DIMES Progress Report
RR0901 - 22 January 2009
We have started a
hydrographic survey line at 50 S, 105 W, and will head due south as
far toward the ice edge around Antarctica as weather and time will
allow. The survey is done with instruments that measure the
conductivity, temperature and pressure of the water from which depth,
salinity and density can be calculated. There is also a sensor for
dissolved oxygen, and there is a separate instrument to measure the
velocity of the water within a few hundred meters below the package
using the Doppler effect acting on pulses of sound. There are
bottles that can be closed when signaled to capture water to
calibrate the salinity measurement (see first figure). All of this,
called a CTD for short (for Conductivity, Temperature, Depth), is
lowered on a conducting cable to the bottom, which is 4 to 5.5
kilometers (2.5 to 3 miles) beneath us.
We will occupy around
30 stations with this system on our line south. These will be
supplemented with 20 or so Expendable Bathythermographs (XBTs), which
measure temperature and relay it back to the ship via a fine wire. The
positions of the stations will be guided by measurements of the
sea surface height from altimeters orbiting the earth in satellites.
The second figure below shows estimates of the sea surface height,
relative to where it would be if the ocean were still, on 13 January
2009. The surface is generally higher in the north. In the southern
hemisphere this means that the currents are generally to the east,
i.e., with the higher pressure to the left. The sea surface height
map, though, is filled with excursions and swirls that are analogous
to the high and low pressure areas in the atmosphere, but occur at a
smaller scale in the ocean. The Antarctic Circumpolar Current (ACC –
acronyms seem unavoidable) is a turbulent stream, and our aim is to
release neutrally buoyant drifters and a tracer in it at about 1500 m
depth (a little under a mile) to measure the rate at which seawater
components, heat, and momentum are transported across it.
Our CTD system being
lifted over the rail of the ship and about to be lowered into the
ocean. The sensors are toward the bottom, on the left. The acoustic
current meter is in the yellow cylinder behind one of the lead
weights, and the PVC tubes with their caps cocked open are the water
samplers, ready to be tripped when the operator sends the signal. The
cable carries these signals down to the CTD, and also carries the
readings from the sensors up to the ship as the CTD is lowered. This
particular package is relatively small and is well weighted with the
lead so that the cable does not go slack as the ship moves relative
to the water, especially during deployment and recovery.
A map of the sea
surface height, relative to an equilibrium surface, obtained from
satellites equipped with altimeters. The scale is in millimeters so
that the surface in the northern region here is 1500 mm (5 feet)
higher than in the far south. Maps like this are prepared for us
every day by Valery Kosneyrev at Woods Hole, and emailed to us.