Floats that sink

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.

For more information about the Argo program, visit http://www.argo.net.