Tuesday, 8 July 2008

Oooo check out those waves!

Today we’ve been going around in circles… well an oval really – an oval over the edge of the bank and back up again, one circuit every 6th of the tidal cycle for 25 hours – so we get two repeats of the tidal cycle over one day-night cycle. During these circuits we’ve been towing an ‘MVP’ – a Moving Vessel Profiler – which basically goes up and down the water column (hopefully not hitting the bottom on its way down!) measuring temperature and salinity through the water column. Also, we’ve been collected fish data using the echosounder & the birders have been busy with their binoculars… in a bit of sunshine! (well briefly – enough to make Mark get a wee bit sunburnt).

The physicists got very excited seeing some of the internal waves visible on the fish acoustics (I got excited too… check out the waves dude! Those fish are surfing the internal wave!).

What is an internal wave?… hmmm, well instead of a wave at the surface, it’s a wave at the thermocline – so where the warm light water sits on top of the cold heavy water (different densities – think of oil sitting on water) – at the boundary waves can travel (try it out – half fill a beaker with water & with oil, then jilt it a little and see if you can make a wave travel across the boundary layer between the oil and the water… I’ve not tried it – just thought it up so it might not work… but fun to try!). Anyway, these waves are basically caused by water flowing across bumps on the sea bottom and creating waves at the thermocline that travel, and cause mixing where they occur (turbulence), and so allow the nutrients from the cold deep layer to mix with the warmer surface layer, which is great for the phytoplankton (mini sea plants) & hence for the other critters in the sea. So well… that’s why we got all excited to see them on the fish echosounder… along with lots of fish (still trying to work out what species they are – but we’ll have fishermen coming along after us to fish & see what’s out here).

After all this excitement with the internal waves, I spent an afternoon with the birders to see if all this sub-surface activity was resulting in lots of birds and marine mammals. Alas not (still no dolphins, sorry)! We had a fun afternoon eating icecream and talking about sharks in supermarkets and dogs eating hotdogs (I guess you had to be there!), and were occasionally interrupted by the flitting storm petrels (look like bats darting across the surface – they’re tiny!), a few gannets & fulmars, a big fat bonksy harassing a young gannet, and some lovely Cory’s shearwaters (not seen them before – they come up from the Azores… goodness knows why in this July weather!) – but very elegant twisting and turning over the waves. The birders take this business all very seriously (check out Andy & Adam in the photo). And I enjoyed my relaxing birding afternoon, very peaceful (though I came in before the heavy rain!).

More fun tomorrow – more horrible weather on the way…

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