Aquabumps.com - where the waves live

Tuesday, January 23, 2007

Damn hot’n’sticky in the jungle this morning. Could cook things in my pants. That water temp has crept up after yesterday’s howling southerlies. So you’re safe from a heart palpitation as you lap up 19 degrees on your morning dip.

Wind is non-existent early, but will swing to the E and then NE this afternoon (strong NE 15/20 kts) . South swell kicked up yesterday producing consistent 3 footers, now a tad smaller but still fun. Not much power seeing its locally generated swell. Expect the swell to swing to the east by the afternoon and pick up another notch (as the period increases).

So why has the water been so cold eh? Kellie from Bondi Beach Toni&Guy ( 70 Hall Street) has donated a prize for the best explanation – yep a haircut. Um got 146 responses. 90% are blaming the thousands of cold blooded pommes in the water at the moment. Here are two others…

1. esky ice.
"Every weekend when we finish our barby and beers at Manly, we throw the ice out of the eskys off near the Bower. It appears that the constant north-easters have been blowing the floating ice down to Bondi and cooling down the local waters. Normally in the southerlies it would float up to Longy." Ben Grauer

But here is a more scientific reason: (from Allan Young who wins the haircut)

2. ekman transport.
"The short answer is that these conditions affect much of the NSW coastline, and are caused by several days in a row of fresh to strong NE winds.

You need the winds to blow for a few days because it takes a lot to peel off the top level of warm water and let the deeper cold water rise up. This is an ‘upwelling’ and can bring cold water from hundreds of metres below the surface.

Thats the easy part. What most people can’t get their head around is why a wind from the north east (ie hotter regions) doesn’t blow warm water onto the beach.

So here is where the phenomenon of "Ekman transport™" kicks in. Ekman figured out that the end effect of several days of a northerly or nor’easter is to push surface water 90 degrees to the left of the wind direction. To save you the maths – it means surface water gets transported east and south east. Thats out to sea. But it doesn’t transport the deeper colder water nearly as much. Result – warm surface water sitting offshore; cold water at the beach.

We have had heaps of strong nor’easters this month – more than usual. Thats why its particularly cold. It will take a fairly strong and persistent southerly change to counteract the conditions." or read Ben Matson’s weather guru summary::uge