Visions in the mist

(written 7/3/07 16:00 GMT, i.e. before my previous blog entry)

Hello. While switching my body’s battered internal timepiece from the old-style day shift (1200-0000) to our new-and-improved night shift (2000-0800), I have a short period of ‘spare’ time. This is unheard of on ship, and I was planning on having a lovely time up on the Monkey Island capturing gigabytes-worth of pixels in different shades of white for you to enjoy later. However, Madame Nature has once again thwarted my plans by producing an acrid grey fog of the purest Antarctic lint, so my trusty sidekick Mr. Canon has gone to bed in a huff. Thus, I am now confined to my cabin with a warm laptop and the acorn of a plan for (shock! horror!) an afternoon nap. If that got out around the ship I would probably be run up the foremast for relaxing too much. Before I turn in, I thought I might spend some time on a blog entry that I have planned for a while, viz. a quick tour of the scientific measurements that we are here to make. The aim is to tell you briefly what we measure, not why we are here measuring it – that will have to be saved for another year.

Obviously I am biased, but it seems to me that our knowledge of the oceans is not in as good a state as perhaps it might be. I once heard ocean-data kingpin Sydney Levitus mention that, while we know the surface topography of Mars to an accuracy of 1 metre (vertical) everywhere on a 1-kilometre grid (horizontal), there are areas of the world ocean up to 500 square kilometres in area that have never been surveyed by a ship at all. This means that there are large areas of the only planet we’ve got where we don’t have even the most basic information on what the sea bed looks like, let alone what the currents are doing, etc. When you consider that the oceans carry the majority of the heat, mass, and momentum in the climate system, that lack of information becomes a climate-predicting problem for humankind as a whole.

As a small example, have a look at this photo of the output from the bridge’s navigational equipment. We are the red dot in the middle of the screen, apparently floating above Charcot Island! Obviously, the chart is wrong. Unless this is like The Sixth Sense or something, but I can’t see Bruce Willis hanging around anywhere (though Binns our winch-driver is probably a reasonable approximation).

The JCR is a Destroyer in the war against lack of oceanographic knowledge, bristling with the (almost) latest weapons in the scientists’ arsenal. In the rest of this entry I will briefly list only the ones we are using; obviously, there are many many many (many^3) other devices that could be deployed, and lots of them are sitting around in pieces somewhere on the ship. I will arbitrarily divide the instruments into CTD-borne and ship-mounted kit. We are also doing sea-ice work, of course, but that’s another story. If you want to know more about any aspect of oceanography then this page is always a good bet.

CTD-borne

Snow Petrel-eyed readers will remember that the CTD is the piece of kit that we lower over the side of the ship on a winch. Worship and cringe ye savages as the horned CTD god is lowered into the smoking fiery pit that the southern ocean becomes at sunrise when the air temperature is –20C:

And here are some slightly more sensible pictures of it :


The metallic bits at the bottom are the sensors and the grey bottles are for sampling. We run the thing all the way down to (hopefully not quite) the bottom and then back up again, recording data continuously all the way. This is what it measures:

Pressure (D): we use pressure (the force arising from the weight of water above the sensor) to determine the depth that the CTD is at, using some mystical knowledge of the ocean properties.

Temperature (T): Astoundingly, we use thermometers to measure temperature. Once again BAS is at the forefront of global scientific developments. We also calibrate the continuous temperature recording by taking very-high-precision readings at selected depths using a different sensor.

Conductivity (C): We measure the electrical conductivity of the water. This is a function of, amongst other things, the salinity of the water, so we use it to derive that. In the polar oceans salinity variations usually have the biggest impact on the density of the water, which drives the flow, so it is important.

Dissolved Oxygen: We measure the amount of oxygen dissolved in the water. It gives us a rough idea of the amount of time since a given bit of water was near the surface and tells us about what the wildlife are up to, apparently.

Flourescence: A UV light is shined through the water and the fluorescence produced is measured. This is related to the concentration of phytoplankton (microscopic plants, basically green slime) that the water contains.

Transmissivity: A light is shined through a section of the water and the amount reaching the other side is measured. This gives us an idea of how clear the water is, revealing its murkiness or otherwise as a result of floating particles etc.

Altimetry: The CTD has a downward-firing echo-sounder to make sure we are aware that it is about to crash into the sea bed. That would be more expensive than crashing a Ferrari and therefore a career-limiting move.

Water Sampling: The 12 grey bottles in the sensible photo above are called Niskin bottles (after a famous oceanographer’s cat) and stay open as the CTD is lowered or raised through the water until, at chosen depths, we press a button on our computer that snaps the lids shut. This traps the water inside, and it is brought kicking and screaming to the surface to be poked and prodded by us scientists. This is what we measure:
Salinity: Measured to calibrate the continuous conductivity readings. By far the biggest source of pain for novice cruise monkeys like me as it has to be bottled (cold fingers) and then fed into the hungry salinometer (dulled brain).
Dissolved Oxygen: Again, measured to calibrate the continuous reading. Yawn.
Oxygen-18: Aha! An interesting one. The samples we take are sent away to some laboratory or other, where they measure Oxygen isotopes. These isotopes are affected by evaporation and subsequent precipitation (snow or rain), so that glacial ice has a different signature to seawater. Therefore, we can use these measurements to get an idea of how much glacial meltwater is in each bit of seawater.

Ship-Mounted

Acoustic Echo-Sounder: Measures the depth of the ocean by firing a burst of sound downwards (a ping) and then listening out for its echo as it bounces back off the sea bed. The depth is then derived by combining the time taken for the echo to return and knowledge of the speed of sound in seawater (mystical again). Witness the depth increase suddenly as we change from shelf-seas to the deep ocean:

Multi-beam Swath Echo-Sounder: The suave older cousin of the above. Uses an array of instruments to fire and receive each ping in such a way that, for every ship position, the depth can be determined at a line of points perpendicular to (across) the path of the ship. This means that for every ship position, a line of depth-soundings are taken (running Port-Starboard). If the ship is moving, this results in a 2-dimensional map of the sea floor and lots of pretty colour plots.

Acoustic Doppler Current Profiler (ADCP): Used to measure ocean currents beneath the ship. It transmits high-frequency acoustic signals that are scattered back by bits and pieces in the water. It then estimates ocean velocity at each depth by using the Doppler effect to measure the radial relative velocity between the instrument and scatterers in the ocean.

GPS and navigational systems: The ship has many positional and navigational systems to tell where it is and where it’s going, but it’s not quite that simple. The acoustic systems mentioned above also need to remove the pitch, roll, and heave from their data and all of the above need to remove the average movement of the ship too. Cue the techy (usually) men in white coats and spectacles.

Underway Meteorological and Oceanographical systems: The air temperature, relative humidity, and solar radiation are recorded continuously from the foremast of the ship. The ocean’s temperature, salinity, and flourescence are recorded continuously by a sampling device that sucks water from underneath the ship (6 metres depth). When it’s not frozen solid.

That’s the end of the list. I have tried to write the entry in lay-person’s language but I’m sure that confusions and inaccuracies remain, despite the sage advice of wizened seafarer Mark Brandon on a number of matters. Any errors that remain are entirely the responsibility of the British education system and my slow internet connection. No correspondence on subjects I know nothing about will be entered into.

• The data purists out there will note with glee that the definition of science in the title does not explicitly include modelling. Discuss.