre-summer days of that heat.
The city has also notched four days above 28 degrees in a row, including Tuesday, a tally that could stretch to seven or eight before the hot spell ends. The current record for November is six such consecutive days, Dr Trewin said.
Nights, too, are likely to break records, with the number above 15 degrees in a row on course to reach at least 14. The existing record for November is nine such consecutive nights of 15 degrees or warmer.
Hobart may collect its most days in a row above 25 degrees for any time of the year. Including Tuesday, the city clocked up its fourth such days, matching the November record.
The current run of Hobart days above 25 degrees could reach six or more, setting a record for any time of the year, Dr Trewin said.
But the same system is directing onshore winds along the east coast, keeping a lid on temperatures such as in Sydney.
Daytime temperatures in the Harbour City have been running about a degree below the November norm, although a late burst of warmth can close the gap, and maintain Sydney's run of above-average months for maximums.
The last time Sydney had a below-average month for maximums was in March 2012, almost six years ago, Dr Trewin said. February 2013 and February 2014 matched the average.
Links
- BOM Climate outlook overview December to February
- La Nina likely to develop in the Pacific by December and last until February: Australia's Bureau of Meteorology
- BOM announces short, weak La Nina probability for summer lacking rainfall but packing heatwaves
- One Ocean, Two La Nina Forecasts: A Look Behind the Numbers
- Australia's weather bureau issues La Nina alert
- La Nina event almost certain to occur in southern Pacific: scientists
- Unusual climate mix has summer tilting to warmer times
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