Thursday, 11 April 2013

Day 27: CTDs near Neptune's Palace

Hi All

The weather has blessed us this week. Cruising North from Antarctic waters, the horizon has been near glassy. Despite being 4 weeks into our journey the surprises and excitement hasn't let up. Pairs of whales have enjoyed krill feasts. Seals, penguins and birds seem to remain as curious about us us we about them. Yesterday we saw an island of ice. A tabular iceberg 6 miles long. That means it would take you about 2 hours to walk it's length...if it where perfectly flat...which it not...and if you could get on top of it...Anyway the berg was so big it formed a line of cloud on the horizon and we saw no end to it.

Photo1: The Iceberg extended as far as we could see into the horizon.

The VMPs have begun to work again (although we remain on our toes!)
and tonight we do our last CTD before landing in South Georgia early in the morning. As John gave you complete picture of the CTD sampling process yesterday, I thought I'd talk about what the CTD crew do while its in the water.

Photo2: Andrew - one of my fellow nerds - interpreting the Temperature and Salinity data streaming through (on the far right yellow is oxygen, pink is salinity, green is density and red is temperature, the horizontal lines are where we have triggered the bottles to close and trap the blob).

The CTD measure temperature, salinity and oxygen. As it descend to the sea floor, often 3-4km down on a very long wire, what it is measuring is displayed in real time on a bunch of screens we have around the place. Although I'd like to pretend it is like NASA headquarters in is a bit more like the basement of a nerdy computer hacker. And the nerdyness doesn't stop with the décor. As the temperature and salinity are displayed we can gauge where the water is from. Going down in the water column we see cold water which was put there last winter when the ocean was churned up, below that we see salty water that came from the North Atlantic and below that...anyway we enjoy it.

We know roughly at what temperature and salinity the blob of tracer was injected into the ocean at. If it didn't mix up much at all we'd expect it to still be the same temperature and salinity. So the CTD crew are looking keenly for the right numbers to come up. When they do we sample  close the plastic bottles at and nearby. It is actually quite remarkable how little the blob is mixing. When the Tracer was in the pacific ocean it mixed vertically at such a slow rate you could have done a better job with a hand blender (Nerds only: my colleague Bill estimates that mixing a typical cubic kilometer of the deep ocean at a rate of 10 cms squared per second-which is what was measured-requires as much energy as a common hand mixer). It seems to be stronger where we are now but it is fascinating just how little it mixes. Brian, who was on the voyage that injected the blob keeps saying: "who would have thought that you could put 75kg of this stuff in the middle of the ocean and still go out and find it again 4 years later".

Photo3: One of our last CTDs before South Georgia with a not-so-Southern-Ocean-like swell.

When tonight's CDT came out of the water we where treated to perhaps the most majestical iceberg yet. It most likely melted from below then flipped over. The result, as Gwen suggested, looks like Neptunes Winter Palace.

Photo4: A special one.

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