‘Like summer’s come early’: Sea temperatures surge off Sydney

1 week ago 5

Sea temperatures off Sydney are up to three degrees above normal for this time of year, making for ideal swimming conditions even as scientists sound the alarm about warming oceans.

Sydneysiders have been making the most of unexpectedly warm weather driven by a temperature spike above Antarctica that has upended forecasts for a rainy spring, flocking to beaches and ocean pools around the city.

Sascha Stone, Eliana Finke Alvarez and Tanya Jones enjoying swimming at Fairy Bower at Manly.

In recent weeks, they have also found the water much warmer than usual for this time of year.

“It feels like summer’s come early,” says Sascha Stone, 19, of Manly, who has been swimming at Fairy Bower and other favourite local swim spots.

“It is delightful to swim right now,” says Stephanie Hart of Mosman. “Even on these glorious days, you think how amazing it is and so close to perfection, but it does feel a little bit wrong that it’s so warm so soon.”

The current sea temperatures of more than 20 degrees are ideal for bathing to cool off or swimming for exercise, but also a symptom of a marine heatwave.

Professor Moninya Roughan from Coastal and Regional Oceanography Lab at the University of NSW said satellite maps showed sea surface temperatures around Sydney were three degrees above normal.

Her team also contributes to and analyses long-term data from Australia’s Integrated Marine Observing System, which includes manual temperature readings at intermittent depths from the sea floor to the surface at sites including Bondi, Eden and Coffs Harbour in NSW.

Stephanie Hart has grown to love cold water swimming, but says the warmer water is delightful too.

The average September temperature for 2006 to 2024 was 17.63 degrees, about three kilometres off the coast of Bondi near the sea surface. The average September 29 reading at the same location was 18.03 degrees.

The water temperature on Monday at Bondi, according to the Surf Life Saving Australia BeachSafe website, was 20.29 degrees.

UNSW Scientia Professor Matthew England said temperatures were often colder closer to shore because further out to sea, the East Australian Current brought warm water from the north.

“Twenty degrees is crazy-warm for this time of year,” England said. “I went for a swim [on Sunday] at Bronte and it felt like summer. September has traditionally been our minimum temperature month for the year [in the ocean] … it takes all of winter to basically suck the heat out of the ocean.”

England said one of the main effects of a warmer ocean was increased humidity in the atmosphere. Warmer waters have also been associated with marine algal blooms and changes in fish migration.

Roughan said the waters off southern NSW were warming at some of the fastest rates because the East Australian Current was moving further down the coast towards the pole, driven by changes in global wind patterns including the Southern Annular Mode. The effect was particularly prominent in the stretch from Batemans Bay to Eden.

Alice Golay with her son Koa and (behind) Stephanie Hart enjoying the warmer water.

The changes also meant more eddies – large whirlpools of warm water – that could get stuck off the South Coast.

At 20 degrees, the water would still feel cool when you enter the water, especially on a hot day, because it is cooler than your skin temperature.

University of Melbourne Associate Professor Joseph Rathner said it did not take long to adjust “even if it’s bracing cold” because the skin cooled rapidly.

“As my skin temperature matches the ocean temperature, my temperature receptors would actually stop noticing that difference, and then I’d start to feel comfortable,” Rathner said.

Cold water swimming

Stone swims throughout winter, enjoying the feeling of invigoration and the mental health boost, but in the cold months “it’s usually just a jump-in, jump-out situation”.

Hart, 55, also swims throughout winter, a hobby she has embraced only since her 50th birthday. She even swims at her second home in Tumut, where the water can be about 12 degrees, saying it takes a minute to adjust and six minutes to feel “amazing”.

Patrick Munro, Sanne Senior and Sophie Macintyre enjoy an early morning swim at Bronte in late August.

Cold water swimming has a devoted following, with many people swimming in the ocean year round, or taking cold showers or ice baths, citing health benefits.

Sydney sea temperatures rarely drop below 16 degrees even in winter – and the research on cold-water swimming defines it as 15 degrees and below.

Ben Singh is co-author on a recent paper that reviews the evidence for health benefits in the general population rather than elite athletes, finding just 11 studies that fit the criteria and mixed results.

The research suggests that cold-water immersion could decrease stress levels around 12 hours later and improve sleep, but results were inconsistent. One large study suggested cold showers reduced sick leave by 29 per cent, but the evidence was weak.

On the downside, cold-water immersion briefly increased inflammation right after and up to an hour later, though Singh said this might be part of a normal stress response and not necessarily harmful.

Rathner points out that while the study on the immunity benefits of cold water has a statistically significant result, it did not mean much in the real world.

“The claim of a 29 per cent reduction sounds impressive, but it’s based on a very small absolute difference – likely less than one day per person over three months,” Rathner said. “If you’re an employer ... is that really a meaningful difference?”

He said some of the cited benefits could be achieved more efficiently through exercise.

Rathner said extremely cold water could increase the risk of a heart attack. Hypothermia was also a risk when water was below 30 degrees – but at most sites in mainland Australia, it would take prolonged immersion beyond three hours.

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