Guest Blogger: Ben Hodges
The Argo program is an ongoing international effort
to maintain a global fleet of 3000 or more autonomous profiling floats
distributed throughout the world ocean.
These expendable instruments drift for 10 days at a depth of 1000
meters, and then, by changing their buoyancy, make a quick trip down to 2000
meters and back up to the surface, measuring temperature and salinity as they
ascend. Once at the surface, they
determine their position and send the data they’ve collected to shore by
satellite. Each float is capable of
repeating this 10-day cycle as many as 200 times or more. Eventually though, they stop functioning—some
break down due to a mechanical or electronic failure; others run aground;
probably the most common cause of death is battery depletion. The float shown below, at the Scripps
Institution of Oceanography, was rescued from a premature demise by the
Instrument Development Group there after it sprang a leak in its external oil
bladder.
As old floats die, new ones must be deployed to replace
them. That’s where we come in. SPURS is not affiliated with the Argo
program, but while we’re out here, we’re happy to do our part to help maintain
the Argo array. Before we set sail from
Ponta Delgada, colleagues from WHOI shipped eight SOLO II floats, similar to
the one pictured above, to the Endeavor. We deploy each one in its cardboard box by
lowering it gently off the fantail after slowing down from cruising speed or as
we pick up steam when leaving a CTD station. The water-soluble tape securing the box
quickly dissolves, and the float is released to begin its mission. The biodegradable cardboard box eventually
disintegrates.
Along our cruise track so far (white line in the figure
below) we’ve deployed four of them (white circles). We plan to continue along the red line, and
deploy three more floats at the red circles (the eighth float is sick, and is
headed back to the lab for a checkup).
The yellow dots are currently active Argo floats (including the ones
we’ve just added), and the shading provides an indication of the density of
floats in the area (thanks to Pelle Robbins for the data and concept for this
figure).
Oceanographers use the measurements made by Argo floats for
many purposes. The floats directly measure current velocity at 1000 meters’
depth, and allow currents at other depths to be calculated based on spatial
changes in seawater density and the rotation of the earth. They also monitor the heat content of the
ocean, which changes with the changing climate.
As valuable as the information they provide is, it always
feels a little strange to drop a 40-lb robot that costs as much as a car into
the ocean and watch it drift away never to be seen again. But gathering this data by any other method
would be impossible—ships like the Endeavor can’t be everywhere all the time,
and by their sheer number, Argo floats can.