SEASONAL winds and currents have given east coast swimmers a shock this summer as cool subsurface water has made its way on to beaches.
In a report in The Australian CSIRO oceanographer David Griffin said strong north to northeasterly winds pushed surface water southwards along the NSW coast and that due to the Coriolis effect (caused by the Earth's rotation) that water took a left turn.
What replaced it on the coast was water from about 50m below the surface - a process known as "upwelling ".
Coastal water temperatures in recent weeks had been in the high teens, against an average in the low to mid 20s.
"There is a cold fringe and warm water further off shore," Dr Griffin said. "The surface water wants to go left of the wind, so if the wind is from the northeast, the water turns off shore and that sucks up cold water from beneath.
The article showed the upwelling using IMOS OceanCurrent imagery.