DIMES Progress Report
RR0901 - 29 January 2009
We made it as far as 58
S, on 25 January, with our CTD/XBT casts, when we found that one of
the crew members was suffering some heart trouble which required him
to be brought back to shore as quickly as possible. We have just now
left him off in Punta Arenas, after a fast 4-day steam with the wind,
seas and current at our back. Now we must return against those same
winds, seas and currents, to return to our site, which might take a
little longer.
At the time we left we
had covered the line from 50 S to 58 S, along 105 W, with our CTD and
XBTs. Also along that line we released 36 “RAFOS” floats in sets
of three. These “floats” don’t actually float at the surface. They are
precisely ballasted to be neutrally buoyant at the
temperature, salinity and pressure found at the density level that
serves as the focus of our experiment, which is about 1500 meters
deep. These floats have acoustic receivers on board which listen for
the sound pulses from the sound sources we deployed on mooring lines
earlier in the cruise. From the timing of the pulses from two or
three sources, the position of the floats can be determined later on.
“Later on” means after two years in most cases, when the floats
come to the surface and relay all their data to a land station via
satellite.
Caption: Three RAFOS
floats, ready for launch. The glass tubes can withstand pressures to
about 20 atmospheres, or about 2000 meters depth in the ocean. There
is a device at the far end which is compressible and gives the floats
about the same compressibility as seawater so that the floats move up
and down roughly with the water they are following. This device is
dropped when the time comes for the float to surface. The scrolled
scale inside the glass tube is used during ballasting. The
electronics in the metallic enclosure near the end enable the floats
to record sound arrival times and the pressure and temperature of the
water as they travel, and also enable these data to be transmitted to
land via a satellite connection when the float is on the surface.
Our plan when we get
back is to finish the CTD/XBT line and float release between 60 S and
58 S. We will have to forgo sampling any farther south due to the
unanticipated trip to Punta Arenas and back, but the essential goals
of the cruise can still be met: to deploy the sound sources (done),
the RAFOS floats (half-done) and our tracer (still to be done). We
also have some special floats to deploy, which will be described
later, and we would like to document the vertical distribution of the
tracer shortly after release. Our CTD/XBT data and the satellite
altimetry should tell us where, relative to the fronts and mean sea
surface elevation, we will have placed the floats and the tracer.