Dips in water temperature are making life unseasonably tough for ocean swimmers across Sydney, and an ill wind is to blame, reports Sarah Whyte.
OCEAN swimmers have had a rude shock with water temperatures dipping as low as 17 degrees this past week - comparable to an average winter's day.
The bracing temperatures have been creating headaches for ocean swim organisers with some participants suffering the initial signs of hypothermia, while regular dippers have been dusting off their wetsuits.
The culprit is northerly winds moving the warmer surface water away from the NSW coast and drawing deeper, colder water to the surface - a scientific phenomenon known as "upwelling".
It has been responsible for wildly fluctuating sea temperatures. Last Wednesday, the temperature rose to 22 degrees before dipping to 17 degrees on Friday. The average temperature for this time of year is around 21 degrees.
The conditions meant 70 swimmers suffered the first symptoms of hypothermia during Coogee's 2.4-kilometre Express Glass Island Challenge ocean swim a fortnight ago, after the water temperature dropped to 16.8 degrees.
"It was unseasonably cold," said event co-ordinator Lisa Bolton. "But in the 11 years the race has been held, this has not been the coldest."
One competitor, Andy, described the conditions on an ocean swimming blog: "Just after one of the last yellow buoys having rounded the island, the cold got to me, panic set in and all co-ordination just went out of the window."
Last week's Bondi to Bronte ocean swim was also hampered by the chill.
"This was the coldest race we've had in five years of holding it," race director Stephen Ford said.
"The water at this time of year is generally 19 degrees, but due to the north-east currents this dropped the temperature ... We have had strange climate conditions this year and this in turn affects the temperature of the water."
Thirty-seven ocean swims are expected to take place between now and June next year.
Sydney's largest ocean swim, the 2.5-kilometre Macquarie Big Swim in January, says on its website that competitors will be able to wear wetsuits if the water temperature is below 18 degrees.
Swimmers may be puzzled by the chilly sea temperatures but scientists and oceanographers say upwelling is a common phenomenon.
A Bureau of Meteorology senior scientist, Dr Gary Brassington, said the cold water had surfaced due to persistent northerly winds. "With the air and sea temperature contrast, swimmers will really notice the sudden drop in the ocean temperature," Dr Brassington said.
And the good news?
"As soon as there is a southerly change, the warmer water will return," bureau forecaster Michael Logan said.
Frequently Asked Questions about this Article…
Why have Sydney ocean temperatures fallen to around 17°C recently?
Persistent northerly winds have caused upwelling — they push warmer surface water away from the NSW coast and draw colder, deeper water to the surface. That pushed sea temperatures down to about 17°C in some spots, well below the seasonal average of roughly 21°C.
What is upwelling and how does it affect ocean swim conditions in Sydney?
Upwelling is a natural ocean process where winds move warm surface water away and bring colder deep water up. According to scientists in the article, it can cause wildly fluctuating sea temperatures and sudden chilly conditions that swimmers will notice.
How extreme were the recent temperature swings at Sydney ocean swims?
The article reports rapid swings — sea temperature rose to about 22°C one day then dropped to near 17°C two days later. Those kinds of rapid changes made some organised swims unusually cold for this time of year.
Did any organised Sydney ocean swims experience hypothermia cases because of the cold water?
Yes. At Coogee’s 2.4-kilometre Express Glass Island Challenge, when water dropped to about 16.8°C, 70 swimmers experienced the first symptoms of hypothermia. Race directors at other events, like Bondi to Bronte, also said this was the coldest race in several years.
Should I wear a wetsuit for upcoming Sydney ocean swims?
Many regular dippers have been dusting off wetsuits during the cold spells. Event rules vary: for example, the Macquarie Big Swim’s website says competitors may wear wetsuits if the water temperature is below 18°C. Check each event’s policy and current water temps before you swim.
How many ocean swims are scheduled in Sydney and could they be affected by the chilly sea temperatures?
The article notes 37 ocean swims are expected between now and June next year. Because upwelling can cause sudden cold conditions, organisers and competitors should monitor forecasts and water temperatures ahead of each event.
What signs of trouble did swimmers report when the water turned very cold?
A competitor described that after rounding a buoy the cold ‘got to me, panic set in and all co-ordination just went out of the window.’ Organisers also reported dozens of swimmers showing initial hypothermia symptoms during the cold spell.
When can swimmers expect warmer water to return to the NSW coast?
Bureau forecasters in the article say a southerly change will push the warmer surface water back toward the coast, and the water should warm up again once that change occurs.
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