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23 Feb - More about CTDs

Date:  Sunday 23 February 2002
Position @ 1200 (UTC -3): 75° 36'S 027° 43'W
Next destination: Off Halley, Brunt Ice Shelf, Antarctica
ETA: 24 February 2003, alongside Creek 2 once more
Distance to go: N/A
Total Distance Sailed this Season: 17217.6 NM (based on 3 kts 'hove to' for the bulk of the week)

Current Weather: Stormy, windy and cold
Wind:  NE x 36 kts
Barometric pressure:  979.8 mb
Sea state: Rough
Air temperature:  -0.8°C.
Sea temperature: -1.4°C.
Click here for ships track

The Weather Window

The weather this week - Click to enlarge
The weather this week
Click to enlarge

Above: This week's view of our world at 12.00 noon on a Sunday. Today's weather is not as bad as yesterday's (Saturdays) - hurricane force 12 winds!

RRS Ernest Shackleton in 'Wait On Weather' Mode

Since last Sunday's non-descript, overcast and miserable weather, we have had all the seasons of the year in the one short week. During our final CTD stations (see below) the weather continued dull and overcast, then as we turned our attention to Halley and Creek 2 mid-week, we were blessed with the very best of weathers. Latterly, we have been forced out of the Creek 2 and to 'stooge' up and down the Brunt Ice Shelf waiting for Hurricane Force 12 winds to moderate and the wintery visibility in sleeting snow to abate ! Weather has very much been the topic of conversation on everybody's lips this week.

Summer weather this week - Click to enlarge
Wintery weather this week - Click to enlarge Wintery weather this week - Click to enlarge

Above: Top - 'Summery' weather this week. Bottom - Wintery weather this week. Click the images to enlarge them.

Not only did the sun come out to welcome us to Creek 2, but so did a contingent of GA's from the base, a small colony of adélie and emperor penguins and a good collection of icebergs and bergy-bits to boot. Captain Antonio deftly persuaded the offending icebergs out of the way of our mooring place on Thursday with a gentle 'shove' from the prow of the ship. However, after a very productive day of work, the promised bad weather arrived and that prompted a tactical 'withdrawal'. Back at sea on Thursday night, the ship 'hove to' during the following days as it saw the winds rise, the sea become rougher and rougher, and the ship become more and more icy as hurricane-force winds lashed about our decks.

It was fairly comfortable onboard actually. With the head into the winds and seas, and with no speed necessitated by having to 'be somewhere', we were able to 'sit it out' quite comfortably around the corner of the Brunt ice shelf which awarded us some degree of protection from the storm. However, regardless of the degree of protection, it did not stop the wrath of the gods from blowing spotlights from their metal brackets overlooking the decks, or ripping the windsock clean away from it's elevated home aloft, nor prevent the windscreen wiper blades on the bridge from being so 'caked' in ice that they bent under the extra weight and broke ! As the weather moderates over the weekend and we resume our call to Halley, there are a good few 'repair jobs' to be accomplished courtesy of the weather !

I am often asked 'How cold is it in Antarctica?', and with a nice warm ship beneath us, we are pretty well snug and free from the minus temperatures outside. But this week was so cold that even our nice comparatively warm 'Ernest' endured temperatures cold enough for the snow to lay thick and fast on the metal decks.

As can be seen above, this provided a very wintery visage to the vessel.


Wavey Davey Has Been Fired !! :-
He said to the Chief Officer, ' Sir, My wheelbarrow is making a funny noise, it is going ...
'squeak,...........squeak.......... squeak!'
'You're fired' said the Ch.Officer, and Davey was left to ask 'Why ???'
'Because your wheelbarrow should have been going 'squeaksqueaksqueaksqueaksqueak....' said the Chief !!?

But don't worry, I'm sure Davey will be re-hired again before next week's witty spot !


There has been a great surge in interest in 'Extreme Sports' in the last decade. People are busy throwing themselves off cliffs and bridges on bungee ropes, or scuba diving with sharks, or jumping out of aeroplanes, but no-one yet has fully explored the untapped avenue of 'Extreme Window Cleaning'! That is where this week's Page 3 Males excel. Murdo Nicolson (AB) and James 'Jimbo' Baker (AB) were out there in the wildest of weathers to show how well ahead of the field they are in this largely unknown and untried recreation. In temperatures cold enough to freeze the very chamois leather to your fingers, they were out there providing the bridge team with better visual ability and the web cameraman with a good laugh !

Window cleaning - Click to enlarge Window cleaning - Click to enlarge Window cleaning - Click to enlarge

Above: Expert Extremist Window Cleaners Jim and Murdo brush and squeegee their way into the webpage !!! Click the images to enlarge them.


Now the Shackleton's science cruise is finished, all the gear has been stowed away, and the scientists are getting used to regular sleeping schedules again. So now there is actually some time to write about one of the most important instruments in physical oceanography today, the CTD. CTD stands for Conductivity/Temperature/Depth, which are the three variables measured by the instrument. The actual CTD is the white cylinder at the bottom of the picture; it has a built-in pressure sensor to measure depth, and temperature and conductivity sensors just underneath it, on the left side. The CTD is attached to the end of 3000 meters of steel sea cable, and every second it sends back 24 measurements, which are plotted on a computer screen in a little blue container on deck. On top of the CTD itself is a water sampler rosette. In the picture there is only one bottle mounted, but on this cruise we usually used four, and the rosette can actually hold twelve bottles! On the top of it are little hooks, on which the wires from the water bottles are placed. Then, when you press a button on the computer, one hook will open, and the top and bottom ends of the bottle attached to it will snap in place, sealing off a water sample, which then can be tapped into bottles when the CTD comes back on deck.

CTD ready to be deployed - Click to enlarge CTD being repaired - Click to enlarge

Above: Left - The CTD as it looks when almost ready to deploy. This bottle is closed, though. Right - Innards of a CTD that has been misbehaving. Scientists Cathy Moore, Bogi Hansen, and Povl Abrahamsen grease up some o-rings before re-assembling. Click the images to enlarge them.

The purpose of the CTD is to measure temperature and salinity in a profile down to the bottom of the sea. The conductivity can be converted into salinity, since the more salt is in the water, the better it will conduct. Salinity is measured in PSU (practical salinity units), which are approximately equal to promilles of salt in the water. Both sensors are extremely sensitive; the temperatures should be accurate within 0.001 deg C! The conductivity is also quite good, but can drift a bit over time, so we use water samples, which are sent back to a lab for analysis, to calibrate the measurements. Other properties can be calculated from these variables, such as density and the speed of sound (which could be useful for adjusting the readouts on the ship's echo sounder, for example).

The CTD container - Click to enlarge
The CTD container
Click to enlarge

Above: Tthe CTD container. Here the scientists can enjoy warmth and shelter while watching the profile appearing on screen. They'll freeze their hands off tapping water samples, though!!

In addition to the CTD itself, we also used a little current profiler on some of our profiles. It is the orange cylinder mounted next to the CTD, and is a so-called ADCP, Acoustic Doppler Current Profiler, which basically works the same way as a Doppler radar, except upside down. However, since it only could be used down to 1000 metres' depth, we did not use it on all stations.

So what can we actually learn from a CTD profile? Down around Antarctica the water is quite cold, and tastes salty - and most people are probably content with knowing that. But even though all the temperatures we measured in our 91 profiles were all between -2 and +1 deg C, there was quite a variation within this narrow range.

Typical CTD profile - Click to enlarge
A typical CTD profile somewhere in the Weddell Sea
Click to enlarge

Typically in the ice-covered areas, we could see a reasonably thin surface layer, with temperatures just above the freezing point and with low salinity. This is probably meltwater from the sea ice: since the ice is less saline than the ocean, it will lower the salinity of the surrounding water, as well as bringing the temperature down to the freezing point. In cold temperatures, density is tied very closely to salinity, so this water is much less dense than the surrounding water, and stays on the surface.

Somewhat lower in our profiles, we found warm temperatures (some almost up to +1 deg C!), along with higher salinities. This water is, not surprisingly, called Warm Deep Water (WDW), and is typically found in the Weddell Sea, where some of it flows southwards under the Ronne-Filchner Ice Shelf. When it comes out again, it has been cooled down very much and the salinity has been raised, due to successive melting and re-freezing processes under the ice. Down at 500 metres' depth, the freezing point is lower than at the surface, so our lowest temperature measurement was -1.98 degrees, in what almost certainly was Ice Shelf Water (ISW). The ISW is some of the coldest water in the world's oceans, and when mixed with WDW and other water masses, it will form Antarctic Bottom Water, a water mass which is so dense that it is found at the bottom of most of the world's oceans.

But why would we want to know all this? Well, one reason is to figure out the general circulation in the Weddell Sea. There have been several similar surveys conducted in the past, and most of our measurements seem to agree with our general expectations. But accurate measurements like this can help monitor any possible climate change that is taking place. There are theories that the temperature of the ISW has risen minutely over the past 30 years. The more measurements made, the more certain we can be that this either is or isn't taking place. But we were primarily trying to answer specific questions like "How far does the coastal current extend from the coast?" or "How does the ISW change as it moves down the continental slope?" Now in the months and years to come, BAS and the Bjerknes Center for Climate Research in Bergen, Norway, with whom BAS are cooperating on this project, will have to try to answer these and many other questions, as well as posing more that will be raised when analyzing our data.

CTD being deployed - Click to enlarge
Launching the CTD. Normally this is done by only two people, not four.
Click to enlarge

Written by Povl Abrahamsen, with helpful comments and suggestions by Bogi Hansen.

At Sunday lunchtime, 23rd February, the ship is taking a slow steam up around the Brunt Ice Shelf to arrive at Creek 2 in the late afternoon to survey what damage the storm has done to our little 'platform' at the foot of the Creek 2. Although all cargo has been completed, there is still an amount of waste and some 600 empty Avtur Barrels that we would have liked to uplift this call. Moreover, there are also some 35 eager people wanting a lift back to the Falkland Islands and their onward flights home to civilization. Some of those leaving this call will be leaving Halley for the last time after 2½ years in residency there. The general feeling at the moment can be summed up in one amusing incident in the height of the storm on Saturday. We were going about our duties on the bridge of the ship when the VHF Radio Channel 06 broke into life. Not a word was spoken, but the refrains of ' We've gotta get out of this place, if it's the last thing we ever do... ' was heard loud and clear ?!!

Forthcoming events: We will uplift all the personnel from Halley base as soon as the weather permits, and then set sail for the Falkland Islands via more base work en route. So far we are anticipating calls at King Edward Point (KEP), and Bird Island on South Georgia and Signy in the South Orkneys.

Contributors this week : Many thanks to Povl for the excellent scientific write-up in terms that even a simple web-editor can understand. Thanks also to anyone who was wielding the web camera this week and apologies for all those hundreds of shots that we unfortunately have insufficient room to display.

Diary 23 will be written on 02nd March 2003 for publication on the website by 03rd March 2003.

Stevie B