You bolt down to the azure ocean across a scorching Sahara of sand and plunge in only to experience near defibrillation and hypothermia because the water, delightful yesterday, is suddenly so cold you get a headache as if you've just eaten an icecream too quickly.
While you lose all sensation in your body below your neckline - what we surfers call "spanner water" (so cold it "tightens your nuts") - allow me to explain.
The cold water we're experiencing up the coast at the moment comes from the deeper ocean offshore, which upwells onto the coast due to a process known as the Ekman Transport. Steady winds in a consistent direction over the ocean move the top layer (to about 30 metres depth) of seawater.
In the southern hemisphere, the seawater layer moves to the left of the wind direction, due to the Earth's rotation (known as the Coriolis effect). The hot land temperatures of summer bring in sustained North Easterly onshore breezes, and the warm top layer starts drifting at right angles to the prevailing wind. This has the net effect of bringing up cold, nutrient rich water from the deeper ocean to replace it. The resultant cyclic vortex is called the Ekman Spiral, first investigated in 1902 by Swedish oceanographer Vagn Walfrid Ekman.
This usually lasts for a few days, and with cold water and very warm Nor’westers bringing in outback swelter we get the genesis of the stunning sea mists we have experienced lately. So when that southerly buster finally arrives, it has the effect of pushing that warm water back onshore and voila, Tahiti is back.
So the Ekman effect is not all bad news- it’s only a few days at worst, and it brings up lots of nutrients for fish to munch on and brings in larger fish and sharks to munch on them. Surfers learn instinctively to slip into a steamer after a prolonged North Easterly blow; swimmers are advised to keep an eye on weatherman Tim Bailey every night and become Nor Easterly spotters.
My personal observation, from living on beautiful Mistral point, Maroubra is that Nor Easters are becoming a lot stronger and more frequent. Whether this due to climate change is an argument for another place, but it wouldn't surprise me. But Ekman is not the only ‘villain’ here. Sometimes giant eddies occur within the East Australian Current (EAC).
The EAC (immortalised in Finding Nemo) brings tropical warm water down from Queensland, and appears to be getting stronger, and bringing warmer water, which has resulted in an upsurge of some tropical fish and soft coral species in Sydney Harbour.
In 2007 a prolonged freezing water event was caused by a giant eddy the size of Tasmania, situated about 100 kilometres offshore of Sydney that was swirling around like a whirlpool with a full rotation of 10 days. At its centre, the eddy caused upwelling of very cold water from depths of almost 1000 m that spun around, eventually hitting the coast. Causing some of the coldest, "spanner water" we have ever known.
Murray Cook is a surfer and marine biologist.