Bathers hoping to cool off with a coastal swim in recent days might have got more than they'd bargained for, with inshore water temperatures well below average for mid-summer Sydney.
Temperatures at Bondi have been just 16.5-17 degrees for the past few days, said Bruce Hopkins, head lifeguard at the city's most popular beach.
That's well shy of the average for this time of year for Sydney's beaches of about 23 degrees, according to Beachwatch NSW.
What's at play is the so-called "Ekman transport" effect – identified in 1902 by Swedish oceanographer Vagn Walfrid Ekman – with wind-driven processes drawing up colder waters along the coast.
For the eastern seaboard, persistent north-easterly winds tend to move the top layer of the ocean eastwards. The warmer surface waters are replaced by an upwelling of cooler waters from below that feel frigid for those taking a seaside dip.
The reverse effect – a downwelling that brings in the warmer, more tropical waters from the East Australian Current – typically follows persistent southerly winds.
"The last real southerly came through on the weekend before Christmas," Mr Hopkins said, adding that since then, there's been steady north-easterly winds and swells.
A spokesman for the Australian Coastal Society said beachgoers can often get caught off guard when the warm air temperatures that draw the crowds aren't matched when people take the plunge.
"These are the times when [the cool water temperatures] are more noticeable," he said, with the possibly for some to experience "icecream headaches" from the sudden chill.
Jordan Notara, duty forecaster for the Bureau of Meteorology, said the next southerly change is not expected to reach Sydney until Sunday.
"That's obviously going to drive (air) temperatures down as well," he said.
Before then, however, Sydneysiders should have ample cause to consider a trip to the beach, especially on Friday and Saturday, when maximums are forecast to reach 31 and 33 degrees, respectively. The western suburbs should again see the mercury climb into the high 30s.
"At the moment, the strength of the north-westerlies will hold out or delay that sea breeze [on Saturday]," Mr Notara said.
The upwelling of cool water has another consequence: the increased chance of getting sea fog, as warm moist air moves over the colder sea waters.
South-eastern Australia will endure some of the region's most intense heat on Friday. "It looks like mid-40s through the Riverina and the lower and upper western districts," he said.
Victoria may give its January record temperature of 47.2 degrees a shake too, while Melbourne is expected to bake in 42-degree heat.
While coastal waters along Sydney's coast may seem to be on the cool side, the same is not true for much of the Tasman Sea.
Sea-surface temperatures are several degrees above normal for this time of the year, according to the US National Oceanic and Atmospheric Administration.
Peter Hannam is Environment Editor at The Sydney Morning Herald. He covers broad environmental issues ranging from climate change to renewable energy for Fairfax Media.