Record run of autumn hot weather for Sydney as waters simmer past cyclone threshold
A stationary Tasman high deflecting cool changes below NSW is one of the factors for the abnormal heat spell. (Supplied: Linlin Mont Alma Smith)
The start of autumn has delivered extraordinary heat to NSW, both over land and in the water.
The warmth has reached record levels in Sydney, including an unprecedented run of days above 28 degrees Celsius and an offshore water temperature nudging 27C — warm enough for a tropical cyclone.
While relief from the unseasonably high air temperatures will arrive later this week, summery weather is likely to return next week and potentially linger well into April.
Spell of abnormal heat stretching to 10 days
After a hot and humid summer, NSW is now enduring an extended spell of warm autumn weather reaching its second week.
Temperatures across many inland regions have already exceeded 5C above average for eight consecutive days, peaking as much as 12C above normal in the state's south.
As a result, monthly averages are running at levels up to 7C above normal, including 35.5C at Narrandera, more than 3C above the town's record for March.
While the heat along the coast has been less extreme, what's unusual is the duration of warm weather.
True autumnal weather is expected to arrive in another few weeks, as Sydney sweats through seven consecutive maximums above 28C. (ABC News: Jessica Rendall)
Sydney has sweated through seven consecutive maximums above 28C (measured at Observatory Hill), already an all-time record for autumn since the station opened in 1858 — with another two days above 28C likely on Wednesday and Thursday.
The record warm spell has lifted Sydney's average maximum this month to 28.3C, 3.5C above the long term average and currently above the March record of 27.6C in 2023.
So what's behind the extension of summer weather? The prolonged heat is due to a combination of factors including:
- A mass of sinking air above the state called an ‘upper high’ but often referred to colloquially as a heat dome
- A stationary Tasman high deflecting cool changes below NSW
- Record warm water temperatures off the coast
- Climate change which is a background influence on our weather
The upper high or heat dome is the most critical feature for intense heatwaves, as sinking air rapidly heats up on descent at a rate up to 9.8C per 1,000 metres.
The above pattern will continue today and tomorrow before a cool southerly change finally arrives – due to filter through the south on Thursday, reaching Sydney late afternoon, then through the north on Friday.
Loading...Tasman Sea resembles tropical Queensland waters
One of the main drivers of the warm start to autumn is record high ocean temperatures off the coast.
During the past week the Sydney wave buoy run by Manly Hydraulics Laboratory has twice registered a temperature of 26.75C, the highest on record for any month with data back to 1992.
This beats the previous record of 26.6C from February 2022, and is around 3C above average for March.
The water is even warmer around 100 kilometres offshore, simmering above 27C which resembles tropical Queensland and could theoretically support the formation of a tropical cyclone, sitting just above the 26.5C threshold for cyclone development, although other cyclonic ingredients are not present.
The sea temperature off the coast of NSW is above 27C as far south as the Illawarra. (Supplied: Bureau of Meteorology)
Moninya Roughan, a professor of oceanography with UNSW, said the record warmth is due to the demise of La Niña combining with local conditions.
"The last few years water temperatures have been down due to La Niña acting like an air conditioner, so we haven't noticed the effect," she said.
"This year the air conditioner has been turned off … and in the last six weeks hot water off Brisbane has pushed down the east coast."
Professor Roughan believes the warm waters will linger for at least another few weeks as the East Australia Current strengthens.
"Currents will get stronger during the coming weeks, so we are unlikely to see short term cooling."
When will it actually feel like autumn?
Despite short term relief arriving, further spells of warm, muggy weather are likely for at least another few weeks before true autumnal weather arrives.
As early as Monday to Wednesday next week modelling indicates a warm northerly airstream will return, which could send temperatures back to near 10C above average.
Although the next warm spell is unlikely to match the duration of the current heat, the simmering Tasman Sea along with the background influence of climate change, should ensure above average temperatures linger through autumn.